Making major strides on my tape cartridge recorder tonight. I have it playing again! Audio comes through fine, and the mechanism seems to do everything it should at the right speed.

However, I'm getting a rumbling nose through the speaker. It comes and goes, but seems to reliably come through as it warms up, then gradually comes through less and less as it's on for awhile. This doesn't affect the audio quality, which seems to drown it out when playing.

Youtube link because I don't think I can upload videos here: https://youtu.be/WcWWbE7dVQI

Also, in the future, should I just make one overall thread for a single project? I know I've been periodically making new threads for individual questions, but it seems like maybe that's not the best option. Sorry if I've been flooding the forum.

It seems like when I tape the 6EU7 tube, it gets better. Not consistently, but I notice a change fairly reliably. Could it possibly be a vibration within the tube?

One thread collects info on the project better.

What you may have is a microphonic tube. Any vibration from the motor, the mech, flicking the tube etc. is being converted in to a signal voltage by the tube and passed on to the other amp tubes.

One thread collects info on the project better.

What you may have is a microphonic tube. Any vibration from the motor, the mech, flicking the tube etc. is being converted in to a signal voltage by the tube and passed on to the other amp tubes.

Is there a way to test for that beyond swapping in new tubes?

With it acting up give each tube a hard tap with a metal screwdriver, keeping your ear close to the speaker. You should hear a 'pluunk' sound in the speaker.

With it acting up give each tube a hard tap with a metal screwdriver, keeping your ear close to the speaker. You should hear a 'pluunk' sound in the speaker.

How loud am I looking for? I definitely hear a plunking noise when I tap on the 6EU7, but it's faint. Also it definitely seems to affect the motorboating when I tap that tube.

The louder the plunk the worse the microphonics are. If you got a spare 6EU7 you may want to swap it in.