Well, there’s a few silly things happening in the circuit, but tell us what the noise sounds like. Go to YouTube and put in “60Hz sine wave”. If you can tune a guitar by ear you should be able to tell if this is the noise the amp is making. If it’s not 60Hz try the same procedure with 120Hz. If it’s neither and sound like a boat engine or a ambulance siren, let me know.
Yep, I can tune a guitar by ear, but I have no idea what this has to do with my (no solved) problem. I got no 60/120Hz problems because I am in a 50Hz country, but joking aside. There is/was no hum problem, I never mentioned that. It was whirring noise, similar to when parts of the housing vibrate loudly with the strings.
Some obvious bad things with the circuit.
1) no grid stopper on the power tube. Helps prevent oscillation. Value is not critical. 10k. 4k7. 1k. Even 100k. Just put something there.
There is a grid stopper in front of the EL84, 3,5K!
2) no grid leak on power tube. Check data sheet for upper limit on this. If I knew the power tube being used I’d tell ya.
It is clearly described in my first post which power tube is used (12AX7+EL84 inside), just read everything before you post.
There is a grid leak resistor now but that was not the problem.
3) a resistor bypassing the tone stack? I mean, why?
Nonsense, the resistor replaces the tone stack which I completely threw out because it had hardly any effect, sounded bad and stole a lot of level. But I've already written about that in my previous posts. Again, read first, then post.
4) A voltage divider on the B+ before the output transformer? Why? Just use the correct power transformer. Iron ain’t cheap. The tube might be able to handle the higher voltage. Dissipation is what matters more.
What voltage divider? What are you talking about?
5) DC heaters with just a single filtering cap? That might be your culprit right there. On such a low gain build, DC heaters are unnecessary. AC heaters would be fine.
Not my design, but I know where it comes from. This amp is a generic Chinese design that is sold under many brand names. Gretsch, Epiphone, Harley Benton and so on and the first models had the usual AC heater. There were problems with hum, according to internet forums, and the people in charge went with a (rather poorly implemented) DC heater. I wouldn't do it that way, none of my guitar amps have DC heating, I get them quiet without it.
The main problem with these amps is the design of the PCB, where local oscillation occurs due to crosstalk.
The amp sounds really good now and is loud as hell...