Hum in Bandmaster

[martin manning]

Remember that when biasing the absolute grid voltage does not matter (ignore the pin 5 voltage), it's the current and the plate voltage together that determine dissipation, and the tubes must be reasonably well matched so that the dissipation is similar at the same grid and plate voltages. If the pin 5 voltages are the same and one tube is red plating, the only option you have is to lower the bias voltage further.

[Oddvar R]

So I found reaason for the hum: Faulty output transformer. Now it works like a charm. Thanks for help.

[martin manning]

Good to hear, thanks for the follow-up!

[Oddvar R]

So, in the end it wasn't the OT, but the only part of the amp i didn't think of: The electrolytic on the bias board. I changed it, so dead silent. The electrolytic was a Spraque one, but I suspect it might have been a possible fake one. I bought two of these, on of the wires fell off right away. But now the Bandmaster works like a charm.