2. amps-preamps
  3. 1276573000

have i diagnosed my hum?

Tube newbie here, looking for a bit of common sense in diagnosing a problem.

I have a stock Jolida 302brc that is producing an audible hum at the speakers, with or without any input applied, and regardless of volume level. I've sorted out the grounding issues, and the amp produces the same hum (again, at the speakers, not from the chassis) regardless of where or how it's plugged in. It's very audible from the listening position.

Resolution Audio here in Seattle was kind enough to loan me a pair of EL34 tubes to swap out for a pair of mine, thinking that I've got one or more tubes that are bad. When I made this change, the amp sounded horrible playing music (like a damaged driver; a very 'torn' and distorted sound), but the hum disappeared completely.

I'm assuming that this means that A) yeah, I've got one or several bad output tubes, and that B) the loaner tubes I used were way out of bias during this brief test.

Does replacing my driver tubes seem like a reasonable next step?

Thanks in advance for any comments or advice.
soundgasm

Showing 2 responses by soundgasm

soundgasm OP
152 posts
 
Detredwings,

The amp is less than a year old, and has maybe 800hrs on the tubes, although I'm really guessing on the hours.
I didn't bias the replacement tubes, just popped them in to assess the hum situation. I guessed that that was why it sounded horrible, even though the hum disappeared.
When I put my own tubes back in (which the amp is biased for) it sounded good again, although the hum was back.
I hadn't tried swapping positions on the 12's, thank you for the tip.
If the hum can be eliminated completely by replacing tubes, doesn't that identify them as the culprit?
soundgasm OP
152 posts
 
(apologies if this double or triple posts - they're not getting through.)

Thank you very much for your help, guys.

Rleff, I did your test sort of in reverse; replacing a pair of my output tubes (I only borrowed a pair), one at a time. With one replaced, the hum was diminished; with two replaced, it disappeared completely.
I have not seen any blueish glow, nor have trouble with it holding bias.

4est, I only had the loaner tubes for a fleeting time, since Resolution kindly yanked them from an amp on their showroom. I wanted to get them back asap, and figured I was just checking for bad tubes. My bad for not biasing during the test.

Does this admittedly flawed test ID my output tubes as the culprit? I'm not averse to replacing all the tubes as part of my initiation into this art...but if it's likely that I'll end up with nicer tubes in an amp that still hums because I didn't diagnose things properly now -- well, that'd be super irritating. :-)

Thank you again for walking me through this - it's very much appreciated.
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