2. amps-preamps
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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
6 responses Add your response
detredwings
162 posts
 
Soundgam,

How old is your amp and approximately how many hours on the tubes? Also did you try to bias the tubes after you put them in? You can also try to change out the 12ax7 and 12at7's if you have an extra set or at least pull them out and clean them and put them back in or reverse sides with them. Sometimes dirty pins will cause odd problems.

Good luck
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?
rleff
1,065 posts
 
I would use the tubes the dealer sent you and then install your orginals one at a time and see if you can find the one or ones that cause the hum.
4est
507 posts
 
Is there a reason why you did not bias the amp for the new tubes? It would seem that is the plase to start, then as Rleff said, swap them one at a time. Do any of the tubes glow red at the plates as opposed to the heaters?
rleff
1,065 posts
 
Soundgasm I forgot to ask when you bias your tubes do you see any problem such as not holding bias or blueish glow in the the dark?
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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