RT2/50 humming (not buzzing)

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Bucksears
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RT2/50 humming (not buzzing)

Post by Bucksears » Sat Dec 25, 2010 7:21 pm

Just started getting a low-pitched hum (not buzzy) that gets louder as I turn up the volume on my RT2/50. I still get a signal and can play, but have to keep the volume lower on the RT2/50 to minimize the hum.

I just installed new power tubes and will swap in some older ones to see if that changes anything to rule that out as the problem or not.

Anyone ever have this problem?

Thanks,
Buck

Bucksears
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Post by Bucksears » Wed Dec 29, 2010 10:10 am

Found that a different set of power tubes LESSENED the hum, but it's still there. Might be time for a trip to the tech.....
Swapped the preamp tubes as well, but that had no effect.

bduersch
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Post by bduersch » Wed Dec 29, 2010 10:43 am

If I recall correctly (and it's been a while), I thought that early RT2/50's were wired in such a way that generated more hum, but the design was modified in later RT2/50's to reduce the amount of hum. What exactly changed in the design, I do not know, nor if it is something that is easily remedied or if it requires taking the amp to a tech for modification.

(Yeah, I realize this is a pretty useless answer) :)

--B

Bucksears
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Post by Bucksears » Wed Dec 29, 2010 5:54 pm

No, actually that helps a lot. I mean, at least that's SOMETHING to go on and it obviously sounds like it has been a real issue. Since I'm off tomorrow, I had planned on taking it in to a local Randall-designated tech and drop it off.

With the new low-noise fans (Silenx 80mm, 32CFM @14db) in my M4 and RT2/50, I'm starting to really warm up to this rig and would love to have this issue resolved.

HughJasol
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Post by HughJasol » Wed Dec 29, 2010 6:56 pm

Bucksears wrote:With the new low-noise fans (Silenx 80mm, 32CFM @14db) in my M4 and RT2/50, I'm starting to really warm up to this rig and would love to have this issue resolved.
Can we assume the hum was present before you swapped the fans? On an unrelated note: You mentioned in a post somewhere else (I can't find it) what the CFM rating was for the stock RT2/50 fan - wasn't it like 42 or 46 CFM? I love the idea of replacing the stock fan with a quieter one, but I'm not sure I'd feel comfortable going with one that generates less airflow . . . does SilenX make higher-output fans?
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Bucksears
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Post by Bucksears » Wed Dec 29, 2010 9:15 pm

The hum started after changing the power tubes to a new set of JJ 6L6s and GT EL34Ls. Afterward (to test) I swapped in a set of TAD short bottle 6L6s in channel 1 and JJ 6L6s in channel 2. Channel 1 was quieter than 2, so I swapped the power tubes to the opposite channels; then 2 became quieter than 1.
I did try the RT2/50 briefly with no fan installed; the hum was still there. I'll take it to a tech and have him/her look it over.

I'm keeping an eye on the fan in the power amp; the space inside the RT2/50 is maybe 1 cubic foot total. If it gets too hot, I'll bite the bullet and get a fan that puts out 46CFM and hope for the best noise-wise. My M4 is noticeably quieter.

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Post by bduersch » Thu Dec 30, 2010 7:33 am

How well matched are the pairs of power tubes? If the hum is following a particular pair of tubes, they may be the culprit.

--B

Bucksears
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Post by Bucksears » Thu Dec 30, 2010 9:31 am

Well that's the interesting thing.
After reading your reply, I just powered it up and tested 1) that they were in the correct operating voltage range and 2) did the red-lead on test point 1 and black-lead on test point 2 to measure how closely they were matched.

The quiet pair was actually much further apart (5.8mv) than the louder pair (1.2mv).

It's cooling down right now - I'm going to swap in the EL34Ls for the lesser-matched/quieter TAD 6L6s and leave the closer-matched/louder JJ 6L6s in there, take some readings on the EL34Ls and then take it to a tech.

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Post by bduersch » Thu Dec 30, 2010 11:16 am

Bucksears wrote:The quiet pair was actually much further apart (5.8mv) than the louder pair (1.2mv).
Freaky... that's totally the opposite from what I'd expect.

--B

Bucksears
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Post by Bucksears » Thu Dec 30, 2010 12:57 pm

Yeah - the EL34Ls that I put back in were actually closer matched than the 6L6s. Both were less than 2mv apart.

It's at the tech's shop now - we'll see what he has to say.

Bucksears
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Post by Bucksears » Sat Jan 01, 2011 10:24 am

Got the RT2/50 back from the shop, he said nothing was wrong with it. The guy does a lot of work on electronic musical equipment in the area and knows his stuff, so I trusted him; he gave me a couple of tips to check on my other equipment and said to call him if it persists & we'll troubleshoot further. He said to check whatever I'm plugging into the RT2/50 (the Egnater M4).
Got home and flipped the ground switch on the M4 from NORMAL to LIFT and that fixed it. I'm only out $24 for him to go over the Randall and I'm perfectly fine with that.

The thing that still puzzles me is that the Egnater manual says to leave it in NORMAL unless it's in a rack. (from the manual - "Typically set for NORMAL when free standing and LIFT when used in a grounded rack setup. Set for the least amount of audible hum.")

Hope I don't fry anything (including myself with this setup).

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