homebrew hybrid

[billybillybilly]

hey folks !
heres what i got ;
i want to make a hybrid . i have a solid state preamp that i really really like the sound of . its from a Road 115B (70s all ss bass amp) that i play regular guitar through . i was thinking of using it to preamp a tube PI/poweramp .
ive got an old peavey 'festival duece two' (hybrid) . figured i could remove the ss preamp/PI board and rewire the output tubes . its ment to be 4x6l6 @ 100w into 4 ohms , but for now ill run only 2 (and double the output ohms , 8 ohms instead of 4 ... right?) and use the extra juice for 2 preamp tubes .

as for the tube wiring part , i looked around , and then asked around , then looked around some more . the end result i was looking for was something to compliement the ss preamp (obviously?) . being pointed in the direction of the fender twins , i chose the silverface twin reverb .
i have more solid state expereince than tube , so i want to keep this as simple as possible ... plus i dont want to buy any more parts than i have to . i took the schematic from the twin reverbs and removed almost everything (i know i know ... sacralige/blastfemy , im commetting em all here) . i want it so that it will act as a poweramp , volume being dependent on the level of the input signal , so ill kept the normal channel w/ the volume control as a 'set and forget' trimpot on the inside .

my biggest concerns are with the power supply and bias arraingement . the peavey psu - rectifier/RC filter board , provides +480v/plate and +470v/screen as is . this should be fine for the 6l6s im thinking .
but how should the bias circuit be ? like that of the fender im mimmicing or could it be left peavey style ?

ive attached the sf twin and peavey psu/poweramp schematics i messed with (i can post the original unaltered versions for reference if anyone needs) .

id really appreciate any comments , pointers and/or constructive critisism .

[6267]

As long as the tubes don't glow red on the plates at power up, you're either not conducting, properly biased, or underbiased. Personally I'd just ground one 6L6 grid, cathode bias them both through a shared *BIG* resistor of about 350-470 ohms and put my signal into the other grid, it's called self-split push pull and needs no PI, just the two tubes. This can be repeated for the other two in the quartet and then you'd have a 4x6L6 output that needed about half of the normal drive voltage, i.e., probably closer to what your preamp is putting out. That's just me though, it sounds like you're on the right path, but it'll take some quackery to get all of that stuff working together as "one thing".

[billybillybilly]

hmmm , that 'self split push pull' is more of a hi-fi design isnt it ? i do like it though , nice and simple .

after some more looking , im thinking about a musicman style tube PI . ill just use my preamp instead .

well , thanks for responding !
but now i must be off to my pseudo science ... quack quack quack

[Steve Conner]

Self-split push pull is probably a really bad idea for a high powered amp. It stops working properly as soon as the tubes get outside Class-A.

Do you know how much voltage your Road preamp puts out? You might not need as many tube gain stages as you had in your Fender-based schematic.

[billybillybilly]

Do you know how much voltage your Road preamp puts out? You might not need as many tube gain stages as you had in your Fender-based schematic.

hey Steve !
thanks for the response !

the road preamp runs on 12v (dont know the ac swing at the output though) . if it will help , i can post the schematic .

but , i think im gonna loose 1st tube , leaving only 1 . i like the musicman style tube PI . one gain stage then PI . i was thinking about setting the power tubes (6l6) like that in the fender running about 460v on the plates/screen , and about 340v to the 12ax7 . does anyone see any major problems , eminant explosions , etc arising from this ?

i get a little lost with the math involved in all of this .