# marshall mod info needed using cathoder follower as a gain stage



## collector (Feb 12, 2017)

hello all , loking for some help via diagram of a gain mod for a plexi circut using the cathode follower as a gain stage with a plate driven tone stack . there is info on it online all over but no one got the wiring schematic for it , was told that bogner has something in there line that uses this setup, i know doug aldrich has his amp modded this way , alot of people just add another tube and use 1/2 its triode to add the stage , but wanted to do this this way as i hear its quite creamy to the ear , 4 stages of gain


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## collector (Feb 12, 2017)

btw for your info the amp is a 50 watt plexi clone 4 holer with el34s , 480v on the plates cascaded input , ppimv master volume , split cathode on v1


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## Jim Jones (Sep 18, 2006)

If you're set on modding the amp that's up to you but with the tinkering I've done over the years I've come to the conclusion that the classic "Plexi" circuit is about as perfect as it's gonna get. If you want more jam there are a million different stompboxes you can goose the front end with. A good chunk of the classic Marshall charm is that clean-but-not-totally-clean sound and I'm pretty sure you'll lose that with an extra 12AX7 stage.

Again, basically "old man yelling at cloud" unsolicited advice.


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## collector (Feb 12, 2017)

No concerned about the marshall tone as such , but trying new ideas on gain. My amp is cascaded , with a split cathode , , wanted more smoother controlled gain , increased the fizz cap to 100pf, switched the 470 pf on bright cap, have 3 way neg feedback taps 16 ohm/25k—8 ohm/47k—-100k/4 ohm , Also have a 4 position snubber across v1 plates 22pf/47pf/100pf/200pf that works very as do the neg feedback switching . I have a hopkins modded jcm800 with 4 stages of gain with jose modded diode clipping in a 3 pos switch Low diode clipping/ high diode clipping/ 4 stages of gain using the 3 existing tubes and plate driven tone stack , his mods are concealed pretty good , cannot seem to wrap my head around what he has done . Lol


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## Jim Jones (Sep 18, 2006)

Alrighty then, you're more of a gain guy that I am I see.  I've done lots of messing around with the classic Bassman/Plexi circuit but never in the search for that kind of gain. My idea of lots is getting the voltages on the first tube down between 160-180!


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## collector (Feb 12, 2017)

Thnx for the tip Cheers


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## collector (Feb 12, 2017)

Havent seen a handwired layout of thiis mod anywhere online , i know its possible amd has been used by splawn/ bogner/ . Its a handwired jtm45 that i Want to use for this mod , I have no problem to convert ithe preamp stage to a jcm800 style if necessary .The tone from this is killer 80’s rock plugged straight in ,If anyone can draw up a handwired layout for the basic .


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## collector (Feb 12, 2017)

Anyone wired up a marshall jtm45 this way . Would like some insight on this if possible


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## dtsaudio (Apr 15, 2009)

Since you're dealing with a hard wired amp, this may be possible. I can't guarantee results but a simulation shows it should work. Gain will be through the roof though. 
FYI - I've not done this, just simulated the circuit.
You will need a coupling capacitor, and three resistors. You also need to make sure the tone stack capacitors can handle the voltage. If they aren't at least 400V caps you may need to replace them as well.
I don't know what your skill level is, so if I ask obvious questions, please don't be insulted. Can you follow a schematic? 
Also are you familiar with safety aspects of this? Voltages are lethal in this amp.


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## jb welder (Sep 14, 2010)

dtsaudio said:


> Since you're dealing with a hard wired amp, this may be possible. I can't guarantee results but a simulation shows it should work. Gain will be through the roof though.
> FYI - I've not done this, just simulated the circuit.


Dan, just wondering if you simulated driving a tone stack? My understanding is that the cathode follower is used because there is such a great loss of signal when the stack is plate driven. So I would think while the distortion character may change, there should not be much gain increase to be had.
The marshall circuits that have cathode follower driving the stack do not need the extra recovery triode that is seen after the stack like in circuits such as the 1966. So the marshall circuits that plate drive the stack need the extra triode after the stack to provide the signal increase necessary to drive the PI stage with enough signal to give full output.


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## dtsaudio (Apr 15, 2009)

And you would be correct up to a point. The cathode follower versions drive the tone stack much more linearly but there is still a 20dB loss because of the stack. The gain in Marshall amps is very high to compensate before the stack and less gain is needed after it. What suffers the most is frequency response. The amp will become considerably more mid-range oriented in sound quality, and distortion rises both from the added stage, and from trying to drive the stack. The loading caused by the stack is actually not that high because of the 1M pots used.
I've posted the schematic of the sim. A couple of notes. Ignore the output stage. I took that from another sim to save time, and is irrelevant here. Also you will notice the pots are made up two resistors as pots are impossible to do in simulation software.
It's basically the JTM45 modified.


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## collector (Feb 12, 2017)

Yes i can follow a handwired type layout , like ceriatones’s . Willing to give it a go on what you have in mind 
, love the learning part , , lately on this amp I have tried various cathode cap/ res combo’s with increases in v1a plate res .


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## collector (Feb 12, 2017)

BTW The attached file has nothing in it


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## dtsaudio (Apr 15, 2009)

Be careful there. 12AX7 tubes don't like a lot of current. Marshall already runs that a bit hot. They also needlessly use a rather large bypass cap on the V1 cathode.


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## collector (Feb 12, 2017)

Hey dan , hope all is well with you . Yes i have it cascaded , split on v1a/b 4k5/5uf on v1a And 2k7 /.68uf on v1b , nos pre tubes Phillips 12ax7 in v1/ v2/v3 has nos electrohome organ grade , sounds pretty good in the hvy gain , 27k nfb , but the sound is not as smooth as a 4 gain stage


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## dtsaudio (Apr 15, 2009)

File should be a pdf. Works fine. Maybe someone else can try. You might hzve to save it.


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## collector (Feb 12, 2017)

Jim Jones said:


> Alrighty then, you're more of a gain guy that I am I see.  I've done lots of messing around with the classic Bassman/Plexi circuit but never in the search for that kind of gain. My idea of lots is getting the voltages on the first tube down between 160-180! [/QUOTE





dtsaudio said:


> And you would be correct up to a point. The cathode follower versions drive the tone stack much more linearly but there is still a 20dB loss because of the stack. The gain in Marshall amps is very high to compensate before the stack and less gain is needed after it. What suffers the most is frequency response. The amp will become considerably more mid-range oriented in sound quality, and distortion rises both from the added stage, and from trying to drive the stack. The loading caused by the stack is actually not that high because of the 1M pots used.
> I've posted the schematic of the sim. A couple of notes. Ignore the output stage. I took that from another sim to save time, and is irrelevant here. Also you will notice the pots are made up two resistors as pots are impossible to do in simulation software.
> It's basically the JTM45 modified.





dtsaudio said:


> And you would be correct up to a point. The cathode follower versions drive the tone stack much more linearly but there is still a 20dB loss because of the stack. The gain in Marshall amps is very high to compensate before the stack and less gain is needed after it. What suffers the most is frequency response. The amp will become considerably more mid-range oriented in sound quality, and distortion rises both from the added stage, and from trying to drive the stack. The loading caused by the stack is actually not that high because of the 1M pots used.
> I've posted the schematic of the sim. A couple of notes. Ignore the output stage. I took that from another sim to save time, and is irrelevant here. Also you will notice the pots are made up two resistors as pots are impossible to do in simulation software.
> It's basically the JTM45 modified.


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## collector (Feb 12, 2017)

Had a look at your schematic , from what i have i understood from guys on the internet , there are 4 stages of gain with 3-12ax7’s ,


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## collector (Feb 12, 2017)

View attachment 273772

this schematic i found is a splawn nitro, close to what Im after In a 50 watt version with kt88’s . My amp has a lamarr II ppimv , that i still want use in this mod if possible , my b+ is 480-500v


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## collector (Feb 12, 2017)

Looking for a hand wired layout for a 4 gain stage hot rodded jtm 45/50 using cathode follower as a gain stage , looked all over the internet for a drawing ,. I know splawn has used this in its nitro build. The amp i have is a ceriatone type 45 with a solidstate rectifier 485 volts b+, running kt88’s . I have done a 1 wire mod on it using multible cathode/ res combos , feedback combos , tubes types, variable plate res values but can’ t seem to get it to sound smooth at high output levels , , I tried using diff fizz cap values, in phase inverter and v1a and v1b , and even across v1a and v1b (5pf-20pf) Takes away the sizzle I find when snubbers are used , Also it has a type 2 ppimv Mod on it as well , would entertain changing it to a pre mv if needed .I have very high gain when cascaded Mode . squealing and oscillation is not too bad of and issue as I have done shielded wiring on all areas needed . Would like to give the 4 gain stage plate driven tone stack a go . Really like that splawn nitro tone , its thick , with the added sizzle But smooth distortion. Any Info or Schematic on a hand wired layout of this design that I can use as a guide Will be greatly appreciated


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