ENGL Screamer 50 Preamp valves - figured out!

[tmjohn]

Ah, just another ENGL Screamer related question, whilst I think about it....
Does anyone have a definitive answer to which pre-amp valve relates to which channel setting?
As far as I know;
V1 and V2 are the input gain stages, V3 is the effects loop and V4 the phase invertor.
I think that all guitar signals pass first through V1a (regardless of channel, Clean, Crunch, soft lead, heavy lead).
(The amp is really a 2 channel with a gain boost available on those 2 channels, giving 4 different gain amounts)
The circuit diagram is complex due to all the switches in the signal path, and being a hobbyist and not an electronics expert I can't figure the rest out!
I looks like maybe, all the guitar signal also pass though V2b at the end before the tone stack, and maybe V1b is the soft lead gain and V2a is the hi gain boost, available to both clean (becoming crunch) and soft lead (becoming heavy lead) but I am really no too sure.
Can anyone clarify?

Schematic for the ENGL Screamer E335 can be download here:

http://www.schematicsunlimited.com/e/engl/e335-ii-schematics-new-

[tmjohn]

OK, I didn't get any response but I think I have figured it out now. Happy to be corrected if I'm wrong...

It does appear that regardless of which channel is selected, all guitar signals pass through valve V1a and V2b, and V1b and/or V2a are added as extra gain stages depending on which channel/hi/lo mode is selected.
After the pre-amp gain stages the signal goes through the tone stack, effects loop circuit and then out to Phase inverter (driver).

So...

Clean, (Lo Gain) V1A to V2B to tone stack etc...
Crunch (Clean, Hi Gain) V1A to V2A to V2B - tone stack etc...    (Note. V2a is added into the "clean" channel signal path)
Lead, (Lo Gain) V1A - V1B - V2B - tone stack etc...   (Note: V1B is added to the original "clean" channel signal path)
Heavy lead, (Hi Gain) V1A - V1B - V2A - V2B - tone stack etc...   (Note: V1B and V2a are both added to the original "clean" channel signal path)

Basically, in addition to the always used V1A and V2B, valve triodes,  V1B gain stage switched in and added when the lead channel is selected. V2a gain stage is switched in and added to make the either the clean channel into the crunch channel, or the lead channel into heavy lead channel.

Hope that makes sense, and is of use when considering changing pre-amp valve types.
Unfortunately, the way the circuit is configured (always using both valve 1 and valve 2, (or at least half of each) changing either of these valve will alter the character of all channels. (unless one uses a valve with dissimilar triodes, like a 12dw7. Still this is limited because in the 12dw7 valve the "A" triode is the 100u high gain (12ax7) stage, and the "B" triode is the 20u low gain (12au7) stage.