Firestorm wrote:As long as we're hijacking I am increasingly fond of a parallel SE with two EL34s into about 2K5. Preamp can have tone controls or not and as long as you have more than two triodes in the pre you can have as much preamp distortion as you want. Kind of Plexi sounding, but only 25W theoretical max. Just a thought.
Firestorm,
What kind of preamp are you running with your parallel SE setup and what plate voltages on the power amp? Cathode or fixed bias? Sounds cool...
Straight common cathode first stage, DC coupled cathode follower into the tone stack, active EFX loop off the follower with a virtual earth mixer set so the dry signal has gain of about 3 or 4, then a parallel triode final gain stage, bypassed. Overall, the circuit has more gain than a Vibrochamp (so the tone controls don't emasculate it) and the paralleled final stage has enough juice to drive two output tubes instead of one. Lifting the tone stack made it so loud, I simultaneously switched out the bypass cap on V1 to tame it.
Last edited by Firestorm on Wed Oct 07, 2015 9:18 pm, edited 1 time in total.
Oh. Separate 500Rs with 25uF for cathode bias. Plate V is about 385 with 2 EL34s. I built this to run 6V6, 6L6 or EL34 in any combination so plate voltage varies a bit depending on what you stick in.
Firestorm wrote:Straight common cathode first stage, DC coupled cathode follower into the tone stack, active EFX loop off the follower with a virtual earth mixer set so the dry signal us has gain of about 3 or 4, then a parallel triode final gain stage, bypassed. Overall, the circuit has more gain than a Vibrochamp (so the tone controls don't emasculate it) and the paralleled final stage has enough juice to drive two output tubes instead of one. Lifting the tone stack made it so loud, I simultaneously switched out the bypass cap on V1 to tame it.
Very cool! Thanks for posting the details. Sounds like an interesting circuit. I have been revisiting SE (on paper) and considering a prototype build so that's why I was interested when you said you have increasingly fond on that design of late.
martin manning wrote:This sounds interesting Firestorm, especially the loop with virtual earth mixer at the return. Any chance you could share a schematic?
It's a commercial product. I could draw something to share privately.
Actually, I never drew it, sort of just cobbled it together from the schematic in my head. I may have prototype photos, but there are a lot of under the board connections. Basically, the loop is cathode send and return with level pots, paralleled with the dry signal . The return triode has feedback from plate to grid and if the dry side has the same resistance as the plate R feedback, it's unity gain. I made it more like 270K/1M to have some dry gain.
This isn't making any sense is it? I'll try to draw something.
Thanks, I'm sort-of getting it. If you could sketch something I'd appreciate it (sub rosa if that suits you better). It's really only the loop that I'm interested in.
Not wanting to hijack the thread, but what project would you suggest for a chassis and transformer set that has the following specs:
4x6L6
4x12ax7
Was thinking making a two channel amp where one side is a clean Dumble style build and the other would be a Wreck style amp, but I feel like I'm wasting the extra triode stage on the Wreck side. Any ideas?
I am increasingly fond of a parallel SE with two EL34s into about 2K5. Preamp can have tone controls or not and as long as you have more than two triodes in the pre you can have as much preamp distortion as you want. Kind of Plexi sounding, but only 25W theoretical max. Just a thought.
