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)
I've had this amp with skunkie mods for many months now and I've been experimenting with many speakers, basically chasing the hifi tube dragon. I think I finally found the magic elixir. Yesterday I stumbled upon an affordable pair of 1980 Klipsch Heresy HIPs (industrial ported). An amazing pairing. This amp absolutely sings when you match it with something with some real sensitivity. 1/4 volume is plenty for my space with these 99db sensitive speakers and they sound amazing. I was happy with almost everything I threw at them previously but felt the amp had unmet potential until now.
Would these in the project photo be 22uf 400v Orange Drop cap?
Just opened my A12 up to start working. Have your photo and the one of the first things I noticed is it looks like the two caps from the pre-amp sockets to the driver sockets are orange caps. Not the Chinese 224K400. Do you change these as well? Maybe the one on the on off switch as well.
I am afraid I must have missed a video, or do not understand what I thought I did, but how did the problem of increasing the cathode current by reducing the cathode resistor not end up overheating the PT? Great series and great channel in general. Thanks for all your efforts.
It will raise the B+ relative to the rectifier it replaces. A 5AR4 doesn't drop the voltage a lot, a 5U4G does. I would NOT use this with the 6V6 tubes and in fact I would stay with the china rectifier. In a stock amp or a moded one with EL34 tubes, it's fine.
I do want to stay with tube rectification. However, it would be fun to swap in a solid-state rectifier. They have drop in solid-state rectifiers that fit the tube socket. They say it raises the B+ by 10 to 20%. Is this safe to try?
Quite a roller coaster ride… I love roller coaster rides! Very nicely done. Both your work and your videos are outstanding.
Well happy ending, with a few mild mods, this amp turned out quite nice :)
I believe so, or maybe 115V? At 121V (my house) the heaters are at 6.5V, which I'm OK with. 110V PT end up with the heaters closer to 7V, which is a tube eater.
Is the power transformer 120v?
Ugh, if so, the lack of a magnetic gap would do exactly that.