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Old 11th October 2008, 04:31 AM   #1
Zap is offline Zap  United States
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Default Chasing my Simple SE problem...

I finally got the courage to try the pair of 6550's again - after noticing a bit of glow I was examining the tube and it sounds like something is loose inside!

So, with the knowledge that the one tube is basically bad I decided I would try and chase down the other problem I'm having (caused by the bad tube being run cherry red for a minute or so the first time I tried them a month or two ago). When I first power up the amplifier there is a static in the left channel (like a noisy record type sound, scratchy and occasional pop). It usually goes away once the amp finishes warming up.

To try and narrow it down I've swapped out the coupling cap, switched the cathode resistors, both the 100 ohm resistors on the tube (the 1/4W and 2W) and also replaced the 220k resistor in the divider at the input to the power tube.

Now I'm wondering if a resistor in the CCS may have been cooked when the tube ran away and probably drew too much current. I'm guessing the CCS is okay since there is no problem once things are warmed up. The values of the resistors all measure okay.

I'd be most appreciative of any suggestions for nailing this problem for good. I don't want to cause damage to my speakers from the cracking/popping but I'd love to be able to listen to the amp regularly again.
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Old 11th October 2008, 04:37 AM   #2
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Have you tried different signal tubes? Perhaps one has a H-K short,or something. Just a thought.
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Old 11th October 2008, 09:11 AM   #3
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I believe I've tried switching out the 12at7 but I will give that a try when the amp is cold. I've been listening all evening and it sounds great after a minute or two passes and the amp is warmed up. I just know that the noise generated can't be good for the speakers.

I remember reading somewhere that CCS oscillation produces the noises in question. I just didn't feel like swapping the CCS between channels since that would entail removing the circuit board yet again to remove the mounting hardware on both channels since I don't have an extra at the moment.
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Old 11th October 2008, 10:07 AM   #4
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It could be bad solder joint that makes a good connection once something is warm and expanded.
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Old 11th October 2008, 01:03 PM   #5
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I've seen a few tubes where there were bits of loose glass floating around inside. I understand it sometimes happens during manufacture. The tubes seemed otherwise normal, but it was annoying to have a tube with a bit of glass inside. Maybe that's what is inside your 6550?

Click the image to open in full size.

Most of my octal power tubes have some amount of "jingle" which I assume originates from the grids. Tapping on them while your ear is close to the tube while usually reveal the noise. I understand this is more or less normal as well. I wouldn't shake any of them real hard, and I usually notice the jingling before I even install the tube in the amp.

I can't imagine how a runaway power tube would damage the CCS. The current running through the CCS is all going through the 12AT7. It has nothing to do with the power tube. Besides, I once looked up the specifications on that IXYS part. The maximum current rating on it is a ridiculous 4 amps or something.
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Old 11th October 2008, 01:57 PM   #6
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Double checked... the IXCP10M45S isn't good for 4 amps, it's good for 40 watts. The spec sheet say 100 mA maximum. Even still, there shouldn't be current running through the CCS into the power tube.
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Old 11th October 2008, 05:28 PM   #7
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Well, thinking a bit more practically I suppose I suspect the CCS out of the simple fact that I've heard they can make these kind of sounds when oscillating - but it doesn't make much sense looking at the schematic again.

A few things got pretty hot when the cap unsoldered itself from the board, maybe another solder joint got screwed up on the tube socket, though I thought I reflowed them all to be sure in the past. I may have to try that again.
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Old 13th October 2008, 06:33 PM   #8
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Well, now I really don't understand - it doesn't seem to make the noise with the tube I'm currently using. But the first few times over the last few days that I applied power it did make a bit of noise (with the newer tube).

Is it possible I have a loose pin in the socket?? Previously, the colder the amp upon startup the longer it took the noise to go away. Powering down and back up shortly after usually resulted in little to no noise...

Any suggestions greatly appreciated - Thanks!
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Old 15th October 2008, 04:00 AM   #9
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I doubt that the CCS is causing the static, but this is a hard one to find. My 300Beast Push Pull amp does the same thing and I have replaced almost every part in it. After 5 years I have learned to accept it.

I have fixed this before in other amps, and I find that it can be caused by just about any component, but some are more prone. I have found noisy components in this order. Sockets, resistors, tubes, capacitors, and semiconductors.

If this was caused by the glowing tube the problem should be limited to the output tube circuitry. The coupling cap seperates the two circuits pretty well.

Try this, remove the 12AT7 and power up the amp from a cold start. Still makes noise? The problem is likely in the output circuitry. No noise? The 12AT7 circuit is likely to blame. Try cleaning the pins of the 12AT7 tube, and putting a LIGHT coat of WD40 on the pins. Sometimes a loose tube socket (try tightning the pins with a pionted object) or dirty pins can make noise.

Radio Shack used to sell "Freeze Mist". It comes in a spay can and will instantly drop the temperature of a component. Assuming that you can still get the stuff, it can be used to find thermally sensitive components. Try spraying each component one at a time while the noise is happening to see if the thermal shock stops the noise, then try each part after the amp is quiet to see if the shock starts the noise. If either of these techniques finds a thermally sensitive component, replace it.

WARNING Don't spray a hot tube! And don't ask me how I know this!
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Old 15th October 2008, 04:53 AM   #10
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Survey Says - The socket has some sort of issue. Why the problem appeared after the failure - possibly just removing/reinstalling the tubes or something.

I can wiggle the tube and loud static is heard in the left channel - weird. Only tapping the tube results in no noise so we're not talking microphonics here. I'm assuming there is a difference in size of the pins or something plus the heat factor between the two preamp tubes.

I just tightened all the pins on the socket and I can hardly wiggle the tube at all anymore, there is a lot more resistance. To boot, even when wiggled there is no scratching in the left channel. Tomorrow I'll try firing up the cold amp with the other 12AT7 and I should know if this was the problem all along.

In the previous thread about this problem mention was made about significant differences in bias between old and new 6550 types. I'd be interested to hear more about this before I officially declare the 6550 tube bad. It may just be sitting on one side of runaway where its mate doesn't. I'll have to ask that a few USA made Tung Sol 6550's be sent my way to verify that this isn't the case.

Kidding aside, I'm happy to possibly have a solution to this problem that doesn't involve declaring my Bugle Boy 12AT7 bad as well!
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