It's DC heaters on the three pre-amp tubes.
The two power tubes and the phase inverter are 6 VAC.
It's a high-gain design, so the idea is to get the AC completely out of the pre-amp tubes to eliminate noise.
Man, I have built and modded a lot of amps and this is the first time I've dealt with oscillation. I didn't even know that's what it was because I had just finished low-power testing with a variac and a light bulb current limiter, so I'm running the amp into a dummy load and tryna bias the fucking power tubes.harddriver wrote: ↑Thu Jun 16, 2022 1:30 amAny ideas on the source of oscillation? Maybe the diodes on the DC heater board?
I'm getting readings off the bias test points that make no sense to me, so I figured I'll reach out for help. I haven't even logged into TheAmpGarage in like 4 years, so I'm thinking, "Maybe it's not the same bunch of angry curmudgeons using KT-88s as butt-plugs."
no such luck.
But they did advise me to hook up the amp to a speaker load and listen for oscillation, and they were right. They were complete assholes about it, but they were right.
harddriver wrote: ↑Thu Jun 16, 2022 1:30 amI am trying to remember what the cause was with somebody's build on Metro had a similar issue and it was like they had two wires reversed causing a polarity phase issue. I just can't remember what it was...
Yeah, and now I know. It's the output transformer leads connecting to pins 3 of the output tubes.
I just woke up from my afternoon nap, watching the sun go down in the valley. Playing with my 6 Great Pyrenees dogs in out in the yard.
I'll pull the board outta this in the morning and go thru everything.
If I need tech help I'll either PM you, or I'll reach out to a couple amp guru guys I have e-mail addresses for.
ffs