What's wrong here? New ODS has strange 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:

http://www.youtube.com/watch?v=2j_XjX--7Hk

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!

[LeftyStrat]

I'd check the choke, or more likely the filter cap after the choke. Check it for DC leakage as well as value.

[greekie]

I tried replacing the choke with a resistor, to no avail. The filter caps are good. Checked them before soldering them in, and I tried subbing in new ones with no luck.

I believe I might have found a work around. It believe it to be a small amount of oscillation. So I will install snubbers, and change the mastervolume so it grounds the input of the PI 100% (there's a 220k+250pF on there now). That way it won't affect the playability of the amp much.

Any suggestions are still welcome, as I bet I will run into this again some time!

[Structo]

I'm not really hearing anything on that video.

[Guitarman18]

I had something similar when one of my power tubes went bad, however if you are sure it is not your tubes, what kind of jacks are you using for the Pre out/Power amp in points. If they are Switchcraft, have a real good look at them! (and clean the leaves). I had an issue with my FET input that I just could not get to the bottom of, but finally after having rebuilt my FET board a number of times, without any improvement, changing the input jack did the trick.

Good luck.

Cheers,

Paul.