Peavey Classic 30 humming, not in a good way

[AJStrummer]

Hello

I'm repairing a Classic 30. It has a louder than usual hum when an instrument cable is inserted into the input. The hum comes up with the volume being increased, and increases as volume/pre/post controls are increased. No hum when the volume is down. No other controls effect the hum. It's making the amp unuseable. When I recieved the amp, the bias was set at -16 vdc, and I've cooled it off to -12vdc. I've replaced all electro caps in power section. Cleaned all jacks. I've retensioned the tube socket pins. No tubes are microphinic. All voltages in power section & at tube sockets are as per schematic spec's. I'm not sure where else to look. Anyone have ideas? The only way I could get the hum to change was when testing the high voltage ac coming out of PT, before the rectifier diodes. It increased the hum when I tested voltage at that point. Other than that, the hum didn't change when I tried the usual poke and prod everything with a chopstick, move leads, remove and clean chassis earth connections.

Any help would be greatly appreciated.
Cheers AJS

[Helmholtz]

Did you try a different V1?

Make sure the safety grounding is intact. Measure resistance from chassis to the grounding contact of the power plug.

Try a different guitar cable or guitar.

[Jon Snell]

Try checking the input jack for a good ground connection.
"Cooling it off" would be increasing the -ve bias voltage, not decreasing the voltage as that will make the valves draw more current at idle.
The -ve bias should be around -14 volts for the best setting, otherwise it will run too hot and take out the EL84s after only a short while.
From your repair method, it seems you have not much experience, take great care!

[AJStrummer]

Hi Helmholtz.

Tried a brand new V1 tube and tried different cables and guitars. Single coils & hamburgers. It's 1am here, so will try the measurement you suggest in the morning. Thanks

[AJStrummer]

Hi Jon Snell

Thanks for your reply and concern. I'm by no way an expert, but I do know the safe ways to test and repair amps. You are correct. -14 is spec. My understanding was it made the tubes run too hot and raising the bias voltage to -12 would increase tube life. I'll re-read my info on that.

Have you come across this problem before?

Cheers

[g1]

The wording around 'raising the bias voltage' is confusing as it is a negative voltage. If someone says that 'raising the bias voltage makes the tubes run cooler', they mean making the bias voltage even more negative.
So something like -16 or -17V will make your tubes run cooler than the -14V stock setting.

[Jon Snell]

Hi Jon Snell

Thanks for your reply and concern. I'm by no way an expert, but I do know the safe ways to test and repair amps. You are correct. -14 is spec. My understanding was it made the tubes run too hot and raising the bias voltage to -12 would increase tube life. I'll re-read my info on that.

Have you come across this problem before?

Cheers

Yes I have. Many times.
The first thing to check, the frequency of the hum.
50/60HZ is mains hum, usually caused by a bad earthing connection.
100/120HZ is a main smoothing issue.

Did you understand the way that -ve Bias works? More negative means less electron flow and less current at idle as the grid becomes more negative with respect to the cathode, the electron flow is reduced and it conducts less current when idle.

[Helmholtz]

Did you check the safety ground as advised? Can save lives!

I don't think your hum problem is related to the power tube bias.

We know that the hum enters before the vol. pot.

[AJStrummer]

Hi all.

Firstly..my bad. I was following bad advise and adjusted the bias the wrong way, as you have all advised. Thank you. I've re-set the bias to -16 and it seems most of my problems have gone. Thanks for all the input. BTW, I did check the ground connections as you suggested Helmholtz. It was all OK, and thanks for your advice.
Cheers
AJS