Hum in Bandmaster

[Oddvar R]

Which Bandmaster circuit is it?

Ab763

[angelodp]

Have you tried swapping the tubes, to see if one tube is the culprit. You say one tube causes the problem, on which tube socket? Or is it one tube with either socket?

I assume you have tightened all your jacks and pots and ground connections.

[Oddvar R]

If I take out all the preamp tubes andt the tremolo tube, the hum is still there.

[pdf64]

… The funny thing is that when I put in just one power tube, the hum goes away, with two it comes back.
…

I suggest to investigate the above some more. As it’s really weird.
Also, if you haven’t already, I suggest to add 1R cathode current sensing resistors to the output valve sockets. Or get a bias probe. As it’s kinda useful / essential to have an easy quick safe way of checking that.

So, with just the output valves fitted, the hum is still there (V4 might be regarded as part of the power amp circuit)?
And what current is each valve drawing, eg measure mV across each 1R resistor?

To fit cathode current sensing resistors, replace the wire link between each pin 8 socket lug and chassis common with a 1R (1 ohm 1/2W or 1W metal film).

[pdf64]

Delete

[Oddvar R]

I wonder a bit about the call for pin 8 to ground, when I take away on of them, the hum lowers. Is that relevant?

[Stevem]

Not to be mean, but if you can’t see why pin 8 of a output tube needs to go to ground, even if thru a low value resistor then you really need to learn more about tubes.

[martin manning]

What happens to the hum as you remove the tubes beginning at the input end of the string and ending with the PI tube?

[Oddvar R]

What happens to the hum as you remove the tubes beginning at the input end of the string and ending with the PI tube?

The hum is constant.

[martin manning]

So the source is the power tubes. They could be very unbalanced, there could be excess ripple on the bias voltage, or something is not right with the output transformer.

[Oddvar R]

I have ordered a new OT, thanks Martin for being a great person.

[martin manning]

Well, let's hope that is the correct fix. PDF is saying replace the two ground connections from pin 8's with 1Ω resistors from pin 8 to ground so you can measure voltage on pin 8 in mV, which will be numerically equal to mA flowing through each tube.

[Oddvar R]

So where do I measure between?

[angelodp]

OT's are expensive, 1 ohm resistors are pennies. Can you hold off on the OT until you check the current in those Power Tubes.

[martin manning]

So where do I measure between?

Pin 8 and ground.