feedback/oscillation only when in cabinet

[Lindz]

I have an 18 watt TMB combo build and 2 x 12 cab for it. I noticed after building it when on the bench I was able to introduce high pitched feedback by diming the gain and master even with no volume on my guitar. Backing off to about 8 on the master would stop it.

Figured I had some oscillation going and with some cues from the wreck pages, played with lead dress, stranded wire on V1, shielded cable from pot to preamp tube and jacks etc. Managed to reduce hum, hiss and now can dime it with no feedback and way less microphonic noise on the bench while tapping with chopstick - v1 will still "ping" a bit if I tap it, as will the chassis around it but all in all a great improvement. It does not feedback, just a metallic ping.

But if I load it in my cab, it will exhibit the same high pitched feedback at very high gain settings, even with the volume on my guitar off. I've done some tube swapping as well and have a couple pretty microphonic resistant tubes but it still feedbacks

Obviously there is going to be way more vibration in the cab and the preamp tubes are only an inch or 2 from the speaker magnet (would this introduce high pitched feedback?) but I am curious what people might suggest to dampen it more or other suggestions to tame oscillation like this even more

thanks!

[Milkmansound]

I've had a problem like this before - not totally the same, but similar.

An amp worked fine, but every time I put it into the cabinet the first preamp tube stopped working! Drove me nuts - the amp worked on the bench, and was dead in the cabinet!!

It was a loose heater connection - took a while to find. For some reason it seemed to be the cabinet that was bad... but obviously thats impossible.

My advice is to check everything that might get jarred even a tiny bit while loading the chassis into the cabinet.