Add Gain Stage in Vacuum Tube Guitar Amp - N-Mosfet Source Follower

Thread Starter

tmknight

Joined Aug 29, 2020
6
The idea:

To "re-purpose" V2 in that V2b (cathode follower) becomes the "added" gain stage between V1b and V2a. This is achieved via N-Mosfet source follower replacing the cathode follower.

In doing some reading, thinking and simulating (not yet tinkered), I have arrived at what I think might work...at least in simulation.

A second set of eyes would be appreciated to poke holes in the concept and design. Schematic of pre-amp to tone-stack attached (original and modified).

I think the design "visually works better" when the output from the source is either direct to source or after the first source resistor (R33) - the waveform more closely replicates the wave of a 12AX7 cathode follower. Placing it at the "traditional" resistor to ground (R13) significantly clips the negative side of the signal.

I'm for sure not certain if I'm biasing the transistor correctly. I ultimately tried to replicate downstream voltages that are produced with a cathode follower by adjusting R33 accordingly...as I'm not certain what would be the correct approach.

Thanks in advance for your insight...
 

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Alec_t

Joined Sep 17, 2013
12,732
Welcome to AAC!
Please post your asc file so that we can compare the circuits.
I'm not clear why you're adding the MOSFET stage. A source follower adds no voltage gain. If you want more gain, can't you just reconfigure V2b (original circuit) as a gain stage rather than a cathode follower?
 

Ian0

Joined Aug 7, 2020
4,824
Have you lost a coupling capacitor between V2B and the tone control? If not, why does V2B have two resistors in parallel between cathode and ground?
V2B's grid is at a higher voltage than its cathode, so grid current will be flowing.
You should be able to replace V2B directly with the MOSFET without any other changes (keep the zener between gate and source). All that should happen is that the voltage at the cathode should be higher than the voltage at the grid, but when the MOSFET is in circuit the voltage at the gate should be higher than the voltage at the source.
If you really want to replicate the valve circuit, use a depletion mode MOSFET (but don't hope to buy one any time soon!
I'd recommend Merlin Blencowe's book, he has quite a few Valve/MOSFET hybrid circuits.
 

Thread Starter

tmknight

Joined Aug 29, 2020
6
Thanks to both. The thinking is that making V2B (currently the CF) the additional gain stage and leverage a fet as follower would be the same as if I inserted a valve triode...without having to do any drilling - just some "simple" re-wiring. The spice sim comparing the two (all valve vs fet follower) shows gain in both. Thinking the valve triode earlier would provide the desired tonal characteristics vs the fet in that position. ASCs attached.

@Ian0, my original design was to insert an LND150 (large Rds resistance aside, easy to come by) between V1a and V2a. Though, after my research, deducted the fet SF would be more to taste...though really don't know...it's all in sim at this time. Attached are two more schematics I've considered (one being the all valve equivalent). No, caps were eliminated - not sure where you're seeing two resistors in parallel - do you mean the 470K voltage divider on the grid and the cathode resistor?

In all, inspiration largely comes from these sources:
http://milas.spb.ru/~kmg/irf_en.html
https://sound-au.com/project167.htm
http://www.geofex.com/Article_Folders/mosfet_folly/mosfetfolly.htm

Thanks again for your input...
 

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Audioguru again

Joined Oct 21, 2019
4,523
I sold my tubes (valves?) amplifier 57 years ago and replaced it with a solid state amplifier.

Does the old amplifier need more gain because its tubes need replacing again and again?
Increasing its gain will not increase its maximum output power if the tubes are still good but it will produce more distortion.
 

Thread Starter

tmknight

Joined Aug 29, 2020
6
I'm tweaking a JCM800 clone (Marshall mid-gain style) to increase gain/distortion/saturation at lower volumes. Yes, there are other ways to achieve a similar result...I wish to use this as an opportunity to learn about tube/valve amp circuits and modification possibilities, not to solve an issue.

Cheers
 

Audioguru again

Joined Oct 21, 2019
4,523
Sorry, I forgot that electric guitar players want the extreme clipping distortion even when the volume is turned down.
Sometimes they use a volume control on the output of the amplifier too doo dat.
 

Alec_t

Joined Sep 17, 2013
12,732
Simulation of the original circuit shows the output is clipping severely with only a 1mV amplitude signal at the amp input. Surely the amp doesn't need any more gain?
 

Audioguru again

Joined Oct 21, 2019
4,523
They like a high gain because they want massive clipping distortion all the time. Only at full blast.
Also, when they strum, feedback from the speaker to the strummed string causes sustain.
 

Thread Starter

tmknight

Joined Aug 29, 2020
6
Maybe I'm posting in the wrong forum - I had hoped to gain insight into the design from a technical perspective and not subjective. Thanks, all, we can close this one as "solved".
 

Ian0

Joined Aug 7, 2020
4,824
No it isn't - it's the DC load of the cathode follower. It takes no part in the tone control as one side connects to the cathode and the other side connects to ground, putting it in parallel with R13
 

Ian0

Joined Aug 7, 2020
4,824
In the attachment, the lower end of the 33k resistor is connected to two 22nF capacitors.
In your circuit, the lower end of the 33k resistor connects to two 22nF capacitors AND GROUND.
 
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