12AY7EH a hummer?

[lmolter]

Is it possible for a tube itself to be a source of hum? I just finished wiring my 5F6A DIY, and I've got a hum problem that I can't trace. It's 60Hz and about 15mV on the plate of V1. If I pull the tube, the amp is quiet (duh?).

My split grounds appear to be ok, but I'm not ready to redo the grounds and heaters yet. I did go ahead a rewire the input jacks and shorten the leads to the grids. The 68K resistors are on the jacks, not the eyelet board. No difference.

I read in another thread in this forum that some tubes can be hummers. I don't have another AY7, but the local music store sells Fender brand tubes. Should I get one?

Other than the hum, the sound is loud and awesome. Man, I wish I could play better. I'm a builder, not a musician.

Larry

[Diablo]

Some folks stick a 12AX7 in V1 to get more gain. So, if you have one of those tubes handy to try, it'll tell you if the problem was your 12AY7 or some other part of the amp. Or if you have a 12AT7 handy, try that. The 12AX7 has a gain of 100, the 12AT7 has a gain of 60, the 12AY7 has a gain of 40.

[jrfrond]

If you want the lower gain tube in there, the more plentiful option would be a 5751. It's gain is only SLIGHTLY higher than the 12AY7, but still lower than the 12AX7, and are readily available as NOS for a reasonable price.

[Chuck H]

It may or may not be the tube itself. It could be that the supporting circuitry for that tube, including grounds and/or layout is responsible. You can check this with a tube swap. Any known quiet 12xx7 tube will do. If it is the tube, and you want the gain of the 12ay7, I would stick with a 12ay7. IMHE it's a very good sounding tube. But it can be hard to find a good one.

Chuck

[elkym]

also... another one many people don't mention,

12dw7-- I wouldn't use this in a Phase Inverter position (it's unbalanced by it's very nature) but I've heard that people have had great results in other preamp slots...

[soundmasterg]

The 12DW7 would work fine as a phase inverter tube if the phase inverter was using the cathodyne or split-load circuit.

Greg

[Enzo]

Of course tubes can be a hum source. The tube could be defective. The heaters might be involved - that's why some run them on DC, some ground the center tap on the heater power, some elevate the center tap to some +DC voltage.

Slap a 12AX7 in the socket. Doesn't matter if the tone or gain isn;t right. Does the hum remain or is it now gone? That tells you about the existing tube.

Do the grids sit at about zero DC? I assume so. Ground each grid with a clip wire to the botom of the cathode resistor. Does that kill the hum - if so it is coming into the grid from outside.

[jrfrond]

Seriously, you better HOPE it's a tube, because induced hum problems can drive you off a cliff!