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Reading up it seems there are a couple of issues with this amp right out of the box. The first issue is the power transformer primary is rated for 110V, which throws off all the internal voltages when used with the 122V my house has, plus makes everything run hot! I setup a 12V bucking transformer to deal with that issue. But also the first cap after the rectifier is WAY too big at 150uf, so replaced it with a 22uf. Then I rolled in some matched pairs of the Russian 6P1P-EB tubes, and it sounds pretty sweet now for such a cheap amp. Seems the early versions had an undersized rectifier tube as well, mine came with a Russian 5Ц4C in place of the Chinese 5Z4, they both have a 2ma filament but this Russian tube has a higher load rating while keeping the B+ the same.
Anyway, I’m interested in trying a mod to replace the output transformers with something a little better, that can also be setup for UL mode. I would probably set it up with a switch for triode/UL and also want to play with a switchable schade feedback loop while I’m modding it to see what that sounds like. I’m listening through some high efficiency Klipsch KB-15 Icon Bookshelf speakers so it does pretty well for a low watt amp. Given the power supply had some design flaws, I’m not sure the supplied output transformers are an ideal Z anyway + they aren’t marked what they even are.
I’ve attached the original schematic (with issues noted) along with the one with my proposed mods. My question is what impedance transformers will I need? I’ve read that parallel tube designs like 1/2 the normal rated impedance for SE use. I’ve also read this tube, and it’s US electrical equiv the 6AQ5, like 5K in an SE amp, so does a 2.5K transformer sound right for this use? I’m also open to suggestions for the resistance for the shade loop, most seem to be 180k-220k so I picked something in the middle.
I don’t need blasting loud sound, but I don’t want to kill the output either. It sounds really sweet with jazzy type music setup in the factory triode mode, but my hope is rock type music will sound punchier in UL mode. I’m not running any preamp or tone controls, just straight out of a Yamaha S300BL CD deck.
I am thinking about a pair of Edcor GXSE10-2.5k at 8ohm as they physically will fit without hanging off the chassis, 10W each should be plenty (I get bigger is better, but this is a sub $300 amp lol), and they have a 40% UL tap to play with. A pair would be less than $80 + shipping.
Again, new to working on tube amps so let me know if I’m off on my thinking here. I’m seriously hooked on how great tube amps sound and have a SEUL KT88 build planned for the future.