66 Fender Bassman hum

[pat.ladner]

I just bought this amp and it sounds great with the exception of a hum. Although I cannot hear it when playing the bass, I know it has to have an impact and it bugs me because I don't think it is suppose to be that way. The amp has an
aa 165 circuit. The hum happens when the amp is turned on and speakers are hooked up BUT nothing else is plugged into the amp. Turning the volume, treble and/or bass knobs has no impact. Same on the other non-bass side. Other information is that there is a grounded wall plug and the ground switch has no impact regardless of the position of the switch. help. Thanks

[MWJB]

Power down, let the amp & tubes cool, then pull out the 12A#7 tube that is nearest the power tubes. Power the amp back up, does it still hum?

[pat.ladner]

Darn, yes it still hums

[MWJB]

Power tubes matched & drawing reasonable current? Do you have a bias probe? Are you familiar with safety protocols when working around tube amp circuits?

[pat.ladner]

The power tubes look the same. They have the same logos on them. I'm not an electronics technician.

[52 Bill]

Maybe it's time for new filter caps. Has the amp ever been recapped?

[MWJB]

Ha ha, you don't need to be, but for now I'm afraid that all arrows point to you needing a tech. If you don't have one locally, you can ship your chassis to them. Unplug from the wall AC, put the standby switch in "play" mode, by the time you have removed the chassis any caps will be drained of voltage & you can handle the amp safely. Pack it well, if sending away, pack the tubes seperately, in their own box...dont leave them in the chassis, they will get smashed.

In the meantime I would recommend buying some bias probes & learning how to install & set bias current on your amp. Lots of outlets sell these, they pay for themselves after a few tube changes. With your amp, rebiasing can be done without dismantling anything, apart from removing the amp head's back panel.

To explain why I think you need a tech...the tube you removed is called the phase inverter (PI), it is the tube that feeds the power tube with the signal from the preamp. With that removed, only the power tubes themselves & the amp's heater circuit should be capable of making any sound. Changing the power tubes in a fixed bias amp like an AB165 really requires the bias to be monitored or adjusted (you could take a gamble, might work, might not help). Hum could be due to incorrect bias, or a tube/socket failure...hence probably not a simple plug in/out cure.

Good luck.

[pat.ladner]

I have located a local tech and I will take it to him. I will also try to get him to teach me how to use a bias probe.

THANKS for your time and for trying to help someone with minimum electronics capability. I was hoping that it may be something simple that I could do.

[pat.ladner]

Hum Update! Since I live on the Mississippi Gulf Coast, I did some research and found a guy in Slidell, La. who is a amp technician as well as a pretty good bass player. I took the bassman amp over to him Friday evening and pick it up today. He called me Sunday and said that he biased the power tubes and no more hum. He also replaced the standby switch and checked everything else out. He said that there was some noise that is cause by preamp tubes that will probably need replacing one of these days. The cost was next to nothing. Just thought I would pass on an update and thank everyone for their time and suggestions.