Yaqin Valve amp hum

[steveinfrance]

Certainly not genius but anyone who has a Yaqin PA can completely eliminate speaker hum. Set the bias to the low end of whatever it should be (500mV for a 100B) when the things been on for an hour then connect headphones to one output or bring the speaker to the amp so you can stick your head in it.
Leave the DVM connected to one of the test points and very carefully turn that bias set trimmer until the hum disappears. All you're doing is making both KT88 current draws identical so any ripple in the HT is cancelled out in the OP transfo.

[anthonyTD]

Certainly not genius but anyone who has a Yaqin PA can completely eliminate speaker hum. Set the bias to the low end of whatever it should be (500mV for a 100B) when the things been on for an hour then connect headphones to one output or bring the speaker to the amp so you can stick your head in it.
Leave the DVM connected to one of the test points and very carefully turn that bias set trimmer until the hum disappears. All you're doing is making both KT88 current draws identical so any ripple in the HT is cancelled out in the OP transfo.

[Rowlf]

I used to own one to try out but it was a EL34. My right speakers would hum audibly but at low volume levels. Wonder if that is the same thing you are experiencing.

Unfortunately for me there was nothing more I could do. I tried changing tubes, bias, check by technicians and power treatment. Someone then took it off my hands and I was happy to let him have it.

Hope you have better luck. Let us know how yours goes.

[steveinfrance]

With some mods to get the bias on the 6SN7s up the 100B is a very nice amp - and bearing in mind I worked on the STA25 I have a bit of experience.
The biggest improvement is swapping the Chinese 6SN7s for something better.
How valves with (supposedly) the same specs can sound different beats me.