Fretting about FETs

[Blackboarcult]

Sure, temol

At drain of last gain stage:


After hi cut control:



Nothing too outrageous, kinda standard-ish JCM but with more gain. Trying to ape 90's King Diamond type of tone, a bit more modern than 90's standard HM, but not quite modern sounding by today's standards, not scooped nor brutal, just gnarly. As I said, not finished yet but so far it sounds quite nice

 

[rossbalch]

I did some sims just to compare side to side, here's some observations.

We have those three circuits, left to right:
- First one is a fairly standard jfet gain stage, with "cathode follower".
- Second one is the same, but with a follower nicked from AMT L2 pedals (those that have it, e.g. S2, D2, M2...)
- Third one has a bootstrapped follower.
All are biased equally, and the follower has a 26k load (akin to the AMT tonestack with bass and treble at zero, mids fully open).
View attachment 98183

Taking the signal at gate, it is the same for all three circuits, since they are biased the same, no resistor on source and 11.2k on drain, featuring positive lobe clipping.

View attachment 98184

Standard circuit: gain stage inverts signal, positive lobe bangs against the positive rail on drain, and negative does not quite reach the negative rail, merely amplifying the previous gate clipping. At the output of the follower, we have clipping on the negative side due to the 26k load + a fairly high value on the follower source (47k).

View attachment 98185

AMT circuit: same as above, except that the AMT diode-to-source contraption compresses and prevents the positive side of the signal from reaching the rail, clipping 2v before it. Not knowledgeable enough to explain the theory behind it, but it does a fairly good emulation of what happens on the last stage of, say, a JCM800.

View attachment 98186

Bootstrapped circuit: we have more gain due to the bootstrapping (see the Merlin Blencowe chapter on it), and negative side clipping much earlier than the others, that "pushes" the negative signal (effectively soft clipping). The follower output looks quite smoothly compressed, and overall different than what you would see on the above circuits or a standard valve stage + cathode follower.

View attachment 98187

If we reduce the source resistor, or increase the load, the negative clipping on the follower will decrease (cleaner signal). If we replace the load with a tonestack, the knob positions will affect the amount of clipping.

As for circuits, this one here is not where I would like it yet, but getting there. Kinda hot rodded marshall tones. LND150 works well as an input stage, since it won't ever feature blocking distortion even when slamming it at capacity, and works nice as a last stage too, clipping a bit softer than jfets and without gate cutoff distortion. Can be biased to clip quite symmetrically, and does a reasonable "power stage" clipping thing.

View attachment 98188

Hope this helps

Thanks so much for this and going to all the effort of creating the sims. Will definitely tinker with some of this!

 

[temol]

Nothing too outrageous, kinda standard-ish JCM but with more gain. Trying to ape 90's King Diamond type of tone, a bit more modern than 90's standard HM, but not quite modern sounding by today's standards, not scooped nor brutal, just gnarly. As I said, not finished yet but so far it sounds quite nice

The first iteration on the breadboard turned out to be unstable - oscillations appeared with gain past noon. I grabbed another breadboard and the whistles disappeared. 90dB of gain is not that little, so the layout is quite important.

Small sample here. Guitar -> breadboard -> audio interface. 4x12 V30 Mesa Boogie IR. Two parts - 18V and 12V. 3 gain posistions in each part: 9 - 12 - 15 o'clock.

 

[Blackboarcult]

The first iteration on the breadboard turned out to be unstable - oscillations appeared from with gain past noon. I grabbed another breadboard and the whistles disappeared. 90dB of gain is not that little, so the layout is quite important.

Small sample here. Guitar -> breadboard -> audio interface. 4x12 V30 Mesa Boogie IR. Two parts - 18V and 12V. 3 gain posistions in each part: 9 - 12 - 15 o'clock.

So f*n cool, mate! Nice to hear it played by someone else than me
I also tend to get whining from the breadboard withthis kind of circuits, I usually rejig some of the gain stages further apart til it stops.
the resistor + cap after the Hi cut control is there to counter the treble bump from my test amp (PV Bandit silver stripe) and kinda make it as if it was going into the tonestack in a "flat" fashion.
Also, I almost never play this kind of circuits without a TS in front. The only reason I do not bake it into the circuit is that I like to have the possibility of using different flavours of boost in front.
Any comments or suggestions are welcome,of course