bassman 5f6a on fritz

[tweedydum]

hi,
I have a sporadic problem with my bassman. Recently, it made a bad buzz and cut out - I immediately turned it off. Checked all the components, chopsticked them, and couldn't find anything wrong. Turned it on again and it worked fine. Played it for about 6 hours total (different days). Yesterday, turned it on to hear a loud buzz again - haven't had a chance to look into it, but what would you guys suggest I look for?

thanks,
Dave

[rfgordon]

First of all, I'd say check all the voltages. If you can get it to act sick, check them then.

When you say buzz, do you mean like 6o Hz or 120 Hz, or something else?

When sick, try pulling and substituting tubes, starting with the 1st preamp tube.

[tweedydum]

thanks for the comments - I'll yank some tubes later tonight if still malfunctioning. This buzz is an extremely loud honking - not a tone really, but quite loud - even with volumes down. Like the sound of the jack being plugged in/out with the volume up. But continuous, as I say.

more later,
Dave

[Alexo]

Is this an original bassman? A clone? kit? Reissue?

I would check out the ground connections in the power supply....

[jaysg]

Please verify that the socket had ground, hot was hot, neutral was neutral.

[selloutrr]

http://www.geofex.com/ampdbug/hum.htm

this should help!

[tweedydum]

ok, on both power tubes here are the voltages -

closest to rectifier: closest to preamp tubes
pin 3: 494 pin 3: 495
pin 4: 493 pin 4: 494
pin 5: -49 pin 5: -49
pin 6: 494 pin 6: 493

now, the crazy thing is, is that the 6L6 closest to the preamp is nearly cool to the touch. Even after being on for a few minutes I could wrap my hand around it. The other one is more what I'm used to - too hot to touch.

Does this make any sense? Also, I'm fairly meticulous with wiring. I see one of the red hi V wires on the rectifier looks burned. I really don't think I would have done this - but don't know for sure.

Of course, the amp seemed to work fine today when it was apart. I did find the power switch had been shorting intermittently, so I replaced it. I'm not sure that's what my problems were however.

Does the difference in tube heat seem strange to anyone? Or the voltages?

thanks - I'd love to get this fixed!

[Structo]

The difference in heat felt from the tube could mean the filaments are going south.
Swap the tubes in the sockets to see if the problem follows the tube or stays at the socket.

Or it could mean a dirty tube pin/socket.
Measure the AC voltage between pins 2 and 7 on the power tubes.

I would pull the tubes and clean/ re-tension the sockets.

Do you have any other 6L6 tubes you can try.


You

[Bob-I]

The difference in heat felt from the tube could mean the filaments are going south.

More likely the tube is shorted or open. Filaments don't put out a ton of heat. Most of the heat you feel from a power tube is the plates.

Swap the tubes in the sockets to see if the problem follows the tube or stays at the socket.

Or it could mean a dirty tube pin/socket.
Measure the AC voltage between pins 2 and 7 on the power tubes.

I would pull the tubes and clean/ re-tension the sockets.

Do you have any other 6L6 tubes you can try.


You

Yep. 100% correct. I'll be surprised is this is NOT a power tube or socket problem

[tweedydum]

well, the coolness of the tube closest to rectifier follows the tube. After switching, the same tube runs cool in a different position. 6.2 V AC across pins 2 and 7, so I guess the filament voltage is getting there.

Is this good enough to say bad tube and junk it, or would I try to measure the AC across cathode and plate while passing a signal?

Should I worry about what caused this tube failure or just assume it was cheapo tubes ( which they are )?

thanks again,
Dave

[M Fowler]

If your having a buzz sound when the tone controls are turned down check the bias supply cap for loose connection.

Mark