ODS Trouble Shooting Advice Requested

[donvan]

I built an ODS a few years ago. I was playing the amp yesterday and all of the sudden there was a loud, obnoxious, constant noise or oscillation. Even with the master volume turned down all the way, it still persisted. Banging the top of the cabinet (not too hard) with my open hand changed the volume of the sound/oscillation. It was a lower frequency sound but I can't say for sure if it was 60/120Hz. I turned the amp off immediately, let it cool down, and then wiggled each tube to make sure it was secure. I turned it back on, the issue was still there, and I turned back off immediately. It seemed like the power tubes were brighter than normal but I can't say for sure since I normally don't really pay attention to their brightness.

I have a signal generator, scope, and DVM. I used to be an electronics tech and I am experienced in trouble shooting low voltage circuits. On this amp, I am confident trouble shooting the power supply, following the signal path, and isolating a problem. What I lack is the experience that many of you have trouble shooting tube amp circuits and I don't want to do anything that will cause more damage so I want to run my ideas by you before I start trouble shooting.

I suspect that this is a power tube issue but that is only a guess. What I would like to do is to remove the power tubes and then see if the power supply and preamp are working correctly. I realize that this will raise the voltages. In your opinion is this a good approach? If it is a good approach, I would then add back one power tube at a time and try to isolate the bad boy. Any advice?

BTW, I don't have a variac transformer.

[martin manning]

The voltage ratings of the components are usually high enough to take unloaded (tubes out) voltage levels, but there are no guarantees. Do you have a lightbulb limiter? A low-wattage bulb would knock the voltage down to a safe level.

[bepone]

pull the PI tube out and verify the oscillation stop

[donvan]

The voltage ratings of the components are usually high enough to take unloaded (tubes out) voltage levels, but there are no guarantees. Do you have a lightbulb limiter? A low-wattage bulb would knock the voltage down to a safe level.

Thanks Martin. This makes me feel better about removing the power tubes. No, I don't have a lightbulb limiter.

[donvan]

pull the PI tube out and verify the oscillation stop

Thanks bepone. This is an interesting suggestion. Do you suspect that the PI tube is causing the oscillation or is this just a good way to isolate the preamp from the power amp?

[WhopperPlate]

pull the PI tube out and verify the oscillation stop

Thanks bepone. This is an interesting suggestion. Do you suspect that the PI tube is causing the oscillation or is this just a good way to isolate the preamp from the power amp?

This is the common troubleshooting process of elimination. The PI tube is most likely suspect if master volume doesn’t effect anything. Pi is downstream.

[martin manning]

Low frequency buzz could be failed/failing rectifier. That will usually take the mains fuse out, but it's something to be aware of.

[donvan]

This is the common troubleshooting process of elimination. The PI tube is most likely suspect if master volume doesn’t effect anything. Pi is downstream.

Got it. Thanks for the response Charlie!

[donvan]

Low frequency buzz could be failed/failing rectifier. That will usually take the mains fuse out, but it's something to be aware of.

Thanks Martin! I will certainly look at that.

[donvan]

Just wanted to follow up. The issue was indeed a bad power tube. Thanks for all of the tips and information!

Don

[donvan]

What do you amp repair guys do in a case where 1 of 4 matched power tubes needs to be replaced. Do you replace all 4 or try to find a single tube the matches the other 4?

Thanks,
Don

[ijedouglas]

What do you amp repair guys do in a case where 1 of 4 matched power tubes needs to be replaced. Do you replace all 4 or try to find a single tube the matches the other 4?

I'd replace with a matched quad. I have got in the habit of buying a matched sextet so I have replacements if needed. I know, this doesn't account for the other 3 getting extensive use and potentially drifting but so far I've been lucky in replacing a bad tube and having them well-balanced.

[donvan]

Ian,
Thanks for your input. I just pulled the trigger and ordered a matched set.