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Old 20th November 2004, 09:08 PM   #1
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Default Pop/crackle in one channel

Hi,

My amp started doing this today, after I dusted it. Mental note - never dust amp again Anyway, it seems like it only happens after the amp's heated up for a while. Mostly it's random pops in one channel, one time it turned into this rushing/crackling sound. I've swapped external cabling and it stayed with the channel. I'll try swapping tubes too to make sure that it's in the electronics, but I suspect it is.

I took a look through the archives. Assuming it's not the tubes (which would be easy to fix/replace) - this sounds like a bad solder connection or a resistor dying, right? The amp's been working fine for several months now with no modifications to it, so could that still be a solder joint? Can a solder joint fail after several months of just sitting in one place and playing music? Is there any way to find out where the problem might be? I guess I could reflow every solder joint for that channel. Is there any way to identify a resistor that might be the culprit? Could it be something else entirely?

One more thing I noticed - if I turn the amp off and then turn it back again within a few seconds, the 5AR4 flashes white for an instant. I don't remember seeing it do that before, and I *think* I've turned it off and on again before. Is that bad for an amp? The last time I had any problem was this, which was several months ago and it disappeared on its own.

Thanks in advance,

Saurav
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Old 20th November 2004, 09:31 PM   #2
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Default Re: Pop/crackle in one channel

Quote:
Originally posted by Saurav
Anyway, it seems like it only happens after the amp's heated up for a while.
There's your key right there

Get yourself a $3 can of freeze-spray from Rat Shack and hit capacitors and resistors until it stops.

To narrow it down, is it the same volume, regardless of volume control? If so, it's after your volume control (assuming you have an integrated amp).
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Old 20th November 2004, 09:36 PM   #3
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Good idea. I'd heard of that before, but forgot about it. It's not an integrated amp.
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Old 20th November 2004, 09:57 PM   #4
SHiFTY is offline SHiFTY  New Zealand
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I would try wiggling the tubes around, i bet you just have a loose tube. 9 pins are esp bad for this, octals never seem to get loose.

Also turning your amp off and on again quickly will destroy the rectifier and output tubes, because there is a massive current surge (no bias and full b+). Wait for it to cool a little.
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Old 20th November 2004, 10:05 PM   #5
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Don't hit tubes with it

I had an old tired 6GH8A, probably removed from a TV, which would do that in Frankenhouse. I think I even tried to record it once. Ah, here we go:
http://webpages.charter.net/dawill/Images/AmpHiss.mp3
Kind of a swishing hissing popping sound. Not really the crackling record it sounds like. Changes a bit when the tube is bumped/moved (as if oxidized) but the pins don't look any different from any others...

Keep swapping tubes and if it follows one, replace it.

Oh, and don't overlook the obvious - the contacts might just be on a spot of crud. Wiggle and reseat the tubes.

Tim
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Old 20th November 2004, 11:30 PM   #6
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Well, when I posted the question I'd removed all the tubes, put the amp upside down on the floor and tapped every solder joint/resistor/cap with a pair of pliers. I also moved apart some of the 9-pin socket tabs that looked like they might touch. It still popped after I did that, and the crackling/rushing sound happened after that too.

I'll try swapping the tubes next. Maybe one of my 2A3s is dying. I'm using Sovteks at a pretty high operating point - maybe they're not quite as super-robust as everyone seemed to believe. Though they have lasted well over 1.5 years now I think. I might drop my B+ back down, go back to the higher cathode resistor, and try fancy 2A3s.

Quote:
Also turning your amp off and on again quickly will destroy the rectifier and output tubes, because there is a massive current surge (no bias and full b+). Wait for it to cool a little.
Thanks. I'd heard that from non-DIY 'audiophile' friends, and I always wondered if there was some basis to that. Makes sense though. My whole reason behind using the 5AR4 was to slow down the initial powering up process.
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Old 21st November 2004, 05:03 AM   #7
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About the surge, that's because the capacitor is discharged by the circuits, but all the tubes (including the rectifier) are still hot and able to conduct. This puts full voltage *directly across the rectifier plates* as the capacitor charges in the first cycle or two of new supply voltage, instead of the usual gingerly climb when starting cold. Most tubes are rated for a hot-switching current in the amperes range, but maybe you've exceeded it.

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Old 21st November 2004, 09:43 PM   #8
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I've had that happen many times in the past. Turned out most of the time to be a bad, loose, or wore out SOCKET. 9 and 7 pin sockets are nortorious for this. Fix: re-tensioning/tightening the pin contacts in the socket with a very small jewelers flat blade screwdriver. If the crackling, popping follows the tube, then the tube's your suspect! If wiggling the tube causes the popping to go away or let's say causes it to go in and out, I'd suspect the socket. And please don't fill you sockets w/s*** like WD40 or some other TWEAK!

Wayne
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Old 21st November 2004, 09:53 PM   #9
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Wayne has a good point there.

Tighten the sockets and leave the WD40 or other crap in the garage where it belongs.

Note* Make sure the B+ is discharged before sticking objects into sockets.
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Old 21st November 2004, 10:15 PM   #10
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No WD40 in my sockets I'd never heard of that tweak before.

The problem didn't follow the tubes. I found a grid stopper resistor on the bad channel that seemed like it was close to the lead separating from the body (you know when the lead starts to twist a little too freely right at the end-cap?). Replaced that, in the process found another resistor that looked close to breaking so I replaced those on both channels. It's playing clean now, let's hope that's all it was.
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