I think this amp might kick my butt lol. The main issue is, besides the distortion, it has no low end. Measuring the turns ratio in the OT, it's 50% which suspiciously is what it would be if it was a push pull OT. I'm trying to think about what else could do this but the coupling caps and resistors all seem reasonable and shouldn't be blocking the low frequencies. I'm starting to think these OT are just garbage...
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All I can offer is my findings......... so far.
I'm running a pair of Tang Band horns along with a 15 inch (150w) Polk audio powered sub.
https://www.tb-speaker.com/products/d4-1
Upon set-up my A-12 came with Psvane white box tubes. I did look up the overall thoughts on Psvane tubes and the feeling seemed to be "sterile sounding".
I hooked it up, feeding it with eac (exact audio copy) rips through a commercial Toslink DAC.
We all know what I heard, not too bad highs, very distorted minimum midrange and bass.
In the 45-50 hour range the Psvane EL-34's "woke up". Now I had a very strong (but distorted) midrange and bass, with the highs "rolling off" as they should do with guitar amp tubes.
I installed a pair of run-in JJ 6CA7 tubes, which levelled things out, leaving the usual mid/bass distortion.
Before I start modding this thing I want to try a pair of 6V6 tubes in there, they should be here in the next day, or three. (I have a box of parts ready to go, but am holding off until I hear the 6V6s in there)