Hum through speaker with NO TUBES! Help!

[Krwkka]

A bit freaky... Turned on a reissue Ampeg Reverberocket and after a couple of minutes I suddenly get a fairly loud hum. Then the 2 amp slo-blo goes.

I put in a new 1 amp "fast blo" fuse, pull the power tubes and flip the amp on. As soon as I take it off standby - same hum. Back to standby and hum leaves.

What does this mean?

[Chuck H]

Hmmm... Pull the PI tube just because it IS connected to the speaker via the feedback loop. If the hum persists I think your OT may be the problem, unless...

I don't know if these amps hum due to transformer proximity or not. Since the hum seems associated with a blown fuse I'm not hopeful.

With the tubes out, diconnect the OT center tap and test for continuity between the OT primary and secondary. If there is continuity your OT is toast. Now...

It was probably an output tube failure that caused the problem. You'll want to check components to see if anything else is damaged.

If there is no continuity between the OT primary and secondary and pulling the PI tube doesn't kill the hum I don't know what else could cause your problem.

[Krwkka]

Thanks for the reply, Chuck.

The transformer looks good - infinite resistance with all primary leads disconnected from the board. But even with the driver tube out I'm getting the hum after I reconnect the leads! (By the way, it appears that there is no negative feedback loop from the secondary on this amp.)

[Krwkka]

... but maybe the high voltage can jump through the insulation even though the meter doesn't show it?

[Chuck H]

... but maybe the high voltage can jump through the insulation even though the meter doesn't show it?

Good thinking. Connect your meter to read voltage on the secondary when you flip the standby switch and see.

[Krwkka]

Well, I don't quite understand it, but I found a shorted diode - one of two diodes in series for a "anti flyback circuit" or whatever it's called. Replaced the diode, not expecting anything, and the hum disappeared!!

Replaced the tubes and all is good. Or maybe it's that I accidentally bumped into the real problem and temporarily fixed it?

Whatever. I'll remember that for the future, though.

[Enzo]

The flyback diodes should be the first suspects.

ANy time you have hum through the speaker with no power tube installed, the first thing to consider is how could any current be flowing through the output transformer. The B+ has no normal path at this point, other than leakage/short to the secondary or frame, or through those diodes. B+ is pretty much the only source of current available. And blowing fuses cinches the deal.

[Krwkka]

Thanks for the confirmation, Enzo. The diodes will certainly be the first suspect next time!

[Chuck H]

What flyback diodes? I saw nothing like that in any rocket schem???

[Krwkka]

Are you looking at the reissue - Saint Louis Music version?

[Chuck H]

No. But I'll try to find it.

[Enzo]

I do and do for you kids, but does anyone ever call at Christmas?



Ampeg R12R

[Chuck H]

Ha. From another thread:

No wonder Enzo is always so helpful and giving... He's Santa Claus... Think about it. Have you ever seen the two of them in the same place at the same time???

And now a blatant Christmas reference... I think we caught you. You giant sneaky elf.

[Chuck H]

Back on topic... Now that I see the actual schematic, it still isn't clear to me?!? If only one diode was shorted, how was current allowed to flow?

[Krwkka]

+1

That really puzzles me.