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So 2 of the 6SN7 tubes make a glass ringing sound in the speakers when I tap on them lightly with a screwdriver (kind of like a bonk, bonk, bonk sound as I tap).

The other two make no noise.

I have never done this before, so I don't know if they did that when they were new.

I clearly didn't hear any pops or cracks.

Just to clarify something:

Those four 6SN7 tubes are supposed to make no noise when the amplifier is on, and you tap on them?

If they do, then they are microphonic, and that is some sort of developmental defect?


Thanks,

Chris

Your description in the prior post (....boing, ringing,....) is normal and expected.
ALL tubes are microphonic, to some extent.
Microphonic noise produced in the two input tubes will be more noticeable, because there is more gain downstream to amplify it.

I would replace all four 6SN7s. If the crackling continues, I would visually inspect all solder joints for loose joint.

Most tube amps are capacitor coupled between the driver and output tubes. If this is the case for your amp you might try removing the driver tubes to see if the popping continues. If it does, the output stage is likely involved and the driver stage is probably not. Also, identify which channel is popping. If both are popping, perhaps a filter capacitor in the power supply is shorting. If the popping is a regular occurrence, I would be happy to invite you over to my house to see if we can determine what is causing the fault.

David

My strong suggestion is to take David up on his offer. You would be hard pressed to find someone who has more knowledge of tube circuits than David.

Tom

Thanks Dave, It's really sporadic. I t hasn't happened in several days. Let me feel it out for a little while, and I will keep you informed.

Chris

Chris,

I understand your desire to debug the amp yourself. But, when an expert like David offers to help, you should take it. Think of it as an opportunity to learn about the amp from a guru.

_________________
Walt