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

 

[rossbalch]

Also, I found this design interesting...

Mixing JFETs and MOSFETs. On the thread different circuits actually place the FETs and MOS in different locations in the circuit, though always a MOSFET into a JFET in a coupled "cathode" / source follower arrangement.

Why might you choose MOSFETs in this case? Is it purely texture, or are there also current / buffering considerations too do we think? There seems to be some attempt to simulate Miller capacitance here which is interesting.

Obviously this circuit has a lot more going on than the typical "conversions". But I'd love to understand more of the principles of what's going on here. For instance, the MOSFETs are Vr ground biased, but the JFETs aren't. Q3 appears to be biased in a cold clipper configuration just like V2b on a real Soldano, but I don't know how the match works out on the JFET end versus the AX7 math.

(https://www.guitarristas.info/foros/muestras-sonido-messa-soldano-krank-engl-vht/88517/pagina2)
View attachment 97731

@thomasbe86 do you have any insights into the MOSFET aspects of these circuits? Some people in the forum were also talking about how they found the BS170 less harsh sounding than a 2N7000 in this circuit.

 

[thomasbe86]

@thomasbe86 do you have any insights into the MOSFET aspects of these circuits? Some people in the forum were also talking about how they found the BS170 less harsh sounding than a 2N7000 in this circuit.

I havent tried in this circuit but, swapping a 2N7000 for a BS170 can change the feel because the BS170 has a lower output impedance (~4 Ohm difference) and higher inpout capacitance (about 20pF difference I think), lower output impedance drives the next stage stronger (slightly), and the extra capacitance rolls off "some" high end, both can smooth the top end and make it sound less harsh compared to the faster and "brighter" 2N7000. but honestly, the difference is subtle in most circuit, here you might hear something though. then again, you can adjust the circuit to your taste I would say, instead of changing the transistor. For pricing and availability I always use 2N7000 for SMD and BS170 for through hole, dont really hear a difference in most circuits

Edit: I just saw you are using LND150 in there, these are way different and a completely different biasing would be required to swap in enhancement for depletion. Not a drop in replacement so to say

 

[rossbalch]

I havent tried in this circuit but, swapping a 2N7000 for a BS170 can change the feel because the BS170 has a lower output impedance (~4 Ohm difference) and higher inpout capacitance (about 20pF difference I think), lower output impedance drives the next stage stronger (slightly), and the extra capacitance rolls off "some" high end, both can smooth the top end and make it sound less harsh compared to the faster and "brighter" 2N7000. but honestly, the difference is subtle in most circuit, here you might hear something though. then again, you can adjust the circuit to your taste I would say, instead of changing the transistor. For pricing and availability I always use 2N7000 for SMD and BS170 for through hole, dont really hear a difference in most circuits

Edit: I just saw you are using LND150 in there, these are way different and a completely different biasing would be required to swap in enhancement for depletion. Not a drop in replacement so to say

We're talking about two different circuits, a bit confusing I know. The old Russian one was mixing sk30s and 2N7000, where this modern one is using LND150 instead.