Harmony tube amp hum is back - diyAudio
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Old 23rd December 2013, 05:44 PM   #1
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Join Date: Dec 2013
Default Harmony tube amp hum is back

Hello all;
I have a 1965 Harmony 306A tube guitar amp that was humming badly at idle with and without a guitar plugged in. I replaced all the caps except the old disk ceramic ones making sure all were oriented like the originals. They are all newly purchased caps from a popular online supplier. I replaced the can cap with a new one from the same place, making sure the chassis ground solder points were solid. I replaced the original two wire power cord with a new grounded one with ground wire to chassis. Cord ground is eyelet to power transformer mounting bolt with star washer. The hum was reduced a lot. I replaced all the tubes with newly purchased ones and the hum virtually disappeared (6V6 x 2, 6SH7, 5Y3, 12AX7 x 2). The amp played well for two days and sounded sweet.
The hum returned when I turned the amp on the third day. With both channel volumes turned all the way down, the hum remains fairly loud. The new thing now is that the tremolo "modulation" control, when turned up from zero, increases the hum dramatically.
I have checked all solder joints and checked for solder bridges. I re-tensioned all tube sockets. No components were changed out before the hum returned.
I have tested all the new tubes and they test in the "good" range. No shorts show on the tester.
I replaced the tubes, one by one, with the old tubes I removed. The new hum remains regardless of old or new tubes installed.
I'm in the USA and the amp is 110v AC powered.
The chassis is all point-to-point construction with no trace or component board.
If anyone has ideas on how I can troubleshoot this further I would be really appreciative.
Here are some facts about me:
My electronics level, especially with tube amps, is amateur. I know how to solder.
I have a tube amp basic repair DVD and I know how to drain caps and work safely on these.
I have borrowed a good tube tester and have it in my shop.
I have the Harmony 306A schematic, I can read and understand the cap and resistance values.
I have read threads about chasing hum and read things like "check B+ voltage", but I don't know how to identify the test points, so that right now means nothing to me. There are three pairs of wires out of the power transformer to chassis but I don't know what the expected voltages should be. They are not on the schematic that I can see.
I have a good VOM meter. I have no resistance meter.
Our towns only trusted tube amp tech is swamped with stacks of tube amps and refuses to take on any new work for an undetermined amount of time.
I have searched this forum for related threads but none really answer my questions on the return of a once-fixed hum problem.
I appreciate anyone who would like to help me troubleshoot this thing. If I'm in over my head on this deal, let me know and I'll wait for the tech to have a slot open up.
Let me know if there's an existing thread here that can help me that I missed in my search.
Link to schematic:
http://harmony.demont.net/documents/...rmony_306a.pdf
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Old 26th December 2013, 02:04 PM   #2
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Join Date: Dec 2013
Sorry you're having trouble. Ultimately, I'd suspect that you're having an issue with either your power transformer or output tranny, given the amp's age. Heater winding issues often sound like you're describing. Check this article out for some ideas: Power Transformer
Could be a few other causes, tho, so don't bum out too quickly. Any poor solder connection can do this, too.

Seems like you appreciate the need for safety, caution and pre-thinking your moves on this stuff, huh? You are exposed to potentially FATAL VOLTAGES inside a tube amp, just pointing that out. If one doesn't know what's going on, one should stay out...doing a lot of reading about this stuff prior to going in is very important, IMHO.

That said, you should be able to track this down with some patience and some careful diagnostics.

Before diving into the trannies, you could check that the jack next to R35, tremolo mod control, is not screwed up in any way (detached ground wire, broken insulators, loose, etc). With caps discharged/amp unplugged and all, of course! And how is the pot (R35) itself?? Are the grounds in that area - around V5 - in good condition?? A failing pot can hum like that, too. Any poor ground can, in fact. That's where I'd start (with a chopstick, and being SAFE!!!!), then move to the PT/OT....hope this helps!

Remember: it passes sound, so that makes it easier to fix than if it was totally dead!
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