getting overdrive without distorting the pre-amp

[jml77]

I think most of the the preamp distortion is generated in V2b. I also don't like to think of the 'clean' volume control as a master volume, because V2b follows it and it has a lot of gain. With both the drive and volume maxed (and boost engaged) I was getting approx. 15 volts RMS at the send jack, enough to freak out anything in the loop and overdrive V2B into a state of lathered fizz/fuzz (which incidently I quite like). And that was using a fender strat 50s reissue as the input, which has quite low output pickups.

(I'm an unusual soul, as I like a proper master volume and have put one in between V2b and the 6v6 and I don't use the watts control, but ignore this as you don't like preamp distortion)

If I'm going into the ultra jack, then I tend to think of the clean volume control as a fx loop level control or the level control for the final preamp stage V2B. In it's stock config the sp6ultra is really a non-master volume amp. If I'm using the ultra input maxed out then I will generally have the clean volume level set at approx 9-10 o'clock when boosted and 2 o'clock when unboosted. Anymore than that and the loop and V2B is heavily overdriven.

Using a 12at7 in V2, might help, but might make the clean channel input too weak.

If you want to minimise the gain of the V2b stage then, as suggested above replace the plate 220K resistor (R25) with a 100K, then one other possibility I would think of is to use a switch to select either cathode cap bypass C22 (for max gain) or unbypassed for less gain. Which could then be selected depending on which input is being used.

(Put a 1M resistor in series with C22, resistor to ground, and use a switch across the resistor, switch closed = resistor shorted, fully bypassed max gain. Switch open = cap effectively eliminated from circuit, unbypassed less gain. Or use a variable resistor instead and forget the switch, ie VR in series with C22 and ground, variable partial bypass. Sorry I can't supply a diagram, but an internet search for 'switched bypass capacitor' should give some useful results)



Good luck, have fun and keep safe.