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kt88 base temperature

hi , the other day I noticed that when I tapped the side of my tube amp , I could hear my output tubes ringing and rattling, as if they were rather loose. I know that loose
connections are not good for sound , it causes distortion.
so my solution to dampen the tubes was pure silicon tape, rated to at least 500 degrees rapped around the base of the tube, now when I tapp my amp it is dead quite, but I am worried about how hot the base of the tube might get , I dont want melted crap in my amps. has anybody else tried this ? could rapping the metal base at the bottom of a kt88 cause shortened tube life ? If this helps the sound I will let everyone know how It turns out. thanks, chris
chrissain
8 responses Add your response
swampwalker's avatar
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swampwalker
5,132 posts
 
What you heard was microphonics. try tube dampers rated for output tubes.
nsgarch's avatar
nsgarch
2,517 posts
 
Since you were tapping on the chassis, how do you know it is a power tube problem? Usually, microphonics develop in the gain/driver tubes (most often the smaller 9-pin twin triodes like 12 (AX,AT,AU) 7's, etc.

Try tapping the top of each tube individually with a WOODEN instrument like an ice cream stick or a pencil (not the eraser end ;-) to find out which tube(s) it really is.

Personally, I don't like dampers, but . . . . My position is either a tube is microphonic or it isn't. If it is, I replace it.
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4est
507 posts
 
I am with Nsgarch for the most part. Microphonics used to be less of an issue in the day, as they were selected out and often rebranded. If the tubes are truly loose in their sockets (common) you should carefully tighten the pin connections of the tubes. Tube dampers usually shorten the life of the tube by not allowing the heat to escape...
cyclonicman
693 posts
 
Sounds like you've had this amp for a while. If you only hear the noise (microphonics) because you intentionally rapped the side of the amplifier, I would do nothing. If you hear it while the amplifier is sitting quietly, absent of any external forces, than I would consider following Nsgarch's advice. Personally, I would not chance placing anything within the amp that could potentially melt. I have even read somewhere that you should not use tube dampers on output tubes because of the heat factor. In my experience, I have been fortunate in that I have never had microphonic problems with output tubes in my amp. Wish I can say the same for the smaller gain tubes in my preamp.
chrissain OP
425 posts
 
thankyou for your replies, I have done my own research and have found that the base of a kt88 does not get anywhere near 500 deg so I am in no danger of the silicon tape melting, also since the tape is only on the base of the tube and not the glass, it should not effect tube life, however I am really suprised at how it changed the sound, more testing will be needed to see if it makes the sound better or worse, thanks, chris
newbee
4,093 posts
 
When you did your research did you find any mention of the possibility that the tape might inhibit the ability of the metal base to dissapate heat? Maybe that limited dissapation, as small as it might be, could change the internal temp of the tube's components and change its sound. Just a thought, FWIW.

Interesting........
chrissain OP
425 posts
 
hmmmmm, interesting theory, but I would think that the room temp would have a bigger change ie summer vrs winter ect.I think that dampening the tube base would change the resonant frequency of the tube, and that could change the sound. from my understanding any resonance in a tube is a bad thing, it gets amplified along with the signal and causes distortion. in a couple of days I will take the tape off and see what the difference is, If anybody else has any information on tube dampening I would greatly appreciate it ! thanks, chris
benie
147 posts
 
I have used Teflon tape on some metal base tubes with good results. I believe the damping of the metal by the Teflon helps some.
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