# Jvm Mods



## Col

Great news!!! The JVM Forum is back in action thanks to the efforts of the moderator Andy.

For any modding information please go to the Modding Bible section of the forum. You'll need to register to get there but it's definitely worth it.

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Well it’s been some months since the JVForum has been down. Honestly to me it looks like it’s not coming back.

It’s a shame because there was such a wealth of information there - especially for the modding community.

To try and preserve this information let’s post or link as many mods for the JVM series as we can into this thread for future modders.

If the JVMForum ever comes back I’ll just link directly back to that.

In the meantime please post your mods 

Here are links to some of the mods posted below:

Compression Reduction Mod
1959 Plexi Cap Mod
Negative Feedback Mod
Choke Mod
C83 Mod
Gain Reduction Mod
AFD100 Mod
Anti Stiffness Mod
OD2 To OD1 Mod


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## RickyLee

Bummer for me as I just recently got a JVM 410H. I would have had big time fun in that JVM Forum, no doubt.

I am going to try tinkering with the NFB loop on mine. I am curious to how so many people got that big of a change in the power amp, as stock the amp has an 82K NFB resistor. Though it is tapped from the 16 ohm winding. I thought I seen a capacitor in the NFB loop on the schem as well. But I am going to experiment with this one for sure soon.


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## el_bastardo

RickyLee said:


> Bummer for me as I just recently got a JVM 410H. I would have had big time fun in that JVM Forum, no doubt.
> 
> I am going to try tinkering with the NFB loop on mine. I am curious to how so many people got that big of a change in the power amp, as stock the amp has an 82K NFB resistor. Though it is tapped from the 16 ohm winding. I thought I seen a capacitor in the NFB loop on the schem as well. But I am going to experiment with this one for sure soon.


I think most of them are less. I'm pretty sure the JVM had like 27k or 42k or something like that, hence the reason so many people modded theirs for variable NFB. I did mine. I put my NFB on a pot....and still keep it stock 99% of the time. I recently did a choke mod, and that was way more useful to me.


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## Casey_Butt

RickyLee said:


> I am going to try tinkering with the NFB loop on mine. I am curious to how so many people got that big of a change in the power amp, as stock the amp has an 82K NFB resistor. Though it is tapped from the 16 ohm winding. I thought I seen a capacitor in the NFB loop on the schem as well. But I am going to experiment with this one for sure soon.


The winding it's tapped off from is the key. To get the same level of negative feedback as the 'classic' Marshalls you need to increase from 82k. The pot lets you dial it in however you like. Personally, I like less negative feedback (more dynamic and tube-like) but others like a tighter power amp.

The cap is to correct phase differences that's inherent in that type of power amp (including Plexis, 2203s, Mesas, old Fenders, etc). Messing with it won't get you any audible improvements - it's more a phase 'correction' and stability thing.


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## RickyLee

el_bastardo said:


> I think most of them are less. I'm pretty sure the JVM had like 27k or 42k or something like that, hence the reason so many people modded theirs for variable NFB. I did mine. I put my NFB on a pot....and still keep it stock 99% of the time. I recently did a choke mod, and that was way more useful to me.



When you installed the choke, did you change any of the filtering values, mainly on the choke filtering node?

What size choke did you go with?


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## RickyLee

Casey_Butt said:


> The winding it's tapped off from is the key. To get the same level of negative feedback as the 'classic' Marshalls you need to increase from 82k. The pot lets you dial it in however you like. Personally, I like less negative feedback (more dynamic and tube-like) but others like a tighter power amp.
> 
> The cap is to correct phase differences that's inherent in that type of power amp (including Plexis, 2203s, Mesas, old Fenders, etc). Messing with it won't get you any audible improvements - it's more a phase 'correction' and stability thing.



Thanks for chiming in. I have heard/read that you were one of the well knowledgeable fellas on the old JVM Forum.

I see the cap you are referring to. I must have seen something else or am thinking of a different circuit/schematic altogether, as I had thought I had seen a resonance cap in the NFB loop of the JVM. My bad there. Something similar you would see in the SL-X NFB loop or even a traditional Resonance circuit. 

Were you the person that turned his JVM into a Blackface Fender/2203/AFD100/Dual Rec on the four channels? If so, did you have implement any added relay switching schemes? Or were these mods just somewhat "in the ballpark" for those circuits I mentioned?


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## el_bastardo

RickyLee said:


> When you installed the choke, did you change any of the filtering values, mainly on the choke filtering node?
> 
> What size choke did you go with?


No, and 5H.


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## RickyLee

el_bastardo said:


> No, and 5H.



Did you notice any change(s) with that?


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## el_bastardo

RickyLee said:


> Did you notice any change(s) with that?


Yes. The amp is quieter at idle, tighter, beefier lows and mids, less fizz.


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## Col

*Compression Reduction Mod*

This mod reduces compression and allows the amp to sound more open.

*Switchable*

No (is possible though not recommended)

*Affects*

All channels/modes

*Parts list*

- 100kohm resistor (1 or 2x)*

OR

- 200-220kohm resistor (or any value around 200k) (1 or 2x)

*How To Perform The Mod*

- Replace R75 with a 100kohm resistor
- Replace R72 with a 100kohm resistor (optional though recommended)

*Mod Variant*

Parallel wiring
- Solder a 200-220kohm resistor in parallel with R75
- Solder a 200-220kohm resistor in parallel with R72 (optional though recommended)

*Mod Author*

Casey Butt

*Source*

JVM Forum


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## Col

*1959 Plexi Cap Mod*

This mod reduces squeals and glassiness at high gain and rounds out the high end.

*Switchable*

No

*Affects*

All channels/modes

*Parts List*

- .68uF or 1uF polyester capacitor
*
How To Perform The Mod*

- Solder the .68uF or 1uF polyester capacitor in parallel with R97

*Source*

JVM Forum


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## Col

*Negative Feedback Mod*

Changes the way the power supply delivers power and provides more traditional power tube breakup at lower volumes.

*Switchable*

Yes – See Mod Variant below.

*Affects*

All channels/modes

*Parts list*

- One of the following resistors depending what type of amp you want to sound like:

47k -- late '60s Plexi
74k -- early '70s MkII Plexi
82k -- stock JVM
137k -- EVH's Plexi
176k -- JCM800 2203
177k+ -- a Marshall with almost a Vox's lack of headroom in the power amp

*How To Perform The Mod*

- Replace R58 with one of the resistors above depending on what type of amp you want to sound like.

*Mod Variant*

Use a 250k pot instead of a resistor across R58. This will allow adjustment across the whole range of values.

*Mod Author*

Casey Butt

*Source*

JVM Forum


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## Col

*Choke Mod*

Reduces noise in all channels by replacing a resistor with a choke. The amp will feel more organic, and the bass response will be improved.

*Switchable*

No

*Affects*

All channels/modes

*Parts list*

- 1 x 2.5 – 10H 200-400mA choke (5H seems popular) 10H gives the most vintage feel.

*How To Perform The Mod*

- Remove the chassis.
- Remove the tubes and boards from the chassis.
- Mount the choke on top of the chassis in the open area between the transformer and tubes, close to the main caps on the underside of the main board. The choke lead are short and they will need to be soldered across R106 so take that into account when positioning the choke.
- Drill an additional hole in the chassis as feed the choke cable through the board.
- Remove the R106 resistor and solder the lines from the choke there instead.

*Source*

JVM Forum


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## Col

*C83 Mod*

Slight gain reduction to reduce some of the “fizzle” from the gain channels.

*Switchable*

Yes

*Affects*

All channels/modes

*Parts list*

- None

*How To Perform The Mod*

- Remove the C83 cap or clip one of the legs and bend the cap away from the board.

*Source*

JVM Forum


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## Col

I just posted a few mods that I found on various sites (including this one) and have tried to put the details into a standard format. I believe these mods were originally sourced from the JVM Forum. I'll attempt to link all the mods to the first post to make them easier to find in the thread.

I believe these are the easiest ones to find - the rest will be much harder but I'll keep looking. If you have any additional context around these mods please post it here.

Thanks for all the discussion so far


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## Casey_Butt

RickyLee said:


> Were you the person that turned his JVM into a Blackface Fender/2203/AFD100/Dual Rec on the four channels?


Sort of. My Clean channel is voiced like a Blackface, Crunch is after a late-1960s Plexi and OD2 is like a mid-1990s Dual Rec. I had OD1 patterned after a 2203 for awhile but it turned out that I like the stock OD1 better, so I reverted it back to *nearly* stock (1 cap difference). I think Santiago's stock OD1 is an improved 2203, so I decided not to mess with it. The main difference between stock OD1 and a 2203 is that the 2203 is a bit brighter (shriller at high gain, IMO) and stock OD1 is tighter on the bottom end. The 2203 certainly is master of moderate gain levels, but when you start getting into really high gain territory you need those extreme highs shaved and the bottom tightened a bit, IMO.



RickyLee said:


> If so, did you have implement any added relay switching schemes? Or were these mods just somewhat "in the ballpark" for those circuits I mentioned?


The original Blackfaces, 1959s, 2203, Dual Rec and JVM all share surprisingly similar circuit design. The power amps are almost identical and the differences in the preamps amount mostly to the number of gain stages and component values.

The stock JVM Clean green was based almost component-for-component on a Fender Blackface in the first place, so it wasn't too much of a stretch to voice it practically dead-on to a Blackface. Where the JVM Clean green differs is largely because the pots are slightly different values (for instance, 200k instead of 250k) and some of the caps are different. What I did was tweak the JVM component values and tone stack to mimic the same frequency response as an actual Blackface. Stock, the JVM Clean green is brighter with a slightly different mid centre.

Similar with the Plexi Channel mods - I re-voiced Crunch Green to match the gain and frequency profile of a late 1960's Plexi. OD2 was re-voiced to follow a mid-'90s Dual Rectifier.

Those mods were based on circuit simulations and actual frequency and Fourier responses of the actual amps, so it wasn't just guesswork or "sounds like it to me". 

But to answer your question - no, I didn't need to implement any added relay switching schemes. The topologies of those JVM modes are similar enough to the target amps that most of the mods could be confined to each channel's tone and gain "stacks". In the few places where components were added that affected other channels I just put them on switches.

One example of that was the "AFD100" mod. The AFD100 has an extra cathode bypass cap in the preamp to give it that modded SIR#36 sound. I added that to my amp on a switchable pot that also allows me to change the bias of that gain stage (aka the "Randy Rhoads" mod). That was part of a full set of "AFD100 Channel" mods that were designed to voice the OD1 channel like, you guessed it, an AFD100/SIR #36. I, personally, didn't install that full set of mods on my own amp, I came up with them for someone else who wanted to do it. Those mods would affect the whole amp unless additional relays (another circuit board) were put in to switch them in and out.


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## RickyLee

Thanks for the clarification Casey.

Yes, making the amp have the AFD and some other variant tones affects all or most channels/modes. Unless a complex relay switching scheme is implemented. That is what I was wondering if you or some of the other JVM Forum members had went to that extreme.


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## Matt_Krush

*A variant to the negative feedback mod:*

Splice the purple wire coming from the Output Transformer and install a 100k or 250k pot in series. There are 2 wires, you splice into the one coming from pin 2 of the PCB.
I used a 250k push/pull switch pot, so I could increase the negative feedback and use the switch as the bright cap on/off switch.

Tapping into the wire from the OT doesn't allow for reducing the negative feed back from stock, to do that you need to do the above method.
But this to me is an easier way and you don't have to mess with soldering.

From JVM forum: negative feedback amount.._.the values below are compensated for the OT tap of the JVM_
Replace R58 with a suitable resistor. JVMForum gives these guidelines on what values to use for what effect:
R58 = 47k -- late '60s Plexi
R58 = 74k -- early '70s MkII Plexi
R58 = 82k -- stock JVM
R58 = 137k -- EVH's Plexi
R58 = 176k -- JCM800 2203
R58 = 177k+ -- a Marshall with almost a Vox's lack of headroom in the power amp


*Another mod from the JVM forum not listed (and I haven't tried)*
"if you want to adjust the gain/volume of the OD Orange and Red modes, you can wire a 1Meg pot across (in parallel with) R128. The pot will act like an additional gain pot for just the OD1/2 Orange and Red modes and you can adjust it to balance those modes with OD1/2 Green. It also influences the compression level by adjusting the level of signal that's fed into the last two gain stages of the preamp. If you turn the pot all the way off it will silence the OD1/2 Orange and Red modes completely. On my own amp I used a push-pull 1Meg pot and wired it so that when the knob is pulled out it is removed from the circuit completely."


Another site that has consolidated the popular JVM mods: http://charlotte-mijnders.magix.net/website/marshall_amp_mods.75.html
I think this person's blog was during the JVMforum time, since they have links to the JVMforum site that no longer works.


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## Col

Hey Matt - thanks for this great info! I'll add that negative feedback mod variant to the mod. I'll also add that other mod you listed from the JVM Forum. Any idea what it was called there?

Thanks also for that link - it didn't come up in any of the searching I did!


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## Col

OK so there seem to be variations of the Choke Mod - some that change the filter caps - and some that just add a choke. A discussion of the mod that changes out the filter caps is here.

Does changing the filter caps make much of a difference? Should that be part of the mod?


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## el_bastardo

Col said:


> OK so there seem to be variations of the Choke Mod - some that change the filter caps - and some that just add a choke. A discussion of the mod that changes out the filter caps is here.
> 
> Does changing the filter caps make much of a difference? Should that be part of the mod?


I didn't change any filter caps with my choke mod...but I thought about it....not sure if I should or not. I do notice a difference in feel with the mod. It's better, quieter, less fizz, punchier low mids.


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## Col

*Gain Reduction Mod*

Allows adjustment of gain/volume on OD1/2 Red and Orange modes. This can be used to balance the OD1/2 Red and Orange modes with OD1/2 Green mode.

*Switchable*

No

*Affects*

OD1/2 Orange and Red modes.

*Parts list*

- 1M pot

*How To Perform The Mod*

- Wire a 1M pot across (in parallel with) R128.

*Author*

Casey Butt

*Source*

JVM Forum


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## Col

el_bastardo said:


> I didn't change any filter caps with my choke mod...but I thought about it....not sure if I should or not. I do notice a difference in feel with the mod. It's better, quieter, less fizz, punchier low mids.


Thanks for the info!

Casey - is it worth changing the caps with the choke mod? Might help el_bastardo to decide...


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## Col

*AFD100 Mod*

For the Crunch Orange and Red and Clean Orange and Red, it will give you the ability to go from a very "raw" and aggressive distortion character (with the knob turned up) to a smoother, more "classic" distortion with the knob turned down.

OD1/2 are only slightly affected by this mod.

*Switchable*

Yes

*Affects*

All modes except Clean Green and Crunch Green.

*Parts list*

- 1 x 820k 1w resistor
- 1 x 10k linear pot

*How To Perform The Mod*

- Remove R96 and wire the resistor and linear pot in series.

*Mod Variant*

You can also switch a 100 nF cathode bypass cap in and out of the circuit. This gives a gain boost starting at around 200Hz and was/is used on SIR #34, #36, #39 and the AFD100 head.

Use a 10k pot with a built in clickable SPST switch. Wire the switch so that when the pot is all the way to the 10k position (full "on"), you click it and it switches in a 100nF cap across the pot to ground. In other words, it switches in a 100nF cathode bypass cap on that tube stage, ala #34/#36/AFD100

*Source*

JVM Forum


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## Col

I just posted what I could find on the AFD100 mod. Details of this one - especially with the 100nf cathode bypass cap - are not complete. The posts all refer back the JVM Forum for more details.

Does anyone have more details on this mod?


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## Casey_Butt

Matt_Krush said:


> *Another mod from the JVM forum not listed (and I haven't tried)*
> "if you want to adjust the gain/volume of the OD Orange and Red modes, you can wire a 1Meg pot across (in parallel with) R128. The pot will act like an additional gain pot for just the OD1/2 Orange and Red modes and you can adjust it to balance those modes with OD1/2 Green. It also influences the compression level by adjusting the level of signal that's fed into the last two gain stages of the preamp. If you turn the pot all the way off it will silence the OD1/2 Orange and Red modes completely. On my own amp I used a push-pull 1Meg pot and wired it so that when the knob is pulled out it is removed from the circuit completely."


I did this to my amp, but I've since replaced the push-pull pot with one that simply switches out when it's all the way clockwise (more convenient). I have it left on about 12:00-1:00 99% of the time... so much so that I practically forgot it was even there. I initially did it so I could balance the volume jump going from OD1/2 Green to Orange but also found that set to 12:00-1:00 it shaves just enough gain off OD1/2 Orange and Red to make them a little more controllable. OD1/2 Red still have insane amounts of gain and OD1/2 Orange have just enough shaved off to make them a shade less lead-ish and a little more rhythm-ish.



Col said:


> Casey - is it worth changing the caps with the choke mod? Might help el_bastardo to decide...


I wrote the quote that was linked to on the previous page, so that pretty much sums up what I have to say about it. Basically, lowering the caps complements the choke in that it allows the power amp to be more reactive to big current draws (like hitting the strings hard with the volume cranked). To maximise that effect you'd lower all the power filter caps, not just the screens (but lowering just the screens helps the bottom end stay tighter). Technically, the larger caps make better filters and more stability, but they also make the amp "stiffer" compared to the older amps. Some old Plexis with very weak filtering had much spongier responses than typical modern amps - think of early Van Halen (and if the filtering was too low they'd get ghost notes as well).

So whether it's worth it or not depends on what you're after - tight or spongy.



Col said:


> I just posted what I could find on the AFD100 mod. Details of this one - especially with the 100nf cathode bypass cap - are not complete. The posts all refer back the JVM Forum for more details.
> 
> Does anyone have more details on this mod?


I never deliberately kept detailed records of that stuff (other than on the JVM Forum) but I do have the models from when I was developing those mods on a dead laptop. I might be able to rescue them from there. There were also Egnator mods, SLO-100 mods, SRV Cesar Diaz mods and Hendrix mods in the works.


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## Col

Thanks Casey... I might put the screen cap changes into another mod here then and call it the "Anti Stiffness Mod" which would be done in conjunction with the Choke Mod (of course).

And of course any info that you could find off your dead laptop would be awesome to have here!


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## Col

*Anti Stiffness Mod*

This mod is applied in conjunction with the Choke mod.

Applying this mod will make the amp have a more spongy or saggy feel. Some people prefer this over the stiffness of the amp when stock.

*Switchable*

No

*Affects*

All modes.

*Parts list
*
If you applied a 3H or 5H choke when doing the Choke mod:

- 2 x 100uF caps (rated for at least 400V each)

If you applied a 10H choke when doing the Choke mod:

- 2 x 47uF 400V caps.

NOTE: The caps should have a diameter of 22mm and the distance between the legs of the caps should be 10 mm so they will fit on the board correctly.

*How To Perform The Mod*

- Replace C50 and C81 (screen caps) with the 100uF caps if you used a 3H or 5H choke.

OR

- Replace C50 and C81 (screen caps) with the 47uF caps if you used a 10H choke

*Author*

Casey Butt

*Source*

JVM Forum


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## Col

*OD2 to OD1 Mod*

This mod changes OD2 to have the same spec as OD1.

Although this mod works with the JVM410 it is most useful for the 2 channel JVMs as they come with OD2 as standard and some people prefer the character of OD1.

*Switchable*

No (possibly with the right switch).

*Affects*

OD2

*Parts list
*
- 1 x 33k resistor
- 1 x 1n0 capacitor
- 1 x 3n3 capacitor

*How To Perform The Mod*

How to perform the mod (JVM2 Series):
- Change R44 from 47k to 33k
- Change C31 from 1n5 to 1n0
- Change C32 from 1n0 to 3n3

How to perform the mod (JVM410 Series):
- Change R220 from 47K to 33K
- Change C225 from 1n5 to 1n0
- Change C224 from 1n0 to 3n3

*Source*

JVM Forum


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## gsgtrman

*Parts list
*
If you applied a 5H choke when doing the Choke mod:

- 2 x 100uF caps (rated for at least 400V each)

If you applied a 10H choke when doing the Choke mod:

- 2 x 47uF 400V caps.

NOTE: The caps should have a diameter of 22mm and the distance between the legs of the caps should be 10 mm so they will fit on the board correctly.

*How To Perform The Mod*

- Replace C50 and C81 (screen caps) with the 100uF caps if you used a 5H choke.

OR

- Replace C50 and C81 (screen caps) with the 47uF caps if you used a 5H choke

*Author*

Casey Butt

*Source*

JVM Forum[/QUOTE]

How about the 3h MM choke - any changes needed? Thanks for your help.


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## Col

Hey gsgtrman - not sure what values would be good for a 3H choke. Someone might reply here with those values - otherwise I'll dig around and see what I can find out for you


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## gsgtrman

Col said:


> Hey gsgtrman - not sure what values would be good for a 3H choke. Someone might reply here with those values - otherwise I'll dig around and see what I can find out for you



Thanks Col, I appreciate that!


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## FundTheMental

Thanks for making the effort to put this together Col! 

I'm about to mod mine this weekend. Wanting to do the Negative feedback, Plexi and Anti Compression mods but have more in mind for the future depending on the results (Choke, Gain Reduction and maybe Anti-stiffness).

I've got caps and resistors but want to know if the board is accessible without dismantling as far as you need to in order to perform the choke mod or can you get to the resistors and caps to replace them easily? If I'm going to have to strip it that far, I might just order a choke and do it while it's in bits. Thanks for any info in advance! And, is there a particular choke brand and spec that is recommended that I might be able to easily acquire in the UK?


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## Col

Hey FundTheMental - I'm currently running my JVM stock... One day I'll be applying some of these mods but I've only recently started playing a JVM and want to get the feel of the stock amp before I change it. Once thing I can comment on is that the amp very different at volume so applying these mods for home playing levels may sound quite different to what they would sound like at higher volumes in an auditorium of some sort.

Anyway I read through the entire JVM Modding Bible thread before it disappeared and there was some wonderful information there that at this point is just lost. There is still a lot of information out there but it can be quite hard to find. Also Casey Butt - who designed many of these mods - is on this forum and is very generous with helping people out.

So... to answer your question this YouTube video by Phil Chapman may help. I figure that the information in that video was all gleaned from the JvmForum in the first place so it's OK to link that video here


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## Col

FundTheMental said:


> And, is there a particular choke brand and spec that is recommended that I might be able to easily acquire in the UK?



I live in Australia so finding this sort of stuff locally is not easy. I've considered:

Mercury Magnetics
Mouser (Hammond seems popular)

And of course a search on eBay will yield some local results. I have no idea if a cheap or expensive choke would have difference on the sound.

Postage from the US to AUS is pretty horrendous and a mail forwarding company may be a better option. Not sure how that would compare to UK shipping.


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## Col

gsgtrman said:


> How about the 3h MM choke - any changes needed? Thanks for your help.



From digging around a bit it seems that people who have used a 3H choke have used the same screen cap values as the people who used a 5H choke. 

I've changed the mod write-up to reflect this.

You don't need to change the screen caps to add the choke, but you get the most benefit from changing the caps as well as adding the choke - Casey commented on this above.

Hope this helps


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## gsgtrman

Thanks again Col, have great weekend!


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## FundTheMental

Thanks Col, I've found a choke that is made by Hammond on the mouser UK page and it's 6H 200mA 50v I believe. This one stood out as it's half the weight of some on there and almost half the price. Would love for someone to check the actual spec and confirm it's suitability! It's a Hammond 159R.

As for the screen caps, I'm not sure if I'll bother doing those as I believe I've read that it affects the loop and how the wet/dry adjustment works but this might be me confusing it with another mod, I've read about so many! Plus, it's more the benefit of being quieter at idle that appeals rather than making it more spongy.


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## Col

Great news!!! The JVM Forum is back in action thanks to the efforts of the moderator Andy.

For any modding information please go to the Modding Bible section of the forum. You'll need to register to get there but it's definitely worth it.


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## RobLaQuinta

These are great ideas! Are there mods like this for the JCM800s?


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## deanproxy

Does anyone know where to get the resistors and capacitors for the OD2 - OD1 mod? What Voltage are the capacitors? I see a lot of 50v capacitors but I'm not sure if that value is fine for these mods.


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## FundTheMental

deanproxy said:


> Does anyone know where to get the resistors and capacitors for the OD2 - OD1 mod? What Voltage are the capacitors? I see a lot of 50v capacitors but I'm not sure if that value is fine for these mods.


I believe the caps need to be 400v minimum but don't quote me on this. That's what I've bought to do the plexi mod...


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## gsgtrman

I just added a 3H choke to my 205H - excellent addition... I would like to also do the negative feedback mod, and can't seem to find info regarding the preferred composition or wattage of resistors to use. Any suggestions would be welcome, thanks!


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## RickyLee

gsgtrman said:


> I just added a 3H choke to my 205H - excellent addition... I would like to also do the negative feedback mod, and can't seem to find info regarding the preferred composition or wattage of resistors to use. Any suggestions would be welcome, thanks!



The negative feedback loop mod can very simply be a potentiometer. Though you can lower the value of the existing NFB resistor to something a bit smaller, so you have range both ways of less than/more than stock value of NFB using a pot. 1MEG is usually the standard value for most NFB and resonance circuits. But this amp already has a resonance control.

250K or even 100K pots can be used as well. And I would think the JVM Forum modders did not put a capacitor in with that pot either. But I could be wrong.

As for resistor type, I would not worry about that one myself. I usually just go with carbon comp in these type applications.

I have been considering tinkering with this one on my JVM 410H as well.


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## gsgtrman

RickyLee said:


> The negative feedback loop mod can very simply be a potentiometer. Though you can lower the value of the existing NFB resistor to something a bit smaller, so you have range both ways of less than/more than stock value of NFB using a pot. 1MEG is usually the standard value for most NFB and resonance circuits. But this amp already has a resonance control.
> 
> 250K or even 100K pots can be used as well. And I would think the JVM Forum modders did not put a capacitor in with that pot either. But I could be wrong.
> 
> As for resistor type, I would not worry about that one myself. I usually just go with carbon comp in these type applications.
> 
> I have been considering tinkering with this one on my JVM 410H as well.



Thanks for the reply. Is there a concern for the wattage value of the resistor? I have 1/4w, but should I go higher, to say 1w or 2w? Also, if I use a potentiometer, could I use a 250k CTS like the one in my Strat, or is there something preferable? I'm guessing I ground the right lug, and wire center and left to the circuit (again, like a guitar volume pot); is this correct?

Thanks again!


----------



## RickyLee

gsgtrman said:


> Thanks for the reply. Is there a concern for the wattage value of the resistor? I have 1/4w, but should I go higher, to say 1w or 2w? Also, if I use a potentiometer, could I use a 250k CTS like the one in my Strat, or is there something preferable? I'm guessing I ground the right lug, and wire center and left to the circuit (again, like a guitar volume pot); is this correct?
> 
> Thanks again!



1/4W resistors are fine. Not enough voltage there to be concerned with the wattage ratings.

You can use that type of pot you mentioned as well. A linear taper pot might be the better choice as you will have a smoother more even varied resistance through the pots travel. Though with a big 1MEG, you do not notice much change after you get up around 200K of value. 1MEG is better suited to use in a resonance circuit in parallel with a capacitor. 250K might work best just for NFB change.

You would wire up the pot as a variable resistor. Tie the middle lug to one outer end. Check the orientation so that the clockwise turn increases the resistance on whichever outer leg you use opposite the middle/outer lug. I can not remember which outer lugs are used, so just check it so you have the function you want in relation to turning the pot. Maybe some people like clockwise to give more NFB? lol. I like clockwise to remove NFB which then gives more power amp volume.


----------



## dax_the_ax

On the Neg Feedback Mod, can the resistor be removed from the top of the board, or do I have to desolder from underneath? Thanks

I’d love to talk more about JVM mods. Anyone can email me.

dax_the_ax@msn.com


----------



## dax_the_ax

In the process of mods!! Just added the 220K to R72. Can’t bring myself to rip out C83


----------



## dax_the_ax

I’m doing the R72 mod as we speak! Can’t bear to Rip off C83, R58 looks too complicated to screw with, I’m closing her up!!


----------



## Bonz

Can any one tell me where I and get a mod to fix my JVM 205H the sound is trebly , classy ,fizzy I hate the tone of this amp. and by add a choke transformer what will this accomplish as far as tone. Hey guys I just joined the membership. please help me


----------



## RickyLee

Bonz said:


> Can any one tell me where I and get a mod to fix my JVM 205H the sound is trebly , classy ,fizzy I hate the tone of this amp. and by add a choke transformer what will this accomplish as far as tone. Hey guys I just joined the membership. please help me



Welcome to the Forum.

First questions: 

How long have you had this amp?

Are you the original owner?

What year of manufacture is this amp?

How old are the tubes/valves in it?

Have you checked the power tubes bias idle setting?

The tones/sound you are not liking is on all channels and modes?


----------



## RickyLee

dax_the_ax said:


> I’m doing the R72 mod as we speak! Can’t bear to Rip off C83, R58 looks too complicated to screw with, I’m closing her up!!



I am curious to know what reducing R72 did for you? Is it that noticeable?

As for the NFB R58, is yours the 82K value? I tweaked my R58 to be adjustable, similar to the NFB mod, and increasing that makes a bit more poweramp punch but did not magically transform it like others have noticed. But I still need to play it more and experiment with that one.

Lowering the gain/saturation in the OD channels is probably the best move on this amp in my opinion.


----------



## Thai Johnson

What wattage do the resistors need to be for the negative feedback mod and the OD2/1 mod, and is 450v enough for all the capacitors


----------



## Thai Johnson

Anybody, I know this is a necro thread, but I can't register with jvmforum and the board admin hasn't replied to any of my emails about registering


----------



## gsgtrman

Thai Johnson said:


> Anybody, I know this is a necro thread, but I can't register with jvmforum and the board admin hasn't replied to any of my emails about registering


I haven't been able to get on the JVM forum either. If anyone is paying attention at this point, can you please let me know the resistor value to piggyback with R128 for the gain reduction mod? I would prefer to have a fixed value rather than the potentiometer mentioned in this thread. Thanks!


----------



## RickyLee

gsgtrman said:


> I haven't been able to get on the JVM forum either. If anyone is paying attention at this point, can you please let me know the resistor value to piggyback with R128 for the gain reduction mod? I would prefer to have a fixed value rather than the potentiometer mentioned in this thread. Thanks!



That one is up to your ears and how much reduction you want. You could try a 470K and see if that is good.

I used a small trim pot across R128, then dialed it in to what I was looking for and let it be. Not sure what the pot value ended up at. It might even be 220K or less. It depends on how much reduction you want.


----------



## gsgtrman

RickyLee said:


> That one is up to your ears and how much reduction you want. You could try a 470K and see if that is good.
> 
> I used a small trim pot across R128, then dialed it in to what I was looking for and let it be. Not sure what the pot value ended up at. It might even be 220K or less. It depends on how much reduction you want.


Good point RickyLee. I'd like to get OD orange and red a little closer to OD green. I find green to have a really nice rhythm crunch with my Strat, but if I switch to one of the others without changing the gain setting, it's too much. I'll try different values since I don't have a 1M pot. Thanks man!


----------



## RickyLee

gsgtrman said:


> Good point RickyLee. I'd like to get OD orange and red a little closer to OD green. I find green to have a really nice rhythm crunch with my Strat, but if I switch to one of the others without changing the gain setting, it's too much. I'll try different values since I don't have a 1M pot. Thanks man!



Hmmm. I will have to research that. I think R128 might also change OD Green mode? I am trying to remember, but the Green mode removes one stage, correct? If so, then it would not alter OD Green.


----------



## gsgtrman

I seem to recall that it didn't affect green, but I could be wrong.


----------



## gsgtrman

I did wire in a 220k resistor, OD green seems as it was before. I could hear going even lower with the resistor, orange and red are still pretty friggin' hot...


----------



## Matt_Krush

dax_the_ax said:


> On the Neg Feedback Mod, can the resistor be removed from the top of the board, or do I have to desolder from underneath? Thanks
> 
> I’d love to talk more about JVM mods. Anyone can email me.
> 
> dax_the_ax@msn.com



On my 410, I cut the leg of the resistor, soldered an 82k in parallel to drop it to 40k, then connected a wire from the paired resistors to a 250k pot...then back to the leg of the original resistor still in the board.

On my 205, I simply just cut the purple wires and connected them to a 250k pot.

*the 250k pots I get are also push/pull switches that I uses for other mods/functions.


----------



## gsgtrman

RickyLee said:


> Hmmm. I will have to research that. I think R128 might also change OD Green mode? I am trying to remember, but the Green mode removes one stage, correct? If so, then it would not alter OD Green.


Hey Ricky Lee, I can confirm that adding the resistor has no effect on OD Green. I tried several values of resistors today using alligator clips; none changed the amount of gain in OD Green. I settled on a 100k resistor. There is still a good amount of gain on OD Orange and Red, but that value certainly tamed down the stock amount of gain so it is more useable with OD Green, at least for my needs. The stock amount in OD Red is like having a FuzzFace hard wired to the circuit... I play a Strat with lower output DiMarzio Virtual 54's. I can't imagine how crazy humbuckers or P90's would be in this channel.


----------



## RickyLee

gsgtrman said:


> Hey Ricky Lee, I can confirm that adding the resistor has no effect on OD Green. I tried several values of resistors today using alligator clips; none changed the amount of gain in OD Green. I settled on a 100k resistor. There is still a good amount of gain on OD Orange and Red, but that value certainly tamed down the stock amount of gain so it is more useable with OD Green, at least for my needs. The stock amount in OD Red is like having a FuzzFace hard wired to the circuit... I play a Strat with lower output DiMarzio Virtual 54's. I can't imagine how crazy humbuckers or P90's would be in this channel.



Lol. Yeah, this amp can get heavily saturated. If you like the tone to get a bit warmer at higher gain settings, that mod is the ticket. It allows you to have the gain control set higher which starts taking the bright cap out of the circuit a bit more than the lower gain settings.


----------



## Phreedude

Col said:


> *OD2 to OD1 Mod*
> 
> This mod changes OD2 to have the same spec as OD1.
> 
> Although this mod works with the JVM410 it is most useful for the 2 channel JVMs as they come with OD2 as standard and some people prefer the character of OD1.
> 
> *Switchable*
> 
> No (possibly with the right switch).
> 
> *Affects*
> 
> OD2
> 
> *Parts list
> *
> - 1 x 33k resistor
> - 1 x 1n0 capacitor
> - 1 x 3n3 capacitor
> 
> *How To Perform The Mod*
> 
> How to perform the mod (JVM2 Series):
> - Change R44 from 47k to 33k
> - Change C31 from 1n5 to 1n0
> - Change C32 from 1n0 to 3n3
> 
> How to perform the mod (JVM410 Series):
> - Change R220 from 47K to 33K
> - Change C225 from 1n5 to 1n0
> - Change C224 from 1n0 to 3n3
> 
> *Source*


----------



## Phreedude

Col said:


> *Negative Feedback Mod*
> 
> Changes the way the power supply delivers power and provides more traditional power tube breakup at lower volumes.
> 
> *Switchable*
> 
> Yes – See Mod Variant below.
> 
> *Affects*
> 
> All channels/modes
> 
> *Parts list*
> 
> - One of the following resistors depending what type of amp you want to sound like:
> 
> 47k -- late '60s Plexi
> 74k -- early '70s MkII Plexi
> 82k -- stock JVM
> 137k -- EVH's Plexi
> 176k -- JCM800 2203
> 177k+ -- a Marshall with almost a Vox's lack of headroom in the power amp
> 
> *How To Perform The Mod*
> 
> - Replace R58 with one of the resistors above depending on what type of amp you want to sound like.
> 
> *Mod Variant*
> 
> Use a 250k pot instead of a resistor across R58. This will allow adjustment across the whole range of values.
> 
> *Mod Author*
> 
> Casey Butt
> 
> *Source*
> 
> JVM Forum


Seems like a great mod, is there a certain power value the 250K pot should be or is 1/4W enough? And the wire gauge, 22 gauge enough as long as it fits into pc board holes and can allow 600V if needed?


----------



## Phreedude

Col said:


> *Choke Mod*
> 
> Reduces noise in all channels by replacing a resistor with a choke. The amp will feel more organic, and the bass response will be improved.
> 
> *Switchable*
> 
> No
> 
> *Affects*
> 
> All channels/modes
> 
> *Parts list*
> 
> - 1 x 2.5 – 10H 200-400mA choke (5H seems popular) 10H gives the most vintage feel.
> 
> *How To Perform The Mod*
> 
> - Remove the chassis.
> - Remove the tubes and boards from the chassis.
> - Mount the choke on top of the chassis in the open area between the transformer and tubes, close to the main caps on the underside of the main board. The choke lead are short and they will need to be soldered across R106 so take that into account when positioning the choke.
> - Drill an additional hole in the chassis as feed the choke cable through the board.
> - Remove the R106 resistor and solder the lines from the choke there instead.
> 
> *Source*
> 
> JVM Forum


Looking forward to installing a 5H choke in a 205C, but had a couple questions about it...first, would a Classic Tone 5H @ 120mA (dc) (115 ohms) be able to had the current and voltage at that point in the circuit, and will this mod be as effective if I don't reduce the values of C50 and C81 to 100uF from 220uF? Seems like a step backwards to reduce the caps to allow more ripple through in an attempt to reduce noise in other parts of the amp...


----------



## Phreedude

Matt_Krush said:


> *A variant to the negative feedback mod:*
> 
> Splice the purple wire coming from the Output Transformer and install a 100k or 250k pot in series. There are 2 wires, you splice into the one coming from pin 2 of the PCB.
> I used a 250k push/pull switch pot, so I could increase the negative feedback and use the switch as the bright cap on/off switch.
> 
> Tapping into the wire from the OT doesn't allow for reducing the negative feed back from stock, to do that you need to do the above method.
> But this to me is an easier way and you don't have to mess with soldering.
> 
> From JVM forum: negative feedback amount.._.the values below are compensated for the OT tap of the JVM_
> Replace R58 with a suitable resistor. JVMForum gives these guidelines on what values to use for what effect:
> R58 = 47k -- late '60s Plexi
> R58 = 74k -- early '70s MkII Plexi
> R58 = 82k -- stock JVM
> R58 = 137k -- EVH's Plexi
> R58 = 176k -- JCM800 2203
> R58 = 177k+ -- a Marshall with almost a Vox's lack of headroom in the power amp
> 
> 
> *Another mod from the JVM forum not listed (and I haven't tried)*
> "if you want to adjust the gain/volume of the OD Orange and Red modes, you can wire a 1Meg pot across (in parallel with) R128. The pot will act like an additional gain pot for just the OD1/2 Orange and Red modes and you can adjust it to balance those modes with OD1/2 Green. It also influences the compression level by adjusting the level of signal that's fed into the last two gain stages of the preamp. If you turn the pot all the way off it will silence the OD1/2 Orange and Red modes completely. On my own amp I used a push-pull 1Meg pot and wired it so that when the knob is pulled out it is removed from the circuit completely."




Another excellent mod, but was curious...if R58 is default JVM level of 82K, if you add say, a 250K pot in series, even at it's lowest CCW setting, you will still have a total of more than 82K of resistance...how is it possible to attain either the late 60's Plexi equivalent if it is 47K and the early 70's @ 74K? wouldn't you want to actually change the value of R58 to perhaps about 47K and additively with the 250K, albeit a little difficult to tweak accurately, allows you to access all the resistances to give you any of the 6 settings (late 60's Plexi to Vox) that are desired, and as a bonus, you could use a push/pull pot, 'out'/up being the adjustment position and when you return the switch to it's 'in'/down position, it would include a 40.7K (40.7K in parallel with the 250K pot = 35K. 35K + 47K = 82K which is the original JVM R58 value) resistor, which would give you the 'JVM' setting, but the pot/ switch combo and Rp (40.7K) resistor being wired so (to outside lugs on lower pot switch, no wiper) the pot would now make no resistance changes in the 'in'/down position.


----------



## Matt_Krush

Phreedude said:


> Another excellent mod, but was curious...if R58 is default JVM level of 82K, if you add say, a 250K pot in series, even at it's lowest CCW setting, you will still have a total of more than 82K of resistance...how is it possible to attain either the late 60's Plexi equivalent if it is 47K and the early 70's @ 74K? wouldn't you want to actually change the value of R58 to perhaps about 47K and additively with the 250K, albeit a little difficult to tweak accurately, allows you to access all the resistances to give you any of the 6 settings (late 60's Plexi to Vox) that are desired, and as a bonus, you could use a push/pull pot, 'out'/up being the adjustment position and when you return the switch to it's 'in'/down position, it would include a 40.7K (40.7K in parallel with the 250K pot = 35K. 35K + 47K = 82K which is the original JVM R58 value) resistor, which would give you the 'JVM' setting, but the pot/ switch combo and Rp (40.7K) resistor being wired so (to outside lugs on lower pot switch, no wiper) the pot would now make no resistance changes in the 'in'/down position.




After doing the mod on the 410, by paralleling the resistor on R58...I learned I don't like it that low.
But I did learn I liked the Negative feedback resistance at 200k or more.
I am not after any vintage tones or trying to sound like someone else's recording.

Now, if that's what you are after, you could get a 6 way rotary switch and select through the exact resistance value you wanted.

22g wire is fine, there isn't much voltage or current there.


----------



## Phreedude

[QUOTE="22g wire is fine, there isn't much voltage or current there.[/QUOTE]

The 22 gauge wire would be for the choke mod, and I am more concerned about the current heating up the wire near the Choke (inductor). Area components are rated 400V (and 1W). The Classic Tone Choke I am leaning towards installing (in series with the Power TX secondary) is only rated for 120mA which doesn't seem like much. Also with the placement of said choke, has anyone tried placing it at a 45 deg angle on the chassis considering it's proximity to the transformer?


----------



## Phreedude

*Choke Mod*


JVM Forum[/QUOTE]

Still curious about this mod; are you absolutely sure this is a safe mod to replace a 7W dropping resistor in between the power transformer secondary (500mA fused) and the Anodes of the preamp tube section (2, 1W resistor area) with an inductor? (115 Ohm @ 120mA rated 5 H choke)


----------



## Ken Ops

Interested in doing (actually, getting an electrician friend to do) the Choke, Anti-Stiffness and OD2->OD1 mods.

Anyone here vouch for those? Actually done them, or had them done? Recommend them?

Also, any of the others that might go well with those? For more “classic” Marshall sound, or close enough.


----------



## Jeffrock

HI everyone! I'm french so maybe my english is gonna be bad!
I really like my JVM410H and I'm gonna try soon some of the mods list but I have one thing that I'd like to change and I found nothing on this : is there a Mods to put the actual Series/parallel effectloops that is really bad, in a series loop with a volume ajustment? Just like the 410h JS has on it...

Thanks a lot for your help!!


----------



## gsgtrman

Ken Ops said:


> Interested in doing (actually, getting an electrician friend to do) the Choke, Anti-Stiffness and OD2->OD1 mods.
> 
> Anyone here vouch for those? Actually done them, or had them done? Recommend them?
> 
> Also, any of the others that might go well with those? For more “classic” Marshall sound, or close enough.



I have a 205H. I installed a 10H choke from Mercury Magnetics more for better power supply regulation than a fixed resistor. I didn't notice any serious change in tone, but it does seem to run a little quieter in the all but the highest gain settings. I also reduced the gain in OD Orange and Red, and reduced the negative feedback. Those mods improved the responsiveness overall. I find the Clean/Crunch Red to be closest to a classic JCM800 tone and feel.


----------



## Ken Ops

gsgtrman said:


> I have a 205H. I installed a 10H choke from Mercury Magnetics more for better power supply regulation than a fixed resistor. I didn't notice any serious change in tone, but it does seem to run a little quieter in the all but the highest gain settings. I also reduced the gain in OD Orange and Red, and reduced the negative feedback. Those mods improved the responsiveness overall. I find the Clean/Crunch Red to be closest to a classic JCM800 tone and feel.


Interesting, thanks.

Can you recall to what degree the negative feedback was reduced? Also, how that has affected the sound.


----------



## gsgtrman

Ken Ops said:


> Interesting, thanks.
> 
> Can you recall to what degree the negative feedback was reduced? Also, how that has affected the sound.



It's been awhile, but if I recall correctly I upped the stock resistor (R58) from 82k to 180k. It's got a little less headroom; gainier and warmer at lower volumes. I like it better than the stock value.


----------



## MatFnEhUK

Hi. I’ve got the 205H. Love it for the most part, but I find it fizzy and lacking clarity on the OD orange and red modes, and I find it sounds too compressed as well. I was thinking of having some of these mods done at some stage. When my warranty has run out maybe. I’m thinking of getting the choke mod, the NFB mod, compression reduction mod, C83 mod and/or gain reduction mod. Would these be enough to solve my issues? Is it worthwhile to do both C83 _and_ the gain reduction mod, or can I get away with just one of these mods for eliminating fizziness and making the gain more useable on OD orange and red? If so, which would you rather do to achieve this? Thanks.


----------



## RickyLee

As for adding a choke to any 100W amp, I myself have always used those 250mA rated chokes. I am sure the rating on most chokes is quite conservative, but 120mA feeding the screens on down to the end of the B+ rail or all the preamp tubes/phase inverter seems a bit under rated if you are going to be pushing the amp hard cranking it up. The JVM power supply/B+ rail is a bit different than say a 2203, but I am remembering the feed through that 7W resistor or the choke runs all the way through all the rest of the tubes/preamp.

I have been wanting to do some testing myself to see how much current really pulls through a choke in various amp models. 50W and then 100W at idle and then when cranked up.

As for the JVM 410H, it has high filtering values on the screen node at 110uF. That is why I have not installed a choke in mine yet. Seems like you will not get much of a change really. I have been considering adding the Classic Tone 3H 250mA choke into my JVM 410H. But I am also then considering lowering the screens filtering to either 22uF or 16uF. I want to get a bit more of an elastic and chewy grind to the midrange crunch voicing.


----------



## krizrox

Hey everyone. Thought this would be a good place to share the story and video. I recently had a 410H modded. The video is a demonstration of the modded amp. A couple of great local guitarists came in for a test drive. The amp was miked and recorded with Reaper. The video comes from an iPhone 12 Max Pro camera. The video and audio were sync'd up in Reaper and all the final editing was done in Reaper. Learned a lot, about a great many things. I dug the project even in spite of a couple roadblocks. The amp was modified by Chicago-area amp guru Kevin Bourke. Contact info at the end of the video. I think you guys would dig the story. I was considering doing another chapter that was just the stills from the start to finish of the modification process.


----------



## Isaac.t

I remember there being a 1M log method with a 33k resistor but not sure why this method was introduced.
I have that method but my only concern now is that it maybe too much and is causing an abnormal loud noise when the pot is rotated to near max


----------



## Isaac.t

Col said:


> *AFD100 Mod*
> 
> For the Crunch Orange and Red and Clean Orange and Red, it will give you the ability to go from a very "raw" and aggressive distortion character (with the knob turned up) to a smoother, more "classic" distortion with the knob turned down.
> 
> OD1/2 are only slightly affected by this mod.
> 
> *Switchable*
> 
> Yes
> 
> *Affects*
> 
> All modes except Clean Green and Crunch Green.
> 
> *Parts list*
> 
> - 1 x 820k 1w resistor
> - 1 x 10k linear pot
> 
> *How To Perform The Mod*
> 
> - Remove R96 and wire the resistor and linear pot in series.
> 
> *Mod Variant*
> 
> You can also switch a 100 nF cathode bypass cap in and out of the circuit. This gives a gain boost starting at around 200Hz and was/is used on SIR #34, #36, #39 and the AFD100 head.
> 
> Use a 10k pot with a built in clickable SPST switch. Wire the switch so that when the pot is all the way to the 10k position (full "on"), you click it and it switches in a 100nF cap across the pot to ground. In other words, it switches in a 100nF cathode bypass cap on that tube stage, ala #34/#36/AFD100
> 
> *Source*
> 
> JVM Forum


Hey man, 

For some reason back in the day I remember someone stating that the resistor is 820 ohm and not 820K, is this true or is it supposed to be 820,000 ohm?


----------



## spacerocker

Isaac.t said:


> Hey man,
> 
> For some reason back in the day I remember someone stating that the resistor is 820 ohm and not 820K, is this true or is it supposed to be 820,000 ohm?




I'm not sure about it being called the AFD100 Mod? The first part is what I know as the "3rd stage bias mod" - and yes the resistor should be 820 Ohms. You are replacing a 10K resistor with a 820 Ohm resistor in series with a pot. With the pot on minimum, you don't want to go below 820 Ohms. The third gain stage is biased very cold with a 10K resistor as stock. This causes the gain stage to clip assymetrically, as it does in a 2203. However, reducing the cathode resistor from 10K down to 820K (and anywhere in between) causes the gain to increase, and the clipping to be more symetrical.....

Adding the 10onF capacitor in parallel with the above components (i.e across the series pot and 820 Ohm resisor) - increases the AC gain massively - especially in the mid-high mid frequency ranges...

Adding this cap is what I would call the AFD mod - and can be done with a resistor in series with the cap to tame it a bit (it does add a LOT of gain...) - but maybe the AFD100 mod is a combination of a number of smaller mods? I can't remember for sure...


----------



## Igor_torcamaniac

Just found this on facebook group. Someone saved a Copy of the modding bible from JVM Forum. Enjoy!!!


----------



## okgb

Thank you, so much good info there and a real loss.


----------



## Daniel55

Really digging this forum thread! Thanks for the copy of the jvm modding bible, awesome stuff. Still have a few questions about the AFD mod, but i want to get parts before i start messing around with my jvm. Where do you guys typically get parts?


----------



## RickyLee

torcamaniac said:


> Just found this on facebook group. Someone saved a Copy of the modding bible from JVM Forum. Enjoy!!!


Very cool.

Did you notice if that facebook source might have page 2 of that thread saved to pdf as well?


----------



## MatFnEhUK

RickyLee said:


> Very cool.
> 
> Did you notice if that facebook source might have page 2 of that thread saved to pdf as well?


----------



## gregr

Col said:


> *Gain Reduction Mod*
> 
> Allows adjustment of gain/volume on OD1/2 Red and Orange modes. This can be used to balance the OD1/2 Red and Orange modes with OD1/2 Green mode.
> 
> *Affects*
> OD1/2 Orange and Red modes.


I have what I believe to be a marked improvement over this mod.

As it stands currently, only the sensitivity of the final gain stage has been reduced.

I've spread out the reduction of sensitivity by also reducing that of the boost stage. As a result, *the dynamics of the Orange mode has been increased* with the onset of saturation occurring at a much higher setting on the gain control. *Switching from Orange to Red now adds a noticeable amount of saturation without getting muddy * unless the gain control is at an extreme setting. In addition to the balance in level between Green and Orange, *the transition in gain from Green to Orange to Red is much more even.*

Most importantly, OD1/2 Orange, arguably the amp's pièce de résistance, still sounds amazing and feels even better than it did before, IMO.

It seems like a fair amount of top should be lost because of the Miller Effect, but I’m not noticing any issue. Maybe it’s due to all the harmonics being generated as a result of the additional stage of gain.


----------



## em07189

Gregr ,How do you put the 3,3 meg resistor in that place without damaging the pcb?



gregr said:


> I have what I believe to be a marked improvement over this mod.
> 
> As it stands currently, only the sensitivity of the final gain stage has been reduced.
> 
> I've spread out the reduction of sensitivity by also reducing that of the boost stage. As a result, *the dynamics of the Orange mode has been increased* with the onset of saturation occurring at a much higher setting on the gain control. *Switching from Orange to Red now adds a noticeable amount of saturation without getting muddy * unless the gain control is at an extreme setting. In addition to the balance in level between Green and Orange, *the transition in gain from Green to Orange to Red is much more even.*
> 
> Most importantly, OD1/2 Orange, arguably the amp's pièce de résistance, still sounds amazing and feels even better than it did before, IMO.


----------



## gregr

Vertically. I connect bottom side of the 1.5M to ground and leave the top pointing up. The 3.3M and 220k attach there and go to their respective points.


----------



## em07189

Yes, i was also thinking in that solution.


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## martinjuarez23

Col said:


> *OD2 to OD1 Mod*
> 
> This mod changes OD2 to have the same spec as OD1.
> 
> Although this mod works with the JVM410 it is most useful for the 2 channel JVMs as they come with OD2 as standard and some people prefer the character of OD1.
> 
> *Switchable*
> 
> No (possibly with the right switch).
> 
> *Affects*
> 
> OD2
> 
> *Parts list
> *
> - 1 x 33k resistor
> - 1 x 1n0 capacitor
> - 1 x 3n3 capacitor
> 
> *How To Perform The Mod*
> 
> How to perform the mod (JVM2 Series):
> - Change R44 from 47k to 33k
> - Change C31 from 1n5 to 1n0
> - Change C32 from 1n0 to 3n3
> 
> How to perform the mod (JVM410 Series):
> - Change R220 from 47K to 33K
> - Change C225 from 1n5 to 1n0
> - Change C224 from 1n0 to 3n3
> 
> *Source*
> 
> JVM Forum



Hello. I wanted to know if I am understanding correctly ... do I have to change the following components? C32 are two components?


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## gregr

You’re looking at the wrong board. You need to mod the one with all the controls on it.

I just noticed the parts list is wrong. To go from OD2 to OD1 you need a 33k resistor and a 3.3n capacitor. Just pull the 1.5n cap and move the 1n cap over to that spot. Put 3.3n where the 1n cap used to be. Replace the 47k slope resistor with 33k.


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## martinjuarez23

gregr said:


> You’re looking at the wrong board. You need to mod the one with all the controls on it.
> 
> I just noticed the parts list is wrong. To go from OD2 to OD1 you need a 33k resistor and a 3.3n capacitor. Just pull the 1.5n cap and move the 1n cap over to that spot. Put 3.3n where the 1n cap used to be. Replace the 47k slope resistor with 33k.



Ok. Thank You!! Now I think I found the correct pieces. I took a picture of them


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## Igor_torcamaniac

RickyLee said:


> Very cool.
> 
> Did you notice if that facebook source might have page 2 of that thread saved to pdf as well?



No.
If I had found more I would've downloaded it.


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## The Dose of Harmony

Sorry i thought that i had it!


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## The Creator

Hi all Marshall JVM users, I had my JVM410H dropped to the ground from 4 feet , so this is going to be opened and checked. While it is on the bench, I would like to carry out several of the modifications in this forumchain. To get new features, and to tame the beast  
Question: 
Filter Caps mod: Replacing C50 and C81 stock 220uF caps with 100uF electrolytic caps 450V.
Also replace the C42 stock 10uF cap with 22uF 350V.

Can I change the above caps without having installing a Choke?


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## Ken Ops

Hm, might have to finally get a couple mods done on my 210H.

OD2 -> OD1
volume mod for clean green (?)

Still not sure if the volume mod is even possible. Hard to find definitive info for that.

And as for the OD mod, is the info for that here up to date and accurate? Just checking before committing to it all.


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## spacerocker

Ken Ops said:


> volume mod for clean green (?)
> 
> Still not sure if the volume mod is even possible. Hard to find definitive info for that.



What do you mean: "Still not sure if the volume mod is even possible"?

I came up with a version of that mod, and have implemented it on my JVM205C. I also did a mod to convert Clean Green to Crunch Green (as I find that more useful than the true clean mode).....


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## gregr

The JVM has two master volumes which can be used to address this situation.

It isn’t just the green mode of the clean/crunch channel of the JVM2. The OD channel can also have this type of problem (volume drop when switching from either the orange or red modes to the green mode). Modding the amp so that the channel volume also controls the green mode of the clean/crunch channel on the JVM2 can be problematic as well. It wasn’t like the decision to omit the volume control on the green mode came out of thin air.

You could add a 5th volume control. The effects mix knob would be a good place to put it. This doesn’t fix the issue with the green mode using the same gain control as the other two modes.

My solution would be to repurpose the controls for the shitty reverb to be used as gain and volume for clean green. I’d also re-voice this joke of a clean mode so that it sounds better without having to compromise positions of the tone controls for the other two modes.


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## Rusty riffs

Col said:


> *OD2 to OD1 Mod*
> 
> This mod changes OD2 to have the same spec as OD1.
> 
> Although this mod works with the JVM410 it is most useful for the 2 channel JVMs as they come with OD2 as standard and some people prefer the character of OD1.
> 
> *Switchable*
> 
> No (possibly with the right switch).
> 
> *Affects*
> 
> OD2
> 
> *Parts list*
> 
> - 1 x 33k resistor
> - 1 x 1n0 capacitor
> - 1 x 3n3 capacitor
> 
> *How To Perform The Mod*
> 
> How to perform the mod (JVM2 Series):
> - Change R44 from 47k to 33k
> - Change C31 from 1n5 to 1n0
> - Change C32 from 1n0 to 3n3
> 
> How to perform the mod (JVM410 Series):
> - Change R220 from 47K to 33K
> - Change C225 from 1n5 to 1n0
> - Change C224 from 1n0 to 3n3
> 
> *Source*
> 
> JVM Forum


Can anyone elaborate in the values of these resistors/capacitors? 1/4w? 400w? does it matter? Thanks


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## Igor_torcamaniac

Rusty riffs said:


> Can anyone elaborate in the values of these resistors/capacitors? 1/4w? 400w? does it matter? Thanks


Hi rusty,

I don't remember. You should have a look at this part of the schematics. Usually there is a voltage written somewhere beside the capacitor. I think 1/4 Watts would be enough for resistors here. The withstanding voltage should be at least as much as on the original capacitors you are replacing. If it's more than you are on the sure side anyway. I think originally they are between 16 or 50 volts. So yes you can use anything that's above also.

Just checked and the capacitors are rated at 63V. So that's what you need at the least. And of course as I had said before you will be fine with 400V.


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## Rusty riffs

torcamaniac said:


> Hi rusty,
> 
> I don't remember. You should have a look at this part of the schematics. Usually there is a voltage written somewhere beside the capacitor. I think 1/4 Watts would be enough for resistors here. The withstanding voltage should be at least as much as on the original capacitors you are replacing. If it's more than you are on the sure side anyway. I think originally they are between 16 or 50 volts. So yes you can use anything that's above also.
> 
> Just checked and the capacitors are rated at 63V. So that's what you need at the least. And of course as I had said before you will be fine with 400V.


thanks a million!


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## jacksonsoloist556

Hi all. Glad to see some faces from the JVM forum in here!

I need to go back into my JVM410H and redo some stuff. The mod that needs the most attention is the blackface clean, but after moving, I have no idea where my notes are. Anybody still have the instructions?

Thanks!


----------



## donniecrump

Anyone have a source for a 6H 250ma to 300ma choke?

Also what is everyone's opinion on Edcor chokes? There seems to be a long wait, but I can get a Hammond now.


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## jacksonsoloist556

Look at what I found. I guess a few years ago I downloaded the major mods in PDF form, including the Blackface Clean mod I was asking about above. Enjoy!


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## Alnilam

Col said:


> *Compression Reduction Mod*
> 
> This mod reduces compression and allows the amp to sound more open.
> 
> *Switchable*
> 
> No (is possible though not recommended)
> 
> *Affects*
> 
> All channels/modes
> 
> *Parts list*
> 
> - 100kohm resistor (1 or 2x)*
> 
> OR
> 
> - 200-220kohm resistor (or any value around 200k) (1 or 2x)
> 
> *How To Perform The Mod*
> 
> - Replace R75 with a 100kohm resistor
> - Replace R72 with a 100kohm resistor (optional though recommended)
> 
> *Mod Variant*
> 
> Parallel wiring
> - Solder a 200-220kohm resistor in parallel with R75
> - Solder a 200-220kohm resistor in parallel with R72 (optional though recommended)
> 
> *Mod Author*
> 
> Casey Butt
> 
> *Source*
> 
> JVM Forum


When these mods aren’t switchable or not recommend, does it have something to do with the biasing of the amp? I’m still learning. Have learned a lot through this process so far. For instance at first I was going to play around and switch R97 with a 3 way switch. #1 820 ohm, #2 820ohm with 1uF parallel, #3 1K with .47uF parallel. I’ve seen an amp on YouTube with R97 switchable, but am unsure what was done.


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## Daniel55

I have that list of mods, when you try to click page 2 there is none. Is that right? Just making sure.


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## jacksonsoloist556

Correct. For whatever reason, I didn't save the second page.


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## Rusty riffs

Col said:


> *OD2 to OD1 Mod*
> 
> This mod changes OD2 to have the same spec as OD1.
> 
> Although this mod works with the JVM410 it is most useful for the 2 channel JVMs as they come with OD2 as standard and some people prefer the character of OD1.
> 
> *Switchable*
> 
> No (possibly with the right switch).
> 
> *Affects*
> 
> OD2
> 
> *Parts list*
> 
> - 1 x 33k resistor
> - 1 x 1n0 capacitor
> - 1 x 3n3 capacitor
> 
> *How To Perform The Mod*
> 
> How to perform the mod (JVM2 Series):
> - Change R44 from 47k to 33k
> - Change C31 from 1n5 to 1n0
> - Change C32 from 1n0 to 3n3
> 
> How to perform the mod (JVM410 Series):
> - Change R220 from 47K to 33K
> - Change C225 from 1n5 to 1n0
> - Change C224 from 1n0 to 3n3
> 
> *Source*
> 
> JVM Forum


Hey there dumb question but does it matter what kind of capacitor you use for the replacements? There are so many types... thanks


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## Morishidol

Col said:


> *1959 Plexi Cap Mod*
> 
> This mod reduces squeals and glassiness at high gain and rounds out the high end.
> 
> *Switchable*
> 
> No
> 
> *Affects*
> 
> All channels/modes
> 
> *Parts List*
> 
> - .68uF or 1uF polyester capacitor
> 
> *How To Perform The Mod*
> 
> - Solder the .68uF or 1uF polyester capacitor in parallel with R97
> 
> *Source*
> 
> JVM Forum


I would like to make this mod. I only can find a .68uf 100v in a local store. Is it possible to do With that ?


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## Moony

Morishidol said:


> I would like to make this mod. I only can find a .68uf 100v in a local store. Is it possible to do With that ?



Yes, 100v is enough for a cathode bypass cap.


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## Morishidol

Moony said:


> Yes, 100v is enough for a cathode bypass cap.


Good Morning,
Thank you for all helps. I made my Mods.

I had remove c83 cap, I didin't like it. l put it back.
I soldered 1w 150k in R71, 1w 250k in R75. It reduced the gain and made a bit open sound...for the results think like " anti-scoop"
Than I tried Plexi Cab mod. I put a .68uF 100v in R97. I didin't notice any differences. All are paralel Connected.
The end of the day, I like it , the sound is better then stock amplifier. My amp is a 410c. I changed also the speakers, 1 x Jensen Falcon, 1x Celestion g12t75. My tubes are 4xJJ tesla, 1x Shuguang ecc83, powers are 4x S4GB el34. Those Power tubes good for home users. I also have a Captor x Reactive Load.
Thank you for thr help.


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## Morishidol

After the mod, I took a video with my mobile phone. The tube I attached to v1 is very creamy. I will try another tube again;


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## Moony

Morishidol said:


> powers are 4x S4GB el34



If you bought the ones with the brown base those are selected Tung Sol EL34.


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## Morishidol

Moony said:


> If you bought the ones with the brown base those are selected Tung Sol EL34.


Yes, they have brown bottom. I also have 5x S4GB 12ax7 High grade, but when I put all of them, the amplifier sounding a bit weak... So, I have right now on v1 S4GB high grade, rest 4 tubes are JJ Tesla.. After my video, I also put stock speakers back.


----------



## Moony

Morishidol said:


> I also have 5x S4GB 12ax7 High grade



Which ones do you have?
There are different S4GB 12AX7 available. 
The "high gain" ones are selected Shuguang 12AX7B, the "normal gain" are selected JJ ECC83MG. 

I like buying tubes from BTB, it's a good seller!


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## Moony

gregr said:


> I have what I believe to be a marked improvement over this mod.
> 
> As it stands currently, only the sensitivity of the final gain stage has been reduced.
> 
> I've spread out the reduction of sensitivity by also reducing that of the boost stage. As a result, *the dynamics of the Orange mode has been increased* with the onset of saturation occurring at a much higher setting on the gain control. *Switching from Orange to Red now adds a noticeable amount of saturation without getting muddy * unless the gain control is at an extreme setting. In addition to the balance in level between Green and Orange, *the transition in gain from Green to Orange to Red is much more even.*
> 
> Most importantly, OD1/2 Orange, arguably the amp's pièce de résistance, still sounds amazing and feels even better than it did before, IMO.
> 
> It seems like a fair amount of top should be lost because of the Miller Effect, but I’m not noticing any issue. Maybe it’s due to all the harmonics being generated as a result of the additional stage of gain.



Just out of curiosity:
Are you still happy with this mod?
Or did you put back the stock values after a while?


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## Unforgiven

Hey everyone, 

I’ve been trying to figure out a way to describe the tone I’m seeking and I found this video. It looks like this is a modeled 410H but this is basically what I’ve been trying to achieve for 3 years. My 210c with v30s in a 212 never sounds like this. Always too much gain and annoying frequencies. The amp always feels flabby or too flat. Nothing in between it seems. 

What would be the best Mods for me to do get a good metal tone? I bought enough material to do any of the listed mods but before I start going wild trying things I felt I should ask all you first. 



Thanks


----------



## Igor_torcamaniac

Unforgiven said:


> Hey everyone,
> 
> I’ve been trying to figure out a way to describe the tone I’m seeking and I found this video. It looks like this is a modeled 410H but this is basically what I’ve been trying to achieve for 3 years. My 210c with v30s in a 212 never sounds like this. Always too much gain and annoying frequencies. The amp always feels flabby or too flat. Nothing in between it seems.
> 
> What would be the best Mods for me to do get a good metal tone? I bought enough material to do any of the listed mods but before I start going wild trying things I felt I should ask all you first.
> 
> 
> 
> Thanks



Hi Unforgiven,

it is good to have expectations and how you would like to sound but trying to get the same sound as some guy in a video is a fool's errand. You actually don't know how he got and recorded the sound. In description it also mentions profiled. Additional equipment, Guitar and pickups etc.?
You should be experimenting yourself while playing to a loud backing track or rehearsal volume with a band. 
The sound by yourself without any backing tracks will always sound different then while playing to loud music or live situation. Just saying... JVM has so many possible settings it can be overwhelming but also possible to get a variety of different sounds.
For metal you probably can get it to sounding somewhat tighter and more articulation and clearer on gain chanels by doing anti compression and gain reduction mods.


----------



## Unforgiven

Igor_torcamaniac said:


> Hi Unforgiven,
> 
> it is good to have expectations and how you would like to sound but trying to get the same sound as some guy in a video is a fool's errand. You actually don't know how he got and recorded the sound. In description it also mentions profiled. Additional equipment, Guitar and pickups etc.?
> You should be experimenting yourself while playing to a loud backing track or rehearsal volume with a band.
> The sound by yourself without any backing tracks will always sound different then while playing to loud music or live situation. Just saying... JVM has so many possible settings it can be overwhelming but also possible to get a variety of different sounds.
> For metal you probably can get it to sounding somewhat tighter and more articulation and clearer on gain chanels by doing anti compression and gain reduction mods.


I fully understand and agree with you. And btw all I do is create tones and try finding something unique. I actually spend way more time doing that then recording. 
My 5150 is my main amp for metal tones and I get awesome recordings from it but I’m trying to get this 210c to sound right so I can quad track with both amps. My Laney IRT just doesn’t mix the way I want. 

My lingo isn’t very good so thats why I shared the YouTube vid. I probably should’ve just asked you all what you think would be the best mods for metal tones but anyways.  

Yes I was thinking the same thing, anti compression and definitely the gain reduction.

I seen the Mesa/Boogie mod and I’m curious what that sounds like. Has anyone tried it?


----------

