What is wrong here? Newly built amp makes a weird noise!

[greekie]

Hi guys,

I've been building amps for a few years now, and I've been able to solve most of my own problems by my own up till now.

I've built a couple of Dumble-style amps and they've all worked great. But now an ODS-clone that I just built is giving me problems.

There's a weird, fuzzy, sputtering, static noise in the amp. I can play the amp loud enough so it goes away, and I can pull the first two tubes and still hear it loudly, so it must be somewhere in the power supply or the phase inverter I recon.

I've tried swapping tubes, resoldering most connections, adding extra filtering, snubbers over the AC outlet, diodes, etc. to no avail. Pulling the PI so there's nothing but power tubes left kills most of the noise, if not all of it. It might already have driven me crazy, but I think I can still hear it. This leads me to think it's not an oscillation in the PI.

Here's a video of what it sounds like:

If I measure the AC on top of the B+, at the first node I get around 2VAC / 470VDC, and after the choke it goes (wildly!) from 80-350mV - up and down, up and down. My scope can't track it, and the noise is sometimes changing, like a resistor is cooking or something.

Any help is greatly appreciated, and I've got about an hour left to work on it today, so I'll update with my findings.

Thanks guys!

[soundguruman]

arching between parts or a cracked resistor?

[tedmich]

PFB?

a schematic would help GREATLY!

[Gtr_tech]

Sounds like its oscillating to me. Check lead dress, keep plate wires away from grid wires. Could be caused by bad grounding as well, so check that out. Try to keep a single return path, keep input jacks off the chassis.

[Chuck H]

If it's oscillating the amp will be drawing current above idle. You should be able to measure and make a determination real quick.