JFET Preamp, how would one convert to MOSFET, if possible

[bushidov]

Hi Guys,

Below is a working schematic of a Preamp that came from a Tube Amp schematic that was converted over to JFETs.


As JFETs, especially J201 and alternatives are becoming more and more rare, I was wondering if there is a way to use MOSFETs instead? If so, what would the process be? Q3 appears to just be something akin to a buffer (no amplification), so I know how to convert that to a MOSFET buffer, but what about Q1 and Q2, which are J201 amplifiers?

I know people say JFETs "sound closer to tubes" than other solid-state parts (BJTs and Op-Amps specifically), but what about MOSFETs?

Thanks for all your info!

[merlinb]

You would need to bias the gates of Q1 and Q2s positive, so you'd need to add blocking caps and potential divider from +9V to the gates. Eyeballing it, I would start by biasing the gates up to 5V (ish), then add variable source resistors instead of the variable drain resistors. e.g. use 22k fixed drain resistors and add 10k trimmers to the sources.

[R.G.]

As it happens, there is an entire article on geofex on how to bias MOSFETs, hidden under a title about building MOSFET boosters. See:

http://www.geofex.com/article_folders/mosboost/mosboost.htm

Merlin's advice is correct. The difference between JFET and MOSFET biasing is that JFETs have to have their gate (in general, for N-channel devices) at a lower voltage than the source, while MOSFETs have to have their gate higher than the source. You might like to read the article.

You're facing several problems. One is that MOSFETs do no necessarily sound like JFETs and JFETs do not sound like tubes, and none of them sound like bipolars, necessarily. The whole wave of "replace a tube with a JFET" is at least mildly misguided. It's different from bipolars AND tubes, but possibly interesting in its own right.
Second, the J201 is a non-typical JFET. It's another layer of strange on top of JFETs being strange.
Third, MOSFETs present a challenge because of their much higher transconductance than JFETs. Transconductance is the change in channel current per volt of change on the gate-source. MOSFETs inherently have a much higher transconductance than JFETs in general
Couple that with a high drain resistor and you get high voltage gain - much higher than JFETs. You can make the voltage drain smaller by using a smaller drain resistor, but that in general makes the current through the device bigger. You can reduce the effective transconductance by putting in an unbypasses source resistor, but that also linearizes the device, presumably reducing any soft-distortoin tube-y-ness that you were after.

MOSFET amplifier or tube-amp-inspired circuits may have a good sound on their own, but it is by no means clear that it will necessarily sound like the tube amp it was inspired by. Just setting expectations here.

[bushidov]

Alrighty, so merlinb

You would need to bias the gates of Q1 and Q2s positive, so you'd need to add blocking caps and potential divider from +9V to the gates. Eyeballing it, I would start by biasing the gates up to 5V (ish), then add variable source resistors instead of the variable drain resistors. e.g. use 22k fixed drain resistors and add 10k trimmers to the sources.

So, like this?

You're facing several problems. One is that MOSFETs do no necessarily sound like JFETs and JFETs do not sound like tubes, and none of them sound like bipolars, necessarily. The whole wave of "replace a tube with a JFET" is at least mildly misguided. It's different from bipolars AND tubes, but possibly interesting in its own right.

I kind of figured as much, but I felt the urge to ask anyways.

MOSFET amplifier or tube-amp-inspired circuits may have a good sound on their own, but it is by no means clear that it will necessarily sound like the tube amp it was inspired by. Just setting expectations here.

Sure, I get that. I'd just be curious if the above style schematic would even "work". I already know it won't sound the same. The preamp is the default normal preamp on a Sunn T inspired preamp. Just seeing if a "Doom" styled preamp could be done, but with MOSFETs instead of all JFETs.

I left the Q3 a J201, but will change it after I see what you folks meant about Q1 and Q2.

[PRR]

So, like this?

Vbias is not likely to be 10/16 of B+, is not likely to be the same for two MOSFETs, is pretty sure to need changing as the source gain resistor is turned.

JUST to get you breadboarded, many-many MOSFETS in small-signal work will bias-up with reasonable Drain (and Source) resistors, and a 10Meg from Drain to Gate. Try it.

[nocentelli]

So, like this?

That's not too far away from the Catalinbread RAH and WIIO pedals, which are supposed to be emulations of tube amps using mosfets. Zvex "amp in a box" pedals like the Box of Rock, '59 sound etc use similar cascaded mosfet stages albeit with a the vbias provided by a voltage divider directly on the gate (see the SHO schematic)