Pleased to make the acquaintance of, Y'all.
Rather than spread this project over several threads, I thought I would consolidate the discussion, particularly as it is all related, in the end. Here is a link to the amp schematic in question: https://el34world.com/charts/Schematics/files/Peavey/Peavey_transtube_special_212_orig_1995_schematic.pdf
Electronic experience: Electronics and computer tech from 1979 to present. Built tube amps a couple of times. Basic diagnostic skills. Electronic theory, but weak on Op Amps- Which is what bought me here.
Objective: At this stage, I'm believing I'd like to build a stereo version of the power amp section of the Special, for use with my Fractal Audio AX8; An amp modeler. I picked one up in the Pawn shop knowing full well many of the controls on the front were damaged, and needed replacement. It's odd how the Pawn dealer will be reluctant to sell at a reduced price, a piece of broken equipment he was scammed on when pawned for next to nothing. Originally, I was after the speakers, but the power amp section sounded really nice to my ears.
Questions:
Looking at the schematic, I think I see, but I'm not sure how this power amp works. It appears to resemble a tube amp phase inverter circuit, or AB tube output section.
What does VR18a,b change in the circuit to obtain the so called Transtube effect?
This doesn't appear to be very complicated to build. It may even be most cost effective to pick up another for about $100, cannibalize the circuit boards, and solder up a stereo version?
Something odd here; When I cranked this amp up, testing it out, it didn't sound like 130 watts. Perhaps, 40 at best. And for sure, I don't believe this is going to put out 200 watts. I was unable to locate the transistor data sheet, although I didn't spend much time; to determine if the power transistors, combined output, were even in that power rating. C107 on the power amp input could be bad, and I know the Dynamics pot is faulty. Another consideration is to drive the power amp with the output from a mixer. I'm not sure what signal level is needed to drive the power amp to max output.
To get around the faulty Dynamics pot for now, I'm considering simply bridging the two legs of the related 10k pots with 5k resistors, to get a better idea as to the usability of this amp with my Fractal Audio amp emulator. Mid point setting on Dynamics sounded good enough for now.
For now, that about sums up my questions and reasons for joining the forum. I was originally pointed to the site after searching the Special and landing on a, "Post", here that discussed the amp.
Thanks for any assistance in this project. Regards,
Silverfox.
Are you positive the schematic matches your amp? Is there 6 large power transistors mounted on the heatsink?
There are many versions of transtube, some are lower power. The 200W rating is only when second speaker cab. is in use.
The power amp topology is fairly standard, complimentary output style. Using complimentary type outputs (NPN and PNP) means no phase inversion is necessary.
The 2 sides of VR18 are affecting feedback networks.
Quote from: g1 on March 31, 2026, 12:05:31 PMAre you positive the schematic matches your amp? Is there 6 large power transistors mounted on the heatsink?
Yes. There are 6 transistors. Thanks for the response regarding the topology of the output circuit. I'll have to research that.
Best regards,
Silverfox.