FET simulations of tube circuits

[Ed G.]

Let's get REALLY unique and design an all tube circuit that simulates a transistor!!! :idea: ... :lol:

Too late, Peavey did that already with the VTM60!!



RDV

Was that the one with the DIP switches? I don't remember too much about it other than I thought it was dark and muffled sounding.

[Phorhas]

Between the 2N3819 and J201 when biased in this manner, I prefer the J201. Neither are clean, but the tone of the J201 sounds better to my ears. However, the character of the distortion isn't that impressive.

Could someone else please breadboard it and post their findings? I'd rather not be providing the only findings for this circuit!

being unclean is due to unsufficiant head room I think. did you tested and ear tested at 12V or 18V?

[RDV]

Was that the one with the DIP switches? I don't remember too much about it other than I thought it was dark and muffled sounding.

Yes it was, and was it awful! It looked great, but sounded awful, and came with Sylvania 6L6 tubes! I used it along with a Classic 50 head 4 a stereo rig. I later traded it all in on my 1st Marshall.

RDV

[Phorhas]

Was that the one with the DIP switches? I don't remember too much about it other than I thought it was dark and muffled sounding.

Dark and muffled... I think that in Mojo it translates to Deep and Fat

[electrictabs]

Well I breadboarded it too with a 2N3819 and had almost the same results with B Tremblay
The sound was somehow misbiased and distortion was there…
I used 11VDC power supply to power the circuit.I found the best ear pleasing result when drain voltage was at the range of 3.5-4.5V while the voltage at source was near 1.9V.
Distortion was there again but sounded a bit smoother.

[michael_ibrahim]

I haven't listened to the circuit, but I would suggest that a better way to judge it would be to build a whole circuit using this technique, and then to compare the results to the expected sound. A typical guitar preamp is after all, multi stage, and the final effect.

Something like this for a Dual Recto:



note: In this schematic, R60, R58, R49, and R35 are meant to be switches for toggling between the low gain and high gain sounds

Also note that the first stage has been replaced with a simple buffer, and the frequency shaping of that first stage is now done with R59 and C27 (and R60). Also, there's no tone stack on this, but you can grab that from the schematics.

The sim says that this has about 10db too little gain, so perhaps putting a low gain stage out the front instead of a buffer might work better in terms of gain.

[GFR]

Instead of concentrating on the frequency response, try matching the transfer function. If you're using CircuitMaker (it seems from the look of the plots) it's the "DC Analysis". It will give you an idea of the non-linear behaviour of the circuit, like the range where it's linear ("clean"), when it starts compressing, and when it's fully clipping, if it's symmetric or not, etc.

Another approach would be feeding the circuits with a triangle or ramp source of adequate amplitude and match the transient response.

The frequency response analysis in ALL simulators is done with a linear model of the circuit calculated at the operating point. So it only tells you, that with a given bias, if the signal is small enough that you can consider your  circuit perfectly linear, the frequency response is that. That's almost useless if you're trying to emulate "tube tone", specially for guitar processing.

Take a look at the VOX AC-30 simulator page, it's a great resource.

[ragtime8922]

Let's get REALLY unique and design an all tube circuit that simulates a transistor!!! :idea: ... :lol:

Too late, Peavey did that already with the VTM60!!



RDV

:lol:  :lol:  :lol:  I've never heard of that thing...lol.

[mojotron]

Something like this for a Dual Recto:

I would sugest a tube driver circuit. It's one triode with some buffering on the input.. With an ideal tube (GT 12AX7M)    those boxes get pretty raunchy at low settings - nothing clean there. But, with a defective Ei 12AX7 (which is the norm for Ei  :wink: ) - the tube driver will have a much cleaner sound...

Maybe it's the models of either the tube or fet? I'll sim this in SuperSpice and see what I get.