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Problem: Really Bright glowing 6CA7
Problem: Really Bright glowing 6CA7
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Old 24th March 2006, 03:42 PM   #11
commsysman is offline commsysman  United States
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Default intermittent tube problems

My experience is that 90% of the time your problem is a bad /intermittent coupling capacitor connected to the grid, allowing bias to become lost or modified and plate current to increase dramatically. A bad connection at the grid due to a faulty socket is another possiblity, of course; same result.
The first thing I would do is replace all of the coupling capacitors with new ones rated higher in voltage than the originals. In most cases you will no longer have a problem.
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Old 25th March 2006, 06:51 AM   #12
richwalters is offline richwalters
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Just had a new JJ88 fail on me for no apparent reason and yes the anode glowed red hot. Swapping tubes sorted the problem out. A sharp rap on the suspect tube reverted it back to normal but only for a short period. Obviously a poor contact within tube. It was drawing 100uA of bias which is way too high.
Don't aways blame the coupling cap !

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Old 26th March 2006, 11:38 PM   #13
Johan Potgieter is offline Johan Potgieter  South Africa
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I once serviced such an amplifier, and found an ancient selenium rectifier supplying the fixed bias voltage. Do throw that out if it has not been done! Otherwise I would concur with SY (was it SY?)about the tube sockets. Especially for power tubes they are below standard.
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Old 21st July 2006, 02:17 PM   #14
gary f is offline gary f  Canada
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I want to confirm that I seriously think that the problem is the sockets as SY guessed. I swapped tubes, and the problem is not tube related, nor channel related. (i had the problem with both channels at different times, and not the same tubes)

The sockets are corroded a bit, and they hold some tubes loose. Some tube don't fit tightly, but seems to be loose and move a bit, not good.

I will change to gold sockets. They are PCB monted. I guess it's will be hard to unsolder them. I have desoldering braid or something.

I'll post after it's done

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Old 21st July 2006, 09:42 PM   #15
kevinkr is offline kevinkr  United States
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Problem: Really Bright glowing 6CA7
In most cases you can usually tighten the contacts with a jewelers screw driver inserted between the socket body and the pin. (There are some pin types where this doesn't work too well, these look sort of like a tuning fork as an analogy..) Make sure of course that the amplifier is disconnected from the mains and the supply capacitors are all completely discharged. - Use a meter to verify.

Replacing the sockets is never a bad idea, but unless you purchase substantially better quality sockets it is not going to make a big difference. The gold plating on current Russian and Chinese made ceramic pcb mount sockets is very thin and the contacts are otherwise no different than what is used in the cheaper socket.

Good luck!
"To argue with a person who has renounced the use of reason is like administering medicine to the dead." - Thomas Paine
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