troubleshooting problem...

[stompjunkie]

I've been working on an old amp running a pair of 6973 power tubes. It hums very loudly when the rectifier tube is in. It seems very stable without tubes. I replaced the filter caps, becuase they were really leakey. That didn't help any with the hum. What would cause an amp to hum like that with just the rectifier in??? Thanks for the help guys.

[Structo]

Do you have another recto tube you can try?

Tubes are always the first suspect in a tube amp.

[Phil_S]

Random thoughts:
1) Sometimes a new cap is just bad.
2) What rectifier? Maybe try tacking some diodes to the socket for a quick temporary conversion to eliminate that as a problem?
3) Plain bad grounding scheme, maybe a ground loop? Have a good look at it.
Got any pics or can you tell us a bit more about the amp?

[stompjunkie]

It's an old Gretsch amp. I don't know the model # it has two tens and a 4" driver... one of those "It WAS running great when I had it" deals... It's running a 5y3 rectifier. I tried swapping it out with another known good one and it did the same thing. The rectifier is also red plating after about 20-25 seconds. I turned it on to try to measure a few voltages, and it started pretty quick. It was doing that with both tubes....

[stompjunkie]

Oh, I'll try to post some pics as soon as I can, tho it'll probably be tomorrow....

Thanks for all the help guys.

[Phil_S]

...5y3 rectifier. I tried swapping it out with another known good one and it did the same thing. The rectifier is also red plating after about 20-25 seconds. I turned it on to try to measure a few voltages, and it started pretty quick. It was doing that with both tubes....

Hmmmmmm.....makes me think something about the cap job isn't right. Take a close look. You may have a cap in backwards, maybe a missed ground? You may have a bad cap. Probably not the tube if it does it on two of them.

Did you get ACV reading on the HT secondary? Pull the 5Y3 and do that. Easy and quick, not much to loose and everything to gain. If it's the PT then you'll know it. While you are at it, check the 5V output, as well. First eliminate what's on one side of the tube, then go to work on the other side.

[stompjunkie]

The huge hum was there before the cap job, and it hasn't seemed to change at all, but I'll definately be going over that again to make sure.

Thats a great idea. I'll let you know what voltages I get from the PT as soon as I can. It'll probably have to be tomorrow though. Many thanks Phil.

[KT66]

The red-plating rectos indicate that there is an excessive current draw ( a short) somewhere. If I understand correctly, the amp hums loudly when only the rectifier is plugged in and all other tubes are removed ? Any kind of sound coming out of the speaker means that it is being induced by the primary ( or from another source, like stray magnetic flux from the PT, but that usually is not that loud) and if the power tubes are removed, there must be a short that is completing the circuit and causing the current draw that is red-plating the 5Y3 and causing the hum. This could be internal to the OT, at the tube sockets, or along the OT plate leads.

If I am misunderstanding and the hum only happens when the power tubes are in then one of them might be no good.

Some Valco made amps, which I assume your Gretsch is, of this vintage that I have seen have the OT mounted on the baffle. So, be cautious if you do have a short in your OT there is a slight possibility that the B+ might be live on the frame of the OT since it is not grounded to the chassis.

Hope this helps.

[Tubetwang]

I like you web site KT...

Nicely done and some good info...

All the best!

[stompjunkie]

KT...

It looks like your spot on. The rectifier red-plates when it's in by itself. and the hum is LOUD...... That explains why it would be ok on the power side. I'll triple check the leads and sockets, but it looks like I'm due for another OT.

Thank you also for your safety advice. Thankfully this one is chassis mounted. If not, carefull as I try to be I may have gotten a bit of a jolt.

Is there anything special I should do when replacing the OT? Is it recomended to give a "soft" on like with new caps?

Thanks again!