Help! Problem with amp!

[Pavement2o8]

Howdy there, i have a problem with my amp and i cant figure out what it is. Last night i go to plug my fender bassman head into a new avatar cab i had recently bought, after the head was done warming up..i go to flip the stand by switch and all i get is a very loud hum through the speakers..no guitar signal at all..the amps volume knob didnt change anything either..had it at 0 and it still produced this very loud hum. I thought maybe it was the cab..but i plugged the head into another cab i have and it did the same thing..also tried another speaker cable..same thing. Took the amp apart but i dont know much about amps and couldnt really see anything wrong. Anyone know what the problem could be or any advice ?

[drgonzoguitar]

Steps for fixing the Bassman:

1) Replace tubes with known good tubes. If problem still exists, see Step 2.

2) Take amp to qualified tech in your area.

After it is repaired, make sure that the impedance of your cab matches the output of your amp.

[doctorx]

Probably bad power tube(s).

[jamme61]

is it possible you plugged into the ext speaker input, by mistake? I hope you did that would be a quick cheap fix.

[Pavement2o8]

Okay update..plugged it in when i got home tonight..and now i have a guitar signal! I think taking it apart and putting it back together did something..but the hum is still there..just not as loud now....hmmmm

[Pavement2o8]

Also i just discovered if i pull out one of the preamp tubes..the hum goes away completely and it sounds back to normal..tried this with two of them individually..any ideas ?

[jamme61]

which one stops the hum? V1,V2,V3? does the amp hum with no guitar plugged into the input jack?

[RussB]

bad tube

[Pavement2o8]

well the thing is..i pulled out v1 (the one all the way to the right im assuming?) and the hum went away..but then i plugged it back it and pulled the next one out..and the hum went away again. So im not sure if it is a bad tube or what..

[Pavement2o8]

and yes the amp still hums if the guitar isnt plugged in..and the volume control does nothing to the hum also..

[RussB]

...im not sure if it is a bad tube or what..


and neither does anyone on the internet

[slider313]

Time for a cap job?

[SatelliteAmps]

and neither does anyone on the internet

heh heh heh.

[billstets]

Pull the phase inverter and see if it goes away.

[Pavement2o8]

whats a phase inverter ?

[elkym]

It's the last tube before the power tubes... it splits the signal, so that the upper half of your signal gets amplified by one power tube and the bottom half, by the other power tube.

[Pavement2o8]

pulled the phase inverter..no sound or signal at all..

[Pavement2o8]

ok plugged everything back in..now the problem is gone! wtf?! thank god...

[Nolatone Ampworks]

ok plugged everything back in..now the problem is gone! wtf?! thank god...

You may be seeing something intermittent.

If it's a bad tube, unplugging and replugging could jar it enough to where it's not showing you it's problem, but it will likely show you later.

Could be dirty pins on tube or socket that reseating cleared.

OR, could be a grounding problem that comes and goes.

I wouldn't stake too much on that amp before you find the real problem. Problems that just "disappear" without actually fixing something worry me.

[billstets]

pulled the phase inverter..no sound or signal at all..


Earlier in the thread you said you pulled the preamp tube all the way to the right and the hum went away. Is that V1 or the PI? It's confusing because the volumes don't effect the sound, but it seems like it's somewhere in the preamp up to and including the PI socket. MAybe after the volumes. Probably a tube or a loose ground.

[Rossl]

If you had hum without the guitar plugged in, check the input jack. It is supposed to short to ground, it this is not happening the amp could amplify some noise

[Pavement2o8]

When i pulled the preamp tube that was all the way to the right..the hum went away..then i plugged that one back in..and un plugged the next one..the hum also went away..but when they were both in..it hummed..but now it has seem to have gone away

[billstets]

When i pulled the preamp tube that was all the way to the right..the hum went away..then i plugged that one back in..and un plugged the next one..the hum also went away..but when they were both in..it hummed..but now it has seem to have gone away

I'm assuming that's V1 all the way to the right. It's strange that it goes away when you pull V1 and yet it's not affected by the volume control. The volume controls are after V1. I would think that if it were somewhere up to and including the V1 socket, the hum would go away when you pull V1 but the volumes would affect it.

Got me. Try tubes or take it to a tech.

[Pavement2o8]

well the thing is..now the hum disappeared and the amp seems to be working fine..maybe one of the tubes was loose..or like someone said..the sockets or pins were dirty..

[jamme61]

the problem went away and the amp works now, after you pulled the PI tube (preamp tube closest to the power tubes) so maybe dirty pins on that tube, or that tube is bad? If the problem comes back go right to that tube first IMHO.

[billstets]

the problem went away and the amp works now, after you pulled the PI tube (preamp tube closest to the power tubes) so maybe dirty pins on that tube, or that tube is bad? If the problem comes back go right to that tube first IMHO.

It's puzzling though, because the hum went away when he pulled V1 and V2. If it were the PI socket having intermittent connection (dirt pins), wouldn't it send the noise though to the speakers even if V1 and V2 were pulled? Furthermore, a hum sounds more like a ground issue than a connection.