Bassman Hum Help

[paolob]

Hello Everyone

I'm writing this thread because I need help with my Bassman 5f6 / 135 handmade amp.

Suddenly it started to produce some kind of hum.

It starts after few minutes after powering on the amp (power and standby switch) and it increases in time. Volume pots (normal and bright) does not affect the hum and It is 100hz i believe. I forgot to mention that each channel shares the same preamp valves V1 V2 And PI.

Thank you very much to all for your help

 

[MrCurwen]

Have you tried it with different tubes? I've had those symptoms from a tube nearing failure of heater - cathode insulation.

 

[JonSnell Electronic]

Check the bias on the output valves.

 

[paolob]

Thank you guys.

One of the power tubes failed. a friend of mine borrowed me a matched pair of used tubes but tested and functioning. So I tried the amp with four and two tubes but the hum is still there... I tried first with sovtek 6L6 wxt+ then with 5881. I replaced the preamp tubes but no change in hum. I checked the bias and it seems good. The plate voltages are more or les 490v. I used a fixed 2w resistor instead of a trimmer for biasing.

 

[kevinkr]

Moving to the Instrument and Amps forum per forum policy. (See sub headers under forum header for what goes where)

 

[paolob]

Moving to the Instrument and Amps forum per forum policy. (See sub headers under forum header for what goes where)

Sorry I didn't noticed there is a forum for musical instruments... my fault

 

[nigelwright7557]

I would be looking for hum on the power supply with a scope.
100Hz sounds like full wave rectified mains frequency.
Maybe an electrolytic dying or dead.

 

[paolob]

Thank you very much! Here in Italy the power is 50hz 230v. I'm trying to check the power section and the electrolytic caps. Maybe the bias circuit is feeding some ac to the grids?

 

[boobtube]

Yes, I agree with Nigel, any time you have 100Hz(120Hz here in USA), look to the power supply. This is where it's that double frequency because the full wave rectification changes the pulsating(rectified) DC to double the line frequency. Usually the bias is only half wave rectification, so would be 50Hz.