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Tube acting as microphone

8/13/1997 8:13 AM
Pascal Le Sauteur
Tube acting as microphone
Here's the problem I have. The input tube on the preamp (12AX7) is acting as a microphone. My amp reacts when another amp is playing beside it, and when I checked all the tubes by lightly tapping them, the clink could be heard through the speaker for this tube but not for the other tubes. I then switched the preamp tubes around to see if the tube was my problem, but nothing changed, the first preamp tube was always the reacting no matter what tube was in the socket. Could it be a cap related problem, the amp is over 25 years old and I wondered if the electolyte caps needed replacement... my amp is Traynor Guitar mate reverb and seems very trebly, it also feeds back very easily... any help would be apreciated...  
8/13/1997 9:42 AM
Could be:  
- Ceramic cap is microphonic, as most of them are to some degree or another  
- Solder joint or resistor has some crack or other defect that is sensitive  
First, remelt the solder connections in the circuit leading to that tube, and try it again.  
IF that didn't help, tap on the R's and C's like you did the tubes to see if there is one particularly sensitive one.  
If this doesn't turn something up, you could try a damper of some sort on that tube. Automotive silicone gasket goop works well.
9/3/1997 2:23 PM

I have the same model Traynor and after some mods, it is now my favorite amp. Those tan colored coupling caps in the signal paths are causing your problem with microphonics. Replace them ALL!  
Replace the input tube cathode resistor bypass capacitor with a 1 mfd capacitor. These changes should correct the problem. FYI - I changed the tone control circuitry, to a Fender type, in mine. It made the amp come alive! Only do these changes if you are qualified. (You're on your own!) Good Luck.

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