# What To Build . Plexi With One Wire Mod Or Jcm800



## Copperheadroads

I`m wanting to build another amp .
50 watt Marshall with a Lar mar or Ken fisher style PPIMV .
I`m not sure what would be better , I assuming the plexi would be more versatile ,would adding the one wire mod (cascading the gain stages ) make this amp compete with the jcm


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## Kris Ford

OWM!!!


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## stickyfinger

Do the RR OWM and the amp becomes a Bright channel plexi and the normal inputs become a hot rodded gain machine.


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## Kris Ford

stickyfinger said:


> Do the RR OWM and the amp becomes a Bright channel plexi and the normal inputs become a hot rodded gain machine.



Which is just like mine!


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## neikeel

You can make it switchable too, then tune the cap/resistor values to your taste, if you like lots of gain.
Personally I lke the definition and tone of a very slightly warmed up 2204 so have a 2 in 1 plexi/2204 that I use for gigging. It has 8k2 on second stage cathode, 0.68uF/2k7 on first stage (ie stock 2204) and another 0.68uF over 820R on V2a, I like reasonably tight NFB (47k/8ohm).


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## ampmadscientist

Copperheadroads said:


> I`m wanting to build another amp .
> 50 watt Marshall with a Lar mar or Ken fisher style PPIMV .
> I`m not sure what would be better , I assuming the plexi would be more versatile ,would adding the one wire mod (cascading the gain stages ) make this amp compete with the jcm



The one wire mod is Mickey Mouse. (it's a joke)
Use the 800 2203 schematic.
http://www.drtube.com/schematics/marshall/2203u.gif

The plexi has a problem...won't work right until the volume is pegged, producing output tube distortion.. Making the amp waaaaaaay too loud for most applications!

The plexi is a CLEAN amp which has no distortion until the output tubes overdrive. Face reality.

The 2203 adds one gain stage...to the plexi design...(it's the same amp w/ one extra stage).
This allows preamp distortion, to get a better sound at a low volume.

Plug into LOW input: you have a plexi (basically).
Plug into HIGH input: you have the extra stage.
The 2203 is much more practical versatile for lower volume (or all) applications.

Most people who "want" a plexi...don't really want a plexi....because they do not understand all of the above information....which is very important.

Add a master volume to the Plexi: It's a waste, it will never give you what you wanted or expected.

Add a master volume to the 2203: NOW you are talk'in.

And while I am on the subject of ppimv Master Volume: direct coupled (without capacitors) is way more accurate, and works much better.
Use a 1M audio pot, which has an insulation rating of 300-500 Volts. (there is bias voltage on the pot)
Recommended: PEC 1 Meg Pot, which has high voltage insulation.





https://www.tubesandmore.com/products/potentiometer-precision-electronics-audio-slotted-shaft

As long as the pot has high voltage rating, there will be no problem whatsoever.
So...don't use a cheap pot to do this master volume.


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## m1989jmp

ampmadscientist said:


> The one wire mod is Mickey Mouse. (it's a joke)
> Use the 800 2203 schematic.
> http://www.drtube.com/schematics/marshall/2203u.gif
> 
> The plexi has a problem...won't work right until the volume is pegged, producing output tube distortion.. Making the amp waaaaaaay too loud for most applications!
> 
> The plexi is a CLEAN amp which has no distortion until the output tubes overdrive. Face reality.
> 
> The 2203 adds one gain stage...to the plexi design...(it's the same amp w/ one extra stage).
> This allows preamp distortion, to get a better sound at a low volume.
> 
> Plug into LOW input: you have a plexi (basically).
> Plug into HIGH input: you have the extra stage.
> The 2203 is much more practical versatile for lower volume (or all) applications.
> .
> .



Plexi (1987/1959) with one wire mod IS JCM800 with different resistors/caps on V1b and actually has more gain. Oh and a pre-ppi master volume

So basically what you're saying is that a plexi with OWM is a clean sounding amp?


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## ampmadscientist

m1989jmp said:


> Plexi (1987/1959) with one wire mod IS JCM800 with different resistors/caps on V1b and actually has more gain. Oh and a pre-ppi master volume
> 
> So basically what you're saying is that a plexi with OWM is a clean sounding amp?



No I did not say that.
Plexi is a clean amp....(copy of Fender bassman). Until you drive it into maximum, the plexi remains clean. All the way to 100W...
After you reach 100W, then it distorts, and is extremely loud.

The one wire mod is a (mickey mouse) attempt to change a Plexi into a 2203.
Might as well do it the right way...you have the schematic for 2203.

Plexi = 10mv sensitivity
V1 One preamp stage > V2 compression Stage > Tone Stack > Phase Inverter> PPIMV> PAmp

2203 = 5mv Sensitivity
V1A preamp Stage 1 >V1B preamp stage 2 >V2 Compression Stage > Tone Stack > Phase Inverter >PPIMV >Power Amp

What you really want is the 5mv sensitivity.

Your Crunch-crazed brain is actually craving the 5mv, that's what the one wire mod does.
It's just that building it as 2203 does the same thing, just the "right"way; the factory way.


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## stickyfinger

Can you explain what you mean by 5mv sensitivity and 10mv sensitivity please? And why that matters?

I disagree about the one wire mod being a joke as long as there's a master vol after the tone stack or PPIMV. The RR OWM is great as you can still have the Plexi for the gig (or when ever) and when you want great tone to practice, the OWM works great.

The 2203 low input is no plexi either! That stage produces almost no gain. A PPIMV actually works quite well on a plexi because the output section doesn't distort as much as a Metal panel. The tone is more constant on the Plexis I find.


ampmadscientist said:


> No I did not say that.
> Plexi is a clean amp....(copy of Fender bassman). Until you drive it into maximum, the plexi remains clean. All the way to 100W...
> After you reach 100W, then it distorts, and is extremely loud.
> 
> The one wire mod is a (mickey mouse) attempt to change a Plexi into a 2203.
> Might as well do it the right way...you have the schematic for 2203.
> 
> Plexi = 10mv sensitivity
> V1 One preamp stage > V2 compression Stage > Tone Stack > Phase Inverter> PPIMV> PAmp
> 
> 2203 = 5mv Sensitivity
> V1A preamp Stage 1 >V1B preamp stage 2 >V2 Compression Stage > Tone Stack > Phase Inverter >PPIMV >Power Amp
> 
> What you really want is the 5mv sensitivity.
> 
> Your Crunch-crazed brain is actually craving the 5mv, that's what the one wire mod does.
> It's just that building it as 2203 does the same thing, just the "right"way; the factory way.


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## neikeel

A very didactic response.
Also making the erroneous point that lo input is same as plexi is totally untrue - have you ever tried it?
I agree if you want distortion then you have two choices, drive the cascaded pre-amp for some distortion and thus push the PI and thence the output stage depending on the pre-PI MV or push the power stage by cranking an uncascaded preamp. 
But, as has been admitted, a pushed output stage sounds best, even with a 2203/4 preamp.

I like nuance and subtlety as well as distortion hence my assertion that in one amp a switchable hybrid of NMV and MV is an excellent option. The 1 wire mod is just another iteration of a cascaded preamp and it does need a master volume. Choose pre or post PPIMV to taste.


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## Copperheadroads

I have the Ken fisher type PPIMV on both of my Traynor YBA-1's that are modded to marshall plexi & jcm circuits & it totally makes them very usable . lovey tone


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## ampmadscientist

stickyfinger said:


> Can you explain what you mean by 5mv sensitivity and 10mv sensitivity please? And why that matters?
> 
> I disagree about the one wire mod being a joke as long as there's a master vol after the tone stack or PPIMV. The RR OWM is great as you can still have the Plexi for the gig (or when ever) and when you want great tone to practice, the OWM works great.
> 
> The 2203 low input is no plexi either! That stage produces almost no gain. A PPIMV actually works quite well on a plexi because the output section doesn't distort as much as a Metal panel. The tone is more constant on the Plexis I find.



5mv VS 10 mv sensitivity, why it matters:
You keep calling the amp (or the tube) "high gain," but that's not it.
The amps have the same gain.
The sensitivity is different - that's what really matters. (people are always confusing "gain" with sensitivity)

The Plexi has 1 preamp stage - it takes *10mv input signal* (from the factory) to drive the amp to full 100W output.

The 2203 has 2 preamp stages in series, (sometimes called "cascade") *it takes 5mv input signal* to drive the amp to full 100W output.

The 2203 is *twice as sensitive as the plexi. *This is what you really want.
(this is why you do the "one wire" mod. --- It's increasing the sensitivity!)
(this is why you use distortion pedals, it's increasing the sensitivity)

How much input signal does it take to drive the amp to full output?
That's the sensitivity!
Guitar player does not realize that this is the most important of all: the sensitivity.

Second most important: what frequencies compress?
High frequencies compress: this is what makes amp "Crunch."
Low frequencies compress: this is what makes amp sound "muddy."


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## solarburn

Good gawd it is not just power tube that gives a Plexi it's sweet spot. It's both preamp and power tube mixed just right.

And there are many spots along the way you can boost power tube to get great tone and feel.

Some of these techs lack application or proper audio tests. Or lack the ability to use a Plexi throughout its volume and gain range. Speakers and pups mean quite a bit in a clear open Plexi amp. So many ways to affect the front end of a Plexi.

It is not just power tube saturation. It's the mix between preamp and power section. If it was just power tube saturation then it would be farty and the volume would be so loud it would ruin tone and dynamics. Freaking techs sometimes need to play more. I respect them but there are times where they tell stories based on data. Guess what? Data can be a cold place. Stiff, sterile and unforgiving. Not what my plexis sound like at all.


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## neikeel

solarburnDSL50 said:


> Good gawd it is not just power tube that gives a Plexi it's sweet spot. It's both preamp and power tube mixed just right.
> 
> And there are many spots along the way you can boost power tube to get great tone and feel.
> 
> Some of these techs lack application or proper audio tests. Or lack the ability to use a Plexi throughout its volume and gain range. Speakers and pups mean quite a bit in a clear open Plexi amp. So many ways to affect the front end of a Plexi.
> 
> It is not just power tube saturation. It's the mix between preamp and power section. If it was just power tube saturation then it would be farty and the volume would be so loud it would ruin tone and dynamics. Freaking techs sometimes need to play more. I respect them but there are times where they tell stories based on data. Guess what? Data can be a cold place. Stiff, sterile and unforgiving. Not what my plexis sound like at all.



Think I just said that above - we agree absolutely!


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## ampmadscientist

Since I am a guitar player, before anything else...

The plexi is the clean preamp which overdrives the power amp into distortion compression.
(the power amp clips before the preamp does...)
Only drawback being: plexi is too loud when cranked to point of distortion.
Otherwise this tone would be really ideal, if it were not for the loudness...

That is why the factory added an extra preamp stage / master volume to the plexi.
It's the 2203...
Allows preamp distortion, overdriving phase inverter...but when the master volume is down, you lose the output tube distortion.
This was the compromise. The output tube distortion sounds much better than preamp distortion.
BUT it was too loud that way.

The only real difference was that the low frequency is limited on the 2203...to prevent too much compression in the low frequencies. (which sounds muddy overloaded).
This is done by changing the cathode bypass cap of V1A, to 0.68.

Other than that the 2 amps are essentially the same amp, 2203 has the 2 preamp stages of a plexi, but ganged together in series....instead of separated in parallel. (this is also called "cascade.")


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## stickyfinger

In a Plexi the PI clips before the power amp unless you consider that the power amp . I don't as its simply a gain stage paired with a inverting stage.

OWM allows for a lot (and I mean a lot) more "sensitivity" than a 2203. RR OWM it and add a PPIVM. Now you got your Plexi and a hot rod cali amp.

Or you could add a switch. I like the RR mod as the plexi looks stock nut once you plug in you'll know 


ampmadscientist said:


> Since I am a guitar player, before anything else...
> 
> The plexi is the clean preamp which overdrives the power amp into distortion compression.
> (the power amp clips before the preamp does...)
> Only drawback being: plexi is too loud when cranked to point of distortion.
> Otherwise this tone would be really ideal, if it were not for the loudness...
> 
> That is why the factory added an extra preamp stage / master volume to the plexi.
> It's the 2203...
> Allows preamp distortion, overdriving phase inverter...but when the master volume is down, you lose the output tube distortion.
> This was the compromise. The output tube distortion sounds much better than preamp distortion.
> BUT it was too loud that way.
> 
> The only real difference was that the low frequency is limited on the 2203...to prevent too much compression in the low frequencies. (which sounds muddy overloaded).
> This is done by changing the cathode bypass cap of V1A, to 0.68.
> 
> Other than that the 2 amps are essentially the same amp, 2203 has the 2 preamp stages of a plexi, but ganged together in series....instead of separated in parallel. (this is also called "cascade.")


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## Copperheadroads

what is the difference in Randy Rhoads OWM & the regular plexi with the OWM ?


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## stickyfinger

Typical OWM removes one of the inputs, so no 68ks (formally the grid stoppers on the input of the amp) to ground on the second stage. The Vol pot before it is the ground so you can remove the 68ks for more gain. RR OWM lets you choose 2 or 3 gain stages by leaving the 68k grid stoppers in tacked. So classic 4 hole bright channel and hot rod OWM on normal inputs


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## AlvisX

Im not a fan of one wire mods myself . Both my Major & my '73 1959 had it when I got each of 'em .....a squeally mess they were . I wish I'd taken pics
The 100 watter I put back to my version of stock .....shared cathode /2x022 V1 couplers/250pf brite with an older trainwreck PPIMV....I have yet to try version 3 ,looking for a donor

The Major ,a 2203 preamp made it into the most awesome thing

I have no idea exactly what you guys are shootin for . I'd say Im more a classic Marshall guy and tend to tune my amps that way ....not much of a "gainiac"


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## Kris Ford

stickyfinger said:


> Typical OWM removes one of the inputs, so no 68ks (formally the grid stoppers on the input of the amp) to ground on the second stage. The Vol pot before it is the ground so you can remove the 68ks for more gain. RR OWM lets you choose 2 or 3 gain stages by leaving the 68k grid stoppers in tacked. So classic 4 hole bright channel and hot rod OWM on normal inputs



Wonder if this is why the gain is boosted when plugging a blank plug into Channel I HI and LO..mine is cascaded, and all 4 holes work...


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## stickyfinger

Kris Ford said:


> Wonder if this is why the gain is boosted when plugging a blank plug into Channel I HI and LO..mine is cascaded, and all 4 holes work...


Yes should be blocking the signal to ground via 68ks.


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## Kris Ford

stickyfinger said:


> Yes should be blocking the signal to ground via 68ks.


Cool, really does boost the gain!!!


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## Copperheadroads

after doing some research I think I've decided i want to build the jcm but add a switchable gain stage if i can or some switchable mods to add extra gain .
i have a bunch of marshall style amps , Phaez Jube /18 watt plexi /2 70's traynor yba-1/yba-1a/
so adding an extra gain stage like the AFD tim caswell #3? my be something i can do if i can add it on a switch .
I have a blank chassis on the way & I have to order the trannys yet ..either that or something along the friedman mod ....let me know what you think


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## dreyn77

That's a much better choice by you.
Avoid making amps which are pretending to be other amps. 
It won't BE the RR amp.
You either go authentic, or custom. And if you GO custom, then do something which isn't out there for sale already. 

The RR amp is a nod to historical things. So if you haven't worked out what that is yet, then don't build yet!

I think you need to do more research and narrow down what your playing style will be with the historical gear you want to use.


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## Thiez

JCM800 as a base is really nice! Listen to the Rockstah mod 5 for instance, it's based on JCM800 while it sounds like a hotrod plexi. 
Easy gainmod is offcourse the bypasscap on V2, original there is non in the JCM, you can boost it to plexi kinda sounds with 0,68uF or more gain over the whole frequency range with 25uF. Put a switch between the cap and ground and you have a switchable gainboost.


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## ampmadscientist

When you take out/ bypass the grid stopper, the amp does become slightly more sensitive, but
the voltage on V1A grid (is now) not what V1A would prefer...
and V1A will be making some farty sounding overloaded/distortion.
See, for "ideal" operation
the vacuum tube wants to see a certain grid DC voltage, and when you take the resistor out, that ideal voltage is no longer present.

You could probably see this grid voltage with a scope or vacuum tube volt meter.
A regular DMM will probably not be able to see this DC voltage. However, it's definitely there.


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## ampmadscientist

Thiez said:


> JCM800 as a base is really nice! Listen to the Rockstah mod 5 for instance, it's based on JCM800 while it sounds like a hotrod plexi.
> Easy gainmod is offcourse the bypasscap on V2, original there is non in the JCM, you can boost it to plexi kinda sounds with 0,68uF or more gain over the whole frequency range with 25uF. Put a switch between the cap and ground and you have a switchable gainboost.



That is boogie type boost.
But, it changes the input frequency response, thereby potentially overloading V2 (compression stage) with subsonic low frequency. This causes low frequency to become over-compressed. The amp sounds muddy when it distorts instead of crunchy.
That is why the design changed to .68 prevents too much low frequency saturation of V2.

Use this calculator:
https://www.ampbooks.com/mobile/amplifier-calculators/cathode-capacitor/


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## NewReligion

Building the Classics...

Yes why not go ahead and build a high gain 2204… That is to say Friedman BE100 with switchable fourth gain tube stage HBE and additional switchable saturation switch (MPSAO6 Dual transistor).

Have fun. I look forward to your build.


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## ampmadscientist

NewReligion said:


> Building the Classics...
> 
> Yes why not go ahead and build a high gain 2204… That is to say Friedman BE100 with switchable fourth gain tube stage HBE and additional switchable saturation switch (MPSAO6 Dual transistor).
> 
> Have fun. I look forward to your build.



Yeah I like NRs amps a lot...he seems to have a good sense about which tweaks work the best.
I like his tone.


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## Thiez

ampmadscientist said:


> That is boogie type boost.
> But, it changes the input frequency response, thereby potentially overloading V2 (compression stage) with subsonic low frequency. This causes low frequency to become over-compressed. The amp sounds muddy when it distorts instead of crunchy.
> That is why the design changed to .68 prevents too much low frequency saturation of V2.
> 
> Use this calculator:
> https://www.ampbooks.com/mobile/amplifier-calculators/cathode-capacitor/



Yes i know this. That why i said that 25uF will boost the whole frequentie range. And .68 will give you more of a plexi grind to it in the higher frequenties. You can try everything in between, whatever you feel like sounds good for you. If the amp sounds muddy or not will depend on a whole lot more then just the 25uF bypass cap. But yeah sure, it is possible it will sounds more muddy.


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## Thiez

For Friedman sounds, just put a 220K-2K7 gainstage into a 100K-2K7 gainstage and then into the normale 820ohm and cathode follower. Start from there and tweak till you have the sound you like.


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## proxy

How about Mini Major?


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## printer

I never noticed this section of the forum. I am no guitar player, I can strum a few chords but don't expect me to string them together and play a song. Maybe one day. But I was browsing through the threads and reading this one I just had to reply. It bothers me to see people give advice when they really do not seem to understand what they are talking about.



ampmadscientist said:


> No I did not say that.
> Plexi is a clean amp....(copy of Fender bassman). Until you drive it into maximum, the plexi remains clean. All the way to 100W...
> After you reach 100W, then it distorts, and is extremely loud.
> 
> The one wire mod is a (mickey mouse) attempt to change a Plexi into a 2203.
> Might as well do it the right way...you have the schematic for 2203.
> 
> Plexi = 10mv sensitivity
> V1 One preamp stage > V2 compression Stage > Tone Stack > Phase Inverter> PPIMV> PAmp
> 
> 2203 = 5mv Sensitivity
> V1A preamp Stage 1 >V1B preamp stage 2 >V2 Compression Stage > Tone Stack > Phase Inverter >PPIMV >Power Amp
> 
> What you really want is the 5mv sensitivity.
> 
> Your Crunch-crazed brain is actually craving the 5mv, that's what the one wire mod does.
> It's just that building it as 2203 does the same thing, just the "right"way; the factory way.





ampmadscientist said:


> 5mv VS 10 mv sensitivity, why it matters:
> You keep calling the amp (or the tube) "high gain," but that's not it.
> The amps have the same gain.
> The sensitivity is different - that's what really matters. (people are always confusing "gain" with sensitivity)
> 
> The Plexi has 1 preamp stage - it takes *10mv input signal* (from the factory) to drive the amp to full 100W output.
> 
> The 2203 has 2 preamp stages in series, (sometimes called "cascade") *it takes 5mv input signal* to drive the amp to full 100W output.
> 
> The 2203 is *twice as sensitive as the plexi. *This is what you really want.
> (this is why you do the "one wire" mod. --- It's increasing the sensitivity!)
> (this is why you use distortion pedals, it's increasing the sensitivity)
> 
> How much input signal does it take to drive the amp to full output?
> That's the sensitivity!
> Guitar player does not realize that this is the most important of all: the sensitivity.
> 
> Second most important: what frequencies compress?
> High frequencies compress: this is what makes amp "Crunch."
> Low frequencies compress: this is what makes amp sound "muddy."



'The sensitivity is TWICE as much, 5 mV compared to 10 mV' .

Now what is wrong with this picture? Twice as sensitive almost sounds like it would be twice as loud going by AMS. But what is a gain of two? 10 mV / 5 mV = 2, well that is 3 dB. Not a heck of a lot. Turn on your stereo. While playing music disconnect one speaker. The drop in volume is -3 dB. Hardly the difference in gain between a Plexi and a 2203. The 2203 has much more gain, I haven't worked out how much, maybe another day. The poster does not have the understanding to give the advice he thinks he is giving. 



ampmadscientist said:


> When you take out/ bypass the grid stopper, the amp does become slightly more sensitive, but
> the voltage on V1A grid (is now) not what V1A would prefer...
> and V1A will be making some farty sounding overloaded/distortion.
> See, for "ideal" operation
> the vacuum tube wants to see a certain grid DC voltage, and when you take the resistor out, that ideal voltage is no longer present.
> 
> You could probably see this grid voltage with a scope or vacuum tube volt meter.
> A regular DMM will probably not be able to see this DC voltage. However, it's definitely there.



The grid of the stage is biased by the cathode resistor. The grid stoppers have nothing to do with the bias. You can have no grid stoppers and the triode will be biased the same just as it would be with them there. The grid stoppers are there for one thing on the first stage, to create a low pass filter with the grid capacitance to reduce radio frequency interference. In following stages they are also used to limit highs and smooth out overload characteristics and blocking distortion but that is not what we are talking about here. Good old Leo Fender had a great idea using the 68k resistors and switched input jacks to give you a regular and reduced gain hole to plug into. But the poster has it all wrong. And yes you can measure the bias voltage on the grid with a normal DMM. They used VOM's before there were DMM's (yes I am old enough to remember a time when TV's ran on tubes) and tech's measured the voltage with a needle, hopefully with a mirrored display. End of rant.


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