Bandmaster Reverb - Hum problem

[tubetonez]

I have an old Bandmaster Reverb head. I've replaced the filter caps & resistors, the 25uf/25volt caps & plate resistors. The amp will work fine for hours, then suddenly develop a loud hum like a dead short. It's at a constant volume, the volume pots or input jacks don't affect it. There is no input signal when it does this, just the hum. I'd appreciate any suggestion in what to look for.

[stokes]

Did you replace the caps and resistors yourself?Check all the compnents you replaced for bad solder joints or components that are drifting when it heats up.The fact that the volume control has no effect on the hum says the problem is someplace beyond that stage.

[Enzo]

The first three things I would check would be tubes, tubes, and tubes.


Put different power tubes in there and see how it responds.

[tubetonez]

Did you replace the caps and resistors yourself?Check all the compnents you replaced for bad solder joints or components that are drifting when it heats up.The fact that the volume control has no effect on the hum says the problem is someplace beyond that stage.

I replaced them myself, when I first got the amp from ebay it was in sad shape. Someone had used aluminum foil for a fuse :-( I initially replaced the fuse holder and the broken standby switch, and added a grounded AC cable. The amp then worked, but would start sounding weak and compressed after it ran for a while. Then I replaced the caps etc. as they were all original, I followed the Fender guide from hoffmanamps.com. I replaced all the tubes as well and set the bias.

The sound was improved, but still after the amp ran for a while it would start sounding weak and compressed. A few times now it suddenly started making this very loud hum, sounds pretty much like when you touch the tip of an input cable. When it does this, the guitar sound disappears, none of the inputs work and none of the volume pots have any effect.

Could it be the PT? I do have a spare from a Super Reverb that I could put in.

[Jag]

What Enzo said.

[tubetonez]

I replaced the tubes already, any other ideas?

[Dave Curtis, dB AudioTech]

Bad ground somewhere? Tighten/clean the jacks? Resolder chassis grounds? Crystal ball fading... Sound like a connection problem somewhere.

-DC

[Mr. Droopy-Drawers]

Make it act up on your bench, and then hit a few components with some freeze spray. Since you changed all the resistors , start there. Hit all the Cathode resistors. Then maybe hit the resistors in the phase inverter. If you have the capabilities, 'scope it out and check a few voltages after the amplifier cuts out. If it seems to not cut out once it is out of the box and on the bench, I've put a small cardboard box over the chassis and just let the heat build up inside . Whatever it takes to replicate the problem. But to take wild-ass guesses and just shotgun everything is a waste of time and money. Measure a few things once you actually see the amp act up.

[Mark Black]

I replaced the tubes already, any other ideas?

Not to beat on a topic too hard, but does that mean you have tried different new tubes? Or are you referring to the set you mentioned as replacements for the old ones?

Sure sounds like a power tube going sideways. If the old power tubes were weak but not exhibiting the humming symptom you could try putting those back in to verify.

Also if you replaced the bias filter cap(s) make double sure they didn't get in backwards, as they are the only e-lytics in the amp where positive goes to ground.

Good luck!

[Mars Amp Repair]

Don't forget the neg bias supply filter cap. Also remove the 1st & 2nd 12AX7's & see if the hum remains. you can do this all the way up to the P.I. (drivde tube next to the power tubes). If it stops, the you problem in or before those stages.
Tap things with the insulated end of a screw drive...or better yet, something that is entirely insulated. You might be able to find a bad connection that way.
Any red plating of the power tubes when that happens...there's a clue. glen

[Alex R]

Yesterday I had the same thing inside a red-knob Twin. Intermittent hum cutting in and out despite new valves. Chopstick-poking inside made it come and go - eventually I found that one of the wires taking the plate voltage to the 'second' pair of output valves was broken inside its insulation (those wires are too thin anyway). So if it's not the power valves then check out all the connections to them with a chopstick. Best not to use metal chopsticks...

[tubetonez]

Thanks for the replies, yes Alex the hum is intermittent. In your case, did the audio signal drop out when the hum kicked in?

[tubetonez]

Make it act up on your bench, and then hit a few components with some freeze spray. Since you changed all the resistors , start there.

I did not change all of the resistors, I followed these guides, which recommends replacing these components in an amp over 10 yrs old:

http://www.el34world.com/charts/fenderservice7.htm
http://www.el34world.com/charts/fenderservice1.htm

Hit all the Cathode resistors. Then maybe hit the resistors in the phase inverter. If you have the capabilities, 'scope it out and check a few voltages after the amplifier cuts out. If it seems to not cut out once it is out of the box and on the bench, I've put a small cardboard box over the chassis and just let the heat build up inside . Whatever it takes to replicate the problem. But to take wild-ass guesses and just shotgun everything is a waste of time and money. Measure a few things once you actually see the amp act up.

I don't have a scope, just a meter what voltages should I check? I'm not trying to make wild-ass guesses, that's why I'm asking here - hoping that someone had seen a similar problem before. I do know that the hum has occurred immediately, within a few minutes of turning the amp on, other times the amp has run for hours.

I am just a hobbyist, but do have some experience as I've built, repaired & restored a few amps. I am a computer & network tech by trade, I am somewhat familiar with troubleshooting problems. Most of the replies here are implying that I messed up a working amp, which is not the case. I bought the amp from Ebay "as is" and I just want to get it in good working order. I've not been able to work on it lately due to health issues, I hope to work on it this weekend and will report back.

[Alex R]

No it didn't cut out audio entirely. One of four power valves was going down, so there was a noticeable drop. A similar fault could take out one half of the power amp and cause a buzz, also removing a lot of the audio. If the audio completely goes then it sounds more like a signal path issue.