2558 Jubilee repair, humming like crazy

[xtian]

Sounds like you had a power tube arc. Pin 2 is heater, and it's not uncommon for the heaters to arc to ground (pin 1). When this happens, the heater's artificial center tap, if there is one, gets burnt, and the next time you power up, you have a terrible hum because the heaters have no reference to ground.

Is this your situation?

[xtian]

Um, well, two things. You have several coils in the PT. One provides high voltage for the plates, one provides heater voltage, maybe more coils for tube recto, etc. Each of these coils can have a CT or not. So be specific: does the heater coil have a CT, or just the HT coil, or both?

Second, if you disconnect the heaters, NO tubes will function, and you will have no sound. If you are indeed hearing hum under this condition, it could be the PT inducing hum magnetically into the OT, and this can only be fixed by repositioning the transformers.

So what's really going on?

[pdf64]

Without the power tubes in place, have the sockets got a suitable bias voltage on their control grid, terminal 5?

[martin manning]

I have tried several, known good power tubes, but every time I plug one into the socket, immediately a short to ground from the plate occurs.

Is that the socket that you replaced? If so perhaps it's the wiring or the new socket itself causing the problem.

[martin manning]

Onward! What are the remaining issues?

[Stevem]

I have one of these to start on tonight and it has been re-tubed by someone else and is very noisy in both hiss and hum levels on the solo channel, is yours doing the same thing?

[Stevem]

With the amp wide open and jamming a chord hard do you see that 445 volts on the plate drop off even lower?

If you hook up you meter set for AC volts across pin 5 on each output tube and do the same test, how much ac drive signal voltage do you read?

[drew]

Try putting a patch cord in the effects loop.

[pdf64]

The UK standard volume control taper is log / 10% audio, which (when used for a master volume) results in the amp not getting loud until its rotated well past halfway.