Mod JVM or sell it?

[spacerocker]

Before you consider swapping the transformers please watch a few demos of the 2203 KK (Kerry King JCM800) - it has the same transformers as the JVM410H.
I need to point that out every now and then because the 2203KK is considered to be "one of the best 800s ever made" also by guys who usually prefer the "vintage stuff" and say that the transformers of the JVM are "crap".
So it's more the circuit that makes a difference!

Also, a few of the guys on the old JVM Forum tried swapping out the transformers for MM ones. Some said it made no difference, whilst others thought it tightened up the bass (or something...) - but the general consensus was that any small perceived improvement wasn't worth the cost and effort of doing it!

I am also of the opinion that the stock transformers are fine!....

 

[spacerocker]

is 176K (jcm800 value) that loose? I would prefer not to drill into the chassis to mount an extra pot, so i would simply ask my tech to replace the resistor. Thus getting the value right the first time is quite necessary 😅 Also, the tone from the video posted earlier is the ballpark tone I'm aiming for, the guy did have his transformers replaced by MM ones how much would that affect his tone?

Initially I added a 91K resistor in series with the stock 82K resistor to make = 173K (as you say roughly 800 equivalent value) I found this made a big improvement over stock. Later on I used a pot, and had it set to around 500K. The difference going from 176K to 500K isn't all that dramatic....these days I just add a 300K in series with the 86K

I answered the question about "upgrading" the transformers above! Although I have never done it, the consensus from those who have seems to be that it doesn't provide good "bang for the buck"...

Incedentally, I think I may have referred to the neg FB resistor as "R86" it is, of course R58. The value is 82K which is what confused me!

 

[marshallmellowed]

is 176K (jcm800 value) that loose? I would prefer not to drill into the chassis to mount an extra pot, so i would simply ask my tech to replace the resistor. Thus getting the value right the first time is quite necessary 😅 Also, the tone from the video posted earlier is the ballpark tone I'm aiming for, the guy did have his transformers replaced by MM ones how much would that affect his tone?

Assuming the power transformer is sufficient for the task (most amp designs are) it will have absolutely no affect on the tone or feel of an amp. The power transformer does nothing but drop (or raise) the voltage level of the line voltage in order to supply the circuit with what's needed. On the other hand, the output transformer "can" affect the tone and more importantly, "feel" of an amp, as it's directly coupled to the speaker(s).

 

[Moony]

Incedentally, I think I may have referred to the neg FB resistor as "R86" it is, of course R58. The value is 86K which is what confused me!

I just looked at the schematics of JVM2 and JVM4 - it's R58 and 82k (and a small 120p cap in parallel).

 

[spacerocker]

I just looked at the schematics of JVM2 and JVM4 - it's R58 and 82k (and a small 120p cap in parallel).

Yes - you are quite correct! I checked myself and have been going back correcting my posts in this respect!....

 

[Moony]

I checked myself and have been going back correcting my posts in this respect!....

It wouldn't matter if its a 82k or 86k - just wanted to post that lest someone think they have the wrong resistor if the value doesn't match.

 

[marshallmellowed]

is 176K (jcm800 value) that loose? I would prefer not to drill into the chassis to mount an extra pot, so i would simply ask my tech to replace the resistor. Thus getting the value right the first time is quite necessary 😅 Also, the tone from the video posted earlier is the ballpark tone I'm aiming for, the guy did have his transformers replaced by MM ones how much would that affect his tone?

This is just my opinion, but I did the adjustable feedback mod to my 410H, and I wouldn't say it "loosens" the amp up. It does add a noticeable amount of treble and what I'd call "ragged" quality to the overdrive as the NFB is reduced (R value increased). What loosened the amp up for me, was the addition of a 3H choke, as a choke is one of the things stock on a 2203, but absent on a JVM. The JVM 410HJS has a choke, but not the standard JVM's.

 

[Moony]

I'm pretty sure that Santiago left out the choke because he thought it doesn't matter with that design.
It's not a production cost saving thing - even the small Mini Jubilee got a choke. The SC20 and SV20 don't have one - it's debatable if a 20W amp needs a choke anyway.

I mean everyone can do it, it's not that expensive.
But I wouldn't expect too much from it.
Changing the plate resistors or the amount of NFB will have a greater impact to the overall sound.

 

[maxxi]

If i understand correctly, the NFB gives the amp a more 'raw' type of distortion, rather than a looser feel in the bottom end? Could well be the thing i'm aiming for... 🤔 it's just hard to explain Sometimes what sound you have in your head 😅

 

[Moony]

If i understand correctly, the NFB gives the amp a more 'raw' type of distortion, rather than a looser feel in the bottom end?

Higher nfb resistor value = less negative feedback (NFB)
Lower nfb resistor value = more negative feedback (NFB)

The stock nfb resistor value on the JVM410H is 82k at 16 ohms.
The stock nfb resistor value on a 2203 is 100k at 4 ohms (moving that to the 8 ohms or even 16 ohms tap would give you more nfb).
You could calculate an equal value with the factor around 1,4 when going from 4 ohms to 8 ohms and from 8 ohms to 16 ohms, eg. 100k at 4 ohms is ~ 140k at 8 ohms etc.

More nfb --> tighter low end and more frequency bandwith - presence and resonance also have more impact
Less nfb --> less tight, more raw, less bandwith - presence and resonance have less impact (without any nfb they don't work anymore)

 

[spacerocker]

This is just my opinion, but I did the adjustable feedback mod to my 410H, and I wouldn't say it "loosens" the amp up. It does add a noticeable amount of treble and what I'd call "ragged" quality to the overdrive as the NFB is reduced (R value increased). What loosened the amp up for me, was the addition of a 3H choke, as a choke is one of the things stock on a 2203, but absent on a JVM. The JVM 410HJS has a choke, but not the standard JVM's.

Perhaps "loosen" isn't the best way to describe the neg FB mod. What I meant was that it makes the amp more dynamic and open sounding (because the signal isn't being partially cancelled as much by a negative version of itself...) I agree that it also give a more "raw" sound as the negative feedback is reduced. Also as the volume is increased, power valve overdrive occurs more easily than with the standard level of neg FB...

I agree a choke gives a looser feel, especially if combined with a lower value of power-supply capacitance. I use a 10H Choke + reduced capacitance (can't remember by how much off the top of my head...)

If i understand correctly, the NFB gives the amp a more 'raw' type of distortion, rather than a looser feel in the bottom end? Could well be the thing i'm aiming for... 🤔 it's just hard to explain Sometimes what sound you have in your head 😅

Yes - reducing the negative FB gives the amp a more "raw" aggressive feel "Big Dooley" on here coined the phrase "more rudeness" to describe the neg FB mod! As explained above, reducing negative FB also makes the amp more dynamic and less compressed in feel. It's like the difference between driving a car with the hand-brake on, and then releasing it!

It wouldn't matter if its a 82k or 86k - just wanted to post that lest someone think they have the wrong resistor if the value doesn't match.

Yes, absolutely! it is important to get these things right to avoid confusion! In future I will double check (rather than just relying on my memory!....)

 

[maxxi]

Allright, i'm pretty much settled on the NFB mod. The only question is what value to take. I'm considering either 136k (evh) or 176k (2203). 🤔

 

[spacerocker]

Allright, i'm pretty much settled on the NFB mod. The only question is what value to take. I'm considering either 136k (evh) or 176k (2203). 🤔

Definitely go for the higher value! it will have more effect at realistic volumes!

 

[maxxi]

The more I think about it, the more I'm tempted to add a choke as well while the amp is at my tech (the board has to be taken out anyway to do the NFB mod). Any suggestions concerning choke value and Ma's/max voltage ratings?

 

[spacerocker]

The more I think about it, the more I'm tempted to add a choke as well while the amp is at my tech (the board has to be taken out anyway to do the NFB mod). Any suggestions concerning choke value and Ma's/max voltage ratings?

I have a 10H choke in my JVM410. A lot of people like 3H. Bear in mind that (in my opinion at least) a choke affects the feel and dynamics more than the actual tone! A Choke gives the amp a sort of "bounce" particularly on power chords and bass notes...

I bought one of each (3H and 10H) and preferred the 10 (I sold the 3H) - but everybody's different!

 

[maxxi]

I eventually got the JVM modded by my regular tech (40 years worth of experience, and well known around here), and I thought why not write a follow up on the results. You never know it can help someone else on the fence of modding their JVM.

What got modded:
- R58 changed from 82K to 176K (2203 negative feedback level)
- 1uF capacitor in parallel with R97 (plexi mod)
- The entire preamp got modded more towards 2203 spec ( don't know what exactly got changed, but it's around 9 points in the preamp that get modded)
- The bias was adjusted back into spec, as over the years it had drifted a bit

First impressions:
- The amp warmed up A LOT. I don't know what mod has the most effect, but I am guessing it's the plexi cap mod. The cleans are way warmer, and the OD1/2 channels don't sound as shrill as they used to.
- The amp did get a little louder than it used to. That's to say, it gets louder faster. You can still get some great tones at bedroom volume, but it really starts to open up when the channel volume hits 5/10 (master on 1/10) -> It's not stupidly loud, but louder than bedroom volume anyway.

- The clean (green) channel now delivers a warm clean tone, which reminds me of the TSL's clean channel back when I owned one. Before the mods I feared the clean channel would change into an "edge of breakup" kind of tone, but to my surprise it stays clean.

- The crunch channel is actually enjoyable to play now. Before I hated the crunch channel because it was way too stiff, and didn't have any dynamics to it. Now it sounds like how a Marshall should sound, fat and in your face. You can go from plexi edge of breakup right up to roaring JCM 800 tones.
Put an overdrive in front of crunch orange, and you get that boosted JCM800 type of tone.

- OD1/2: this is where the money is at. These channels have warmed up considerably, and have some real oomph to them now. The saturation is now backed up by the power section and work together to deliver a full overdriven sound.
Can you still play ultra tight metal on them? Probably not, but it's not flubby either as I have tried it with actve EMG's and it sounded tight enough for my taste (an overdrive pedal should get the low end back in check even further if needed).

Are their any drawbacks to these mods?
Yes and no.
- First off, I noticed that i keep the resonance knob at 0 for now where before I had it around 3-4 (I use an Orange PPC212 now, which gets boomy rather fast. On my Marshall 4x12 with V30's it was less boomy).
- To get the most out of these mods you need to turn the amp up a little more, possibly resulting in complaining housemates, neighbours (not in my case, but your situation might vary).
- You probably won't put your guitar down for quite a while, because you are amazed at how good the amp sounds 😁

Final thoughts.
- Would I mod it again?
yes, the JVM is just so versatile it would be silly to get 4 different amps when a few simple mods can get you there.
- Are these mods worth it?
Yes, these mods cost me less than €100. For that price you can't buy a new amp.
- Is the JVM the perfect amp post-mods?
For me personally no. There are still a few things that bug me like the overwhelming amount of gain on the OD1/2 channels and the lack of a built-in noise gate. Both these arguments can be worked around of course, but are somewhat of a missed chance by Marshall IMO.

 

[spacerocker]

Excellent! Thanks for the update!

For the most part, it seems that you are happy with the mods you have had done, and feel that (for you at least) it has improved the feel and sound of the amp.

You mentioned that the amp seems louder, or at least "seems to get louder sooner"- that is the effect of the reduced negative feedback. Flat out, the amp would be the same as stock, but volume for volume less neg FB makes the amp seem louder. The stock amp is a bit like driving a car with the brakes on. You have let the brakes off a little and it now seems faster!

Regarding the OD channels still having too much gain - I didn't see a mod in your list that would reduce it (unless I missed it?) There are mods to reduce gain in the OD channels Orange and Red Modes - the simplest of which being to replace V6 (the valve under the can) with an ECC823. This has considerably less gain in the section that is used for OD1/2 Orange and Red

Glad you are (on the whole) happy with the mods!

 

[El Dunco]

IMO, the JVM is a very mod friendly amp. I did little more than pull the C83 and added a couple of extra caps, then got the Fortin mod (didn’t dramatically change the tone I already liked but made it a little smoother, no fizz and is an incredible plug-n-play metal monster!) and now it has my absolutely ideal sound without any extra OD/EQ help.

The stock sound was 90% there, as I mostly use it for heavy industrial thrash metal. The fairly simple mods just pulled back the gain to more useable levels and got rid of the slight “scratchiness” in the high-mids, which was improved even more with the right speakers.

There are even more involved, but not too difficult mods to make it more classic and even less compressed. The JVM forum might not be around anymore, but all the mods people submitted are extensively cataloged other places online.

The cleans are gorgeous, the crunch is absolutely close enough to the earlier Marshall sound and the clean is gorgeous from lush to spanky. You can get so many sounds just from different guitar and pickup combos and settings (there’s a relatively simple Blackface mod for that too.)

The OD2 orange is just an awesome metal sound right out of the box, nice and tight, can hang on its own but just mows down everything in its path paired with a “bigger/less taught” amp like my Dual Rec.

 

[Force235]

Help, I'm handy, I can do the mods, but no where above did anyone explain what the mods are, is there a listing of JVM mods somewhere that describes exactly what to change for each mod?????

 

[spacerocker]

Help, I'm handy, I can do the mods, but no where above did anyone explain what the mods are, is there a listing of JVM mods somewhere that describes exactly what to change for each mod?????

Tell us which mods you are interested in, and I am sure I (and others) will be able to tell you how to do them. Also there is (somewhere on this board ) the list of mods off the old JVM Forum, which also describes how to do them.