I wrote to Sound City support via email and they recommended I post here.
My SC20 combo is making some weird noises. Randomly, it will start popping and clicking and sometimes whistling. It’s totally inconsistent but when it happens, it persists for a long time. Some days, it doesn’t happen at all and some days it happens constantly. It happens with or without anything plugged into the amp. It seems to be present on both channels and in the reverb signal. I haven’t taken the amp out of the house and it’s always on the same power supply. I wonder if you have any ideas about the cause or solution?
It’s definitely haunted… Thanks for coming to the forum. Bye.
Welcome oldangelmignight!
This sounds like a preamp tube going bad (maybe heater-to-cathode leakage) and possibly microphonic. Can you remove the metal cases and tap on them with a pencil to confirm better? It did sound like the noise and clicks became synchronized with your tapping on one or a few of those tubes.
Also, this is a really good example of a well-posed question. Thanks for going to the trouble of uploading a video.
@Gil any other reasons for clicks and pops from your experience?
@dan Thank you for your response! I just had some time to work on it.
I took the tube covers off and it seemed like V4 (reverb driver) was most microphonic. There were a lot of pops and crackles so it was hard to tell if it was happening more when I tapped. V5 (phase inverter) might have also been a problem.
I replaced V4 with a JAN from TubeDepot. I paid the upcharge for low noise and microphonics, for whatever that’s worth.
I’m realizing now that I got a 12AT7WC and the manual calls for a 12AT7A. Is that going to be an issue? I also got a spare for V5, just in case.
Is it generally recommended to leave the tube covers on?
Your NOS JAN 12AT7WC is probably going to be amazing. The extra letters at the end do mean something but it’s the nerd details about construction etc.
Tube covers help to reduce noise by shielding against stray magnetic and electric fields that would otherwise interacting with the electron beam in the tube. Once you have settled in your tubes, I’d fit them and leave them in place.
