Sunn Spectrum II preamp with FET's, plz help me, I need an explanation

[JebemMajke]

So here is the schematic http://www.webphix.com/schematic%20heaven/www.schematicheaven.com/newamps/sunn_spectrum_ii.pdf  I understand what to do with those 2 stages before the tone stack, but what about that tube that comes after tone ( 6AN8 )? How to emulate that part, or even better question do i need it?

[diydave]

6an8 is a small signal pentode + a triode in one tube.
Pentodes deliver a higher voltage amplification then triodes.
In the sunn amp the pentode drives a concertina like phase-splitter to get to the push-pull power-amp.
Maybe use a mosfet-circuit to emulate it? And drive a fet?
Which with no doubt will just push the fet into distortion.

[JebemMajke]

So BS170 or similar mosfet. But as u can see it has 4 connections/lugs 8,9,6 and 7. And as you know mosfets have only 3. So how would i wire it/use it in this situation?

I will be using this with bass and i am always looking forward to finding a good bass overdrive or distortion.

[diydave]

Check http://www.runoffgroove.com, eg Matchbox.
You could wire the mosfet like an sho.
The connections of the pendode are kathode (source), plate (drain), grid (gate). But also a screen, which transistors don't have.
Don't know if the following fet will work though. I see the fet's gate would be connected directly to the drain of the mosfet.
Which in turn will need tweaking to bias it.

[JebemMajke]

Hm, how about a 4 terminal mosfet http://www.muzique.com/news/mosfet-body-diodes/

[teemuk]

I imagine several methods could be used to approximate a pentode but where's really the point? A FET pretty much has characteristic curves similar to pentode tubes and the most prominent effect is not actually crated by the drive stages at all, it comes from overdriving the power tubes (which happens way before distortion begins to occur in the pentode gain stage or following phase splitter).

In bare minimum you can approximate the clipping of a push-pull stage with a pair of diodes but if you really want to dwelve into it you can - for example - pair up a FET phase inverter to a pair of asymmetric clipping stages (any device will really suffice), which's outputs are then summed with a differential gain stage (or even a transformer, makes really no difference at low signal levels). You can even wrap the entire thing to a negative feedback loop and it will start to behave pretty much like the tube stage it intents to emulate. Do not forget the effects high output impedance has to damping, which will alter the amp's response to a speaker load... So, in a nutshell, if you really want to emulate it "to a tee" then you need to

a) emulate the effect of two asymmetric distortion stages summing up together, creating symmetric distortion and often DC bias shifts that create crossover distortion during sustained clipping
b) emulate the effects of power supply sag and how they relate to clipping threshold and gain
c) emulate the effects of negative feedback overdriving the drive stages when output begins to clip
d) emulate the non-linear frequency response of the amp has when it drives a reactive speaker load

So we are really talking about a complex power amp emulation and it's really not a "stick a FET to it" -type simple solution and really requires some careful thought and complex circuitry. ...Or you could just symmetrically clip things up with a pair of diodes and call it a day. Not 100% accurate but nails at least the main idea.  

[JebemMajke]

Mhm. OK, I'll just leave it clean than and boost it with alembic f2b ( fet version ofc ) if in need for some dirt. I am not much of a fan of diode clipping, it goes to the treble/middle side more than on bass side. And I want lots of bass in my overdriven sound.

Thanks