Rectifier damaged?

I was playing with the triode and pentode switch the other day on my VTA ST70. Basically I turn off the amp, switch the mode to triode and turn it on again, but I did it quite quickly within 1 or 2 min of turning off the amp. A couple cracking noises from the tubes was heard when I turn the amp back on but it plays fine. The sound seems to be from the rectifier. When I check the bias again, the bias of all 4 power tubes drops to something around 0.2V from 0.4V originally. If I replace the rectifier with another one, the bias goes back up. Sounds like the rectifier is partially damaged but not totally, any idea is appreciated?

Switching off and on is not want the rectifier likes. At least wait until cathode has cooled down.
Switching triod/pentode don't need to power down ( i think).
Consider that GZ34 toast, don't use it again.

By any chance, did you do Roy's "yellow sheet diode mod"?
It protects from current traveling backwards from the capacitor into the rectifier at power down.

See post 5 here:  https://dynacotubeaudio.forumotion.com/t1518-soft-start-rectifier-for-st70

peterh wrote:Switching off and on is not want the rectifier likes. At least wait until cathode has cooled down.
Switching triod/pentode don't need to power down ( i think).
Consider that GZ34 toast, don't use it again.

Lessons learned. Luckily that is not an expensive NOS rectifier.

New2Tubez wrote:By any chance, did you do Roy's "yellow sheet diode mod"?
It protects from current traveling backwards from the capacitor into the rectifier at power down.

See post 5 here:  https://dynacotubeaudio.forumotion.com/t1518-soft-start-rectifier-for-st70

Unfortunately no. Should get that done earlier...

And yes ... no problem hot switching the TP settings ... power down not required.