Bad Transformer?

[tonemonster]

I've been restoring an old tube guitar amp and have replaced the filter caps, checked the resistors,pots, misc. caps ... After my best efforts to get it sounding as it should, it still has a low frequency hum and strange ghost notes/harmonics as the pitch increases. I normally associate this with bad filter caps. What symptoms does a bad transformer exhibit?

[Enzo]

What amp is it?

COUld be a grounding issue, if there is a bias supply, make sure it is ripple free. SOmetimes we replace the B+ caps and overlook the bias supply.

Scope the output to see if there is some sort of ripple problem.

I assume it has full wave rectification. Make sure both halves of that are working. An open diode or a rectifier tube with one side dead will result in half wave rather than full wave, and the ripple will increase.

One general rule of thumb is that if the hum is 60Hz, it is likely grounding, or radiated hum. And if the bias supply is half wave, check it. If teh hum is 120Hz, it is power supply ripple.

And the key to troubleshooting is to isolate the problem. Does the hum remain if all the controls are turned to zero? If so, then look to the power amp, if not, then something before the controls is at fault. In fact any control that has an effect on the hum is after its source. And any control that has no effect on the hum is before its source.

SOmetimes "invisible" things are at fault. If the cover of the volume pot is not grounded to chassis, it can act like an antenna for hum. Many pots will ground the rear cover via the shaft bushing and nut, but not all. Just like in a guitarthen we would solder a wire to the cover and ground it.

SOmetimes moving wires inside helps. All it takes is for a signal wire to be parallel beside a heater supply wire, and it will pick up the hum.

Many things cause hum.

[Trout]

I would like to know what amp it is.

Enzo hit on a lot of good possibilities and I would try them all.

I would also add possible tube issues. I have a NOS pair of 6L6GC's (Sylvania) I had to pull because for some odd reason, the amp would hum using them. They test fine in the Tester, yet still cause the amp to hum. I dropped in several other new sets without issue.

I have also experienced ghost notes from pre-amp tubes in my combo amp. Brand Sovtek 12AX7 sprial filaments. I attribute it to a microphonic/vibration type condition. In my Head/Cabinet setup they work fine.
More Vibration in the combo and they hang in line with the speaker.

Trout

[Enzo]

If a pair of tubes hums, but they are individually good tubes, the first reason that comes to my mind is that the two are not matched. MAtched pairs will cancel power amp stage hum. Mismatched tubes won't.

Try this: with a pair of tubes in there that don't hum, pull one of them. If it is a quad, pull both from one side. The hum should increase - that is the hum that is native to the power supply of the amp. The balance of the two sides cancels that hum.

A microphonic preamp tube is another good possibility as Trout points out. WHen these hyms and ghost notes occur, have someone grasp each preamp tube firmly. Use a rag or gloves if they are too hot for you. If grasping the tube - which stops it from vibrating - makes the symptom quit, then the tube is microphonic and should be replaced. If the tube is otherwise OK, it might work fine in a lower gain stage, such as the phase splitter.

[tonemonster]

I' recently decided to try a different rectifier tube, re-inserted all the tubes, turned the unit on, started washing some dishes and was pleased (temporarily) that I didn't hear any hum!!! However, I soon heard a the sound of bacon in a fry pan, and smelled ozone. It was the transformer expressing it's dismay at having a power tube switched with the rectifier, and left on for several mintues. (TOAST!)


Thanks for the input. It will be very helpful going forward.