Tube Question
Jan 5, 2005 at 9:14 PM Thread Starter Post #1 of 11

Puddleglum

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Okay I ordered 10 sovtek 12ax7lps for my fisher 400 (1 spare) and four Electro-Harmonix 7868's. When I plugged everything in, I found that there was a scratchy/staticy sound coming from the right channel. That sound is non-existant when I first start it up, and gets louder the longer it's on. So I figured it was one of the 7868's and sure enough, one of them was lit up a little bit brighter than the others. So I turned it off, let it cool off, and then switched the output tubes to see if it was just the tube or the socket. It turned out to be the tube and not the socket because the same thing happened to that particular tube.

Are these the symptoms of a microphonic tube, or is it something else? Should I order another pair of 7868's?
 
Jan 5, 2005 at 10:13 PM Post #3 of 11

Hirsch

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Quote:

Originally Posted by Puddleglum
Okay I ordered 10 sovtek 12ax7lps for my fisher 400 (1 spare) and four Electro-Harmonix 7868's. When I plugged everything in, I found that there was a scratchy/staticy sound coming from the right channel. That sound is non-existant when I first start it up, and gets louder the longer it's on. So I figured it was one of the 7868's and sure enough, one of them was lit up a little bit brighter than the others. So I turned it off, let it cool off, and then switched the output tubes to see if it was just the tube or the socket. It turned out to be the tube and not the socket because the same thing happened to that particular tube.

Are these the symptoms of a microphonic tube, or is it something else? Should I order another pair of 7868's?



In the Fisher 400, that could be a sign of mismatched tubes. One tube could be glowing more because it's drawing more current. This is an unstable situation and could damage the amp. If the tube has an "all over" orange glow, shut the amp off immediately and do NOT use that tube pair. However, it's possible that the tube will be fine if it's paired with one matched to it. The two left 7868's form one push-pull pair, and the two on the right are the pair for the other channel. The two pairs can be different, but the tubes within each pair must be matched, or the amp will not be stable.
 
Jan 5, 2005 at 10:16 PM Post #4 of 11

Len

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Ah, the Fisher 400 is a PP amp (should've figured). You really should buy matched output tubes for PP amps. It's just as much a sonic issue as it is a safety/reliability issue.
 
Jan 6, 2005 at 12:40 AM Post #5 of 11

Puddleglum

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These were matched tubes, and no there is no all over glow. That would have worried me a lot more. It's just that one burns slightly brighter than the others, it's barely noticable. I think it's a bad tube so I'll have to get a new pair.
 
Jan 6, 2005 at 7:19 AM Post #6 of 11

Len

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The brightness really isn't a good indicator of whether the tube is out of wack. But yes, I'd change the pair and see if that fixes the problem (it should).
 
Jan 6, 2005 at 2:09 PM Post #7 of 11

mkmelt

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It could be a bad tube, but it could also be that the tube's pins do not fit in the socket the same as the other 7868s in your amplifier.

If you have some Caig contact cleaner, try cleaning the tube sockets with a drop of the Caig product and a pipe cleaner. Alternately, you can use an old 7868 tube and burnish the tube pins on this tube, then use this tube to clean oxidation from the 7868 tube sockets. I would do the same with the 12AX7 sockets for positions V9 - V12, but a pipe cleaner is too big to fit in these tube sockets, so use an old 12AX7 as with the 7868.

It is also possible that you have some loose pin connectors in your 7868 sockets. Short of replacing the socket, try using a sharp pick to press the open ends of the tube pin connectors closer together.

Try comparing the pins of the offending tube to another 7868. If any of the tube pins are misaligned, that may be your problem. If a pin appears to be bent, use a needle nose plyer and "gently" bend the tube pin back in the opposite direction.
 
Jan 7, 2005 at 9:04 AM Post #8 of 11

Puddleglum

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While I think MKmelt's idea is a good one (and one I should do anyways). I believe that the particular tube is bad. While the other tubes don't make any sound when I lightly shake them, the bad tube does. In addition the Scratching noise gets worse the longer it's on.
 
Jan 7, 2005 at 1:25 PM Post #9 of 11

mkmelt

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Never use any tube that has a loose structure inside the globe. This is faulty/failed constuction that can lead to a short, possibly damaging the amplifier. I suggest that you return that tube pair, marking the bad tube, and ask for a replacement matched pair. In the Fisher 400 you do not need to have both output channel 7868 tube pairs be matched.
 
Jan 7, 2005 at 3:56 PM Post #10 of 11

Hirsch

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Quote:

Originally Posted by mkmelt
In the Fisher 400 you do not need to have both output channel 7868 tube pairs be matched.


I reread the statement. I've experienced some issues with mismatched tubes and my Fisher 400, and prefer not to experience that again. While exact matching isn't critical, the tubes in each pair should be within about 10% of each other. Although the tubes within a pair should be matched, the pairs themselves need not match each other, as I indicated previously.
 

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