Fender Bassman 70 high filament voltage

[cryptozoo]

Quick question for the experts -- I'm getting crazy-high filament voltage in my Bassman 70, 64vac on one lead, 53vac on the other. I've disconnected the heater taps from the PT and tested them on their own to confirm it isn't something in the heater chain. The amp is of course warbly and oscillated, but I subbed in a filament transformer and it solved the problem. My question is, is there something that would cause this kind of high filament voltage other than a damaged PT? Would I be able to get away with just taping off the PT heater wires and using a filament transformer, or will that damage the PT over time?

[g1]

Sounds like you are measuring each side to ground? Measure between the two heater wires, with the heater taps disconnected, and also connected with a full load of tubes in the amp. Post your results. Also post what the AC voltage is coming out of the wall.

[cryptozoo]

Yes, measuring to ground. I'll try measuring between them, but typically I can measure one side of the heaters to ground on other amps and get an accurate reading (3+ vac). Any reason this one would be different?

[R.G.]

It may be that in other amps, the centertap of the heaters is grounded, and in this one, they are not. You could check this by looking at the schematic of the amp or by using your ohmmeter to measure the resistance between the heater center tap to ground.

If the heaters are not referenced to ground in some way, you're going to get lots of hum.

[Enzo]

What RG said.

Your 6v winding has no center tap. Fender used to either use a pair of resistors to ground, usually at the pilot lamp, or a "hum balance" pot, which yours should have.

If your hum balance pot is bad or burnt out, you will have no ground reference, and your readings could be most anything whenmeasured to ground.

If you genuinely had 50-60 volts on your heaters, your tube filaments would be REALLY bright for a moment then burn out. The only thing that matters to the heaters is that they have 6v across them, not to ground.

A grounded center tap will inded give you 3vAC to ground from either end. But some amps do not ground the center tap, they wire it to a positive supply, like output cathodes or something., SO your heater wires then might have +25VDC on them with respect to ground, yet still have 6VAC between them .

[cryptozoo]

Awesome, thanks for the info, fellas. I think the humdinger in this one is bad, as it doesn't seem to have much of an effect. I'll start there.

[Bruce / Mission Amps]

I'm gonna start calling all hum bucking methods, humdingers....

[pdf64]

'The only thing that matters to the heaters is that they have 6v across them, not to ground.'
Hopefully it won't be a problem on cryptozoo's amp, but the other things heaters don't like is too much voltage between them and the cathode; the insulation is generally rated at about 200V.
If it has been damaged, there may be some hummy/buzzy interference that can't be got rid of except by replacing the tube/s with the damage.
Whatever, the first step is to replace the humdinger.
Rather than replace the pot, it may be beneficial to use 2x100ohm resistors, 3 watt minimum, to ensure that a solid ground reference is maintained, hopefully even under fault conditions.
Pete

[Enzo]

Yes, that is true. In the context of the original post, I didn;t want to get too far afield. We were having to explain AC readings on wires not referenced to ground as it was.