I'm the proud owner of a VTA ST-70 (with stepped attenuator and cap upgrades) that I built from the kit about 8 years ago. I did the "yellow sheet" diode mod a couple of years ago, as I did have a few rectifier tubes die up until then. With the new new 1N4007 diodes in place, I tried a JJ GZ34 rectifier, which was cheaper than the more well-regarded Gold Lions I had been running. This tube lasted about 2 weeks, and then failed with quite a light show that also took out the fuse on the amp. I switched back to the Gold Lions and everything was ok for a year or two until recently, when one of these failed in similar fashion to the JJ (light show and blown fuse). Since then, I haven't been able to get my tubes (Gold Lion KT77) up to the proper bias reading. With the trim pots turned full clockwise, I'm only seeing a max of ~0.42 volts on my multimeter at all of the test points. I suspected one of the rectifier tube failures might have damaged something, so I checked the 1N4007 diodes (yellow sheet mod). These were indeed both dead (failed closed), so I swapped in new diodes. This didn't fix the bias issue though. I thought the KT77 tubes might just be old (work and taking care of kids has caused me to be less attentive to this sort of thing lately, and I haven't been keeping up on checking bias and remembering how long tubes have been in place in the amp), so I got a fresh quad of Gold Lion KT77s. Upon installing these, the max bias reading I am seeing is now even lower than it was with the old tubes in the amp.
I've tested all the 10 ohm resistors at the bias test points, and these all read ~9.8 ohms on my DMM. Does anyone have suggestions of other things I should test in the circuit to try and track down the problem? The failure of the JJ rectifier did involve a loud buzzing before the fuse blew, so I'm a little worried that a transformer might have been damaged. I think the faint sound of transformer vibration when the amp is running became more noticeable after this failure, but hopefully I'm imagining this...
Thanks in advance for any advice anyone can offer!!