Ampex 601 mic preamp volume drop during use

[mrtnasty]

Hi, Ive got an ampex 601 im using as a mic preamp, it has had the signal caps replaced and worked great for years. However, recently it it has been dropping volume during use, and if I yell into the mic, something pops and full volume is restored instantly. I have included an audio sample of this phenomenon. Any ideas what could be causing this?
View attachment cutInAndOut.mp3

 

[pucho812]

1.Have you tried it with different mics?
2.possible bad solder joint or loose tube socket
2.test with a sine wave and trace the circuit to see where it drops signal.

 

[mrtnasty]

1.Have you tried it with different mics?
2.possible bad solder joint or loose tube socket
2.test with a sine wave and trace the circuit to see where it drops signal.

Yes it happens with all mics. Yea It might come to tracing the circuit. However I noticed that if I pull the tubes from the transport electronics, the signal seems to always stays loud.

 

[pucho812]

Yes it happens with all mics. Yea It might come to tracing the circuit. However I noticed that if I pull the tubes from the transport electronics, the signal seems to always stays loud.

Bad tube?