18 Watt Lite HUM

[rockybottom16]

Got a 120hz hum going on in my latest 18 Watt Lite build. I'm assuming it's 120hz because if I disconnect the filament lead going to the pilot light, the hum continues. It stops instantly when I turn off the standby. The hum is there even with the 2 - 12ax7's (preamp & PI) pulled out. It's there with all the controls at zero, and goes up a little as you turn up the volume. The filter caps are 32/22/16. I would suspect a ground loop somewhere, except that I've build a couple of "18 watters" with 6V6's and a couple with 6L6's with identical grounding and they're all very quiet. I used a used Weber 22913 princeton OT I had laying around for this amp(8K Primary). The power tubes are a supposedly matched pair of TAD EL84's. The hum is not noticable when playing, but it bugs the hell out of me because my other builds have, for the most part, been basically hum free.

Could the OT be out of balance? Tubes mismatched? Bad filter cap? I know this topic has been beat to death, and I've done a ton of reading; but I'd appreciate any help you guys can give me with this.

[Jazz P Bass]

A schematic would be nice,
Here is one.

When confronted with a new build problem you would be best served to start at the beginning.

Pull all of the tubes and measure each power supply node looking for Vdc stability & minimum Vac ripple.

Put in the output tubes: does it hum?
If it does & would suspect the OT.

No? put in the phase inverter tube..Hum.
Bad coupling cap or it is miswired.

Start there.

[rockybottom16]

Thanks for the advice, Jazz. (The schematic I used is nearly identical to the one you posted, except that I put in a PPIMV.) No tubes, no hum. Power tubes in, yes hum. I have no scope at home, so I can't check for ripple. But I paralleled an extra 16uf cap to the first filter cap and there was only a very slight reduction in hum; not enough to want to permanently attach it.

[Diablo]

Try a different set of output tubes. One or both may be hummy, or they might be very mismatched.
You can check for ripple with a good digital multimeter.
Measure AC volts at different nodes.
Post a picture of your build.

[Jazz P Bass]

If the output tubes are humming all by themselves, the OT may be bad.

Measure the grid pin dc & ac voltage.
There should be no Vac.

Measure the voltage on the cathode resistor to see how it is biasing up.

[rockybottom16]

Problem solved. The extra tubes I was waiting for arrived an hour ago. I put a couple new power tubes in and the hum disappeared. The tubes I removed were fairly new TAD's. So much for high-dollar tubes. You pays yer money and ya takes your chances. Thanks for your input!

[Chuck H]

OR... The new tubes are imbalanced and so is the OT. Install them right, no hum. Install them wrong, hum. If the TAD's are balanced and the OT isn't you get hum with either tube in either socket. I'd be interested in this test.

[eschertron]

Problem solved. The extra tubes I was waiting for arrived an hour ago. I put a couple new power tubes in and the hum disappeared. The tubes I removed were fairly new TAD's. So much for high-dollar tubes. You pays yer money and ya takes your chances. Thanks for your input!

OR... The new tubes are imbalanced and so is the OT. Install them right, no hum. Install them wrong, hum. If the TAD's are balanced and the OT isn't you get hum with either tube in either socket. I'd be interested in this test.

Chuck's got a great point! We can state a null hypothesis "the new tubes are imbalanced" and test that simply by swapping their positions. If the new tubes ARE imbalanced and are sufficient for correcting an OT imbalance, then the swap will make the hum problem worse in one orientation than the other. If the tubes are PERFECTLY balanced, then the swap will result in no change. Perform the experiment and know conclusively whether or not the new tubes are balanced. And by extension, the OT. If there's any imbalance in the OT, then the first set of tubes might perform 'better' with one orientation than another. Not sure if you mentioned trying that in a prior post.