Hum on bass channel of Blackfaced Bassman

[01redcobra]

Well, I finally fired up a Silverface to Blackface Bassman project that I brought back from the scrap heap. The problem is that there is a hum on the Bass channel. I never heard the amp prior to this. It didn't even have sockets for the 6l6's when I found it. I modified the bass channel to be a better guitar channel by changing the tone circuit when I blackfaced it.
The normal channel is super quiet. But there is a good hum on the bass channel. I have the amp on a bench with the chassis open and I know the fluorescent lights are causing some of it. Could this hum be because of the extra preamp stage on the bass side? It has a lot more gain than the normal side, definitely louder on the bass side. I've replaced all the resistors and capacitors on the bass circuit and it's still there. Any ideas?

[Enzo]

Sure. FLip off the lights, your ears work in the dark. Now you can tell how much of it is the lighting.

Does the volume control turn the hum down?

When you plug a guitar into the channel and turn the guitar volume to zero, does the hum remain?

Have you tried a different preamp tube in the end socket?

Is the heater supply grounded? Either a center tap grounded or the two 100 ohm resistors to ground?

Back the nuts off the input jacks, move the star washers, and retighten the nuts. Does this fresh chassis contact for the jacks help?

[01redcobra]

Thanks for the feedback Enzo. It does still hum with the lights turned off.
And the hum gets louder and changes pitch as you turn the volume up. (The normal channel doesn't, it just hisses at higher volume) I'll have to try the guitar in it tomorrow cause I don't remember if it affects it or not. The channel is much louder than the normal channel. I didn't try it with a guitar but the signal is much louder on the bass channel. I have tried different tubes in the v1 position and it affects it slightly. If you have no tube in v1, the hum is diminished and if you pull v2 the amp is almost silent. The center tap is grounded and I will try the input jack retightening when I try the guitar. I had forgotten to mention the thing about pulling the v1 tube when I initially wrote this thread. Maybe that will help.