Loud hum

[txstrat]

While I played my recently rebuilt dual channel amp at quite a loud volume, it suddenly started to hum (quite loud too). I pulled the guitar cord but there was no change.
When I switched the stand by off, and back on again the hum was gone until I played loud again.
So far I found that whacking the top of the amp can reproduce the hum.
Turning the master volume down (pre PI) didn't change anything. Therefore I would start to troubleshoot the power section including PI (haven't pulled the PI tube yet).
Does anyone have an idea what I should have a look at first?

[R ski]

Pull single power tube, watch, they are hot. If the hum doesn't seem as noticible, then that power tube went bad.

[Enzo]

Same here, the failing power tube is far more likely than the PI tube. Of course anything is possible.

[txstrat]

Just to mke sure. The hum was as loud as if you plug a guitar cord in and touch the tip of it with your finger.

[tubeswell]

Yep - pulling each of the power tubes in turn is a quick way to eliminate one potential problem. If whacking it stopped or started the hum, its could be something loose inside one of the tubes, or dodgy heater-to-cathode insulation that makes intermittent contact when bumped or heated and warped etc

[txstrat]

If whacking it stopped or started the hum, its could be something loose inside one of the tubes

I'm gonna pull the power tubes one by one to check that

, or dodgy heater-to-cathode insulation that makes intermittent contact when bumped or heated and warped etc

That's what I suspected first. I've looked over the inside amp for things like that but found nothing.
Haven't checked with a chopstick under live conditions, yet.

[Robert M. Martinelli]

Hi Matt,
+1 on what Enzo, tw and R ski said.

I would say one of the power tubes has suffered some kind of mechanical damage and some electrode is loose and moves with vibrations causing the problem ( if one power tube goes off the PP becomes unbalanced and no longer able to "kill" hum )

Cheers

Bob

[txstrat]

if one power tube goes off the PP becomes unbalanced and no longer able to "kill" hum

Hi Bob,

thanks for helping me to look behind the scenes.
I didn't think of the power tubes first, cause they're quite new (actually I didn't WANT to think about them ).
Anyway, I'll check the pair of 6L6's next time I'm at my workbench.

cheers

Matt

[txstrat]

It was the PI tube (12AX7).
I changed both the power tubes (one at a time with known good ones) to no success.
Each time I hit the chassis slightly with my flat hand the hum came on.
Changed the PI tube and the problem was gone.
Have another issue with it but I'll open a new thread for it.
Thanks to all for your help.

added:
Inspected the PI tube a little closer and found the plate structures hit the glas container when I knocked on the tube with my knuckle. Seems all a bit loose inside.