vta st-120 question

Good evening Bob.

Here are my findings from pulling the three driver tubes in different configurations. All KT88's are in place as well as the rectifier:

- all driver tubes out..dead quiet.
- center tube in only (groove tube). dead quiet.
- center tube still in and right Electroharmonix in..right speaker hum but fairly low in sound.
- center tube in and right and left EH tubes in... Loud hum left. Hum on right side but not as loud.
- no center tube. Left and right tubes in (Electroharmonix). Right channel hum. Left channel almost dead quiet.
- center tube out. swapped the left and right driver tubes. Hum still on the right. Left channel almost dead quiet.

All tests done with the attenuator at zero and inputs shorted. And as before, as the amp warms up the hum seems to quieten down to almost no hum..

Are your suspicions still leading towards the center tube area? Mine were until the hum switched channels. Should I still be looking at the center tube area when I open her up again? Any other members please chime in. ..Cheers...Leigh

I just put the center tube back in and left and right's as well. The hum has switched sides again by inserting the center tube. Right channel very low and left channel quite loud. It's slowly getting quieter as the amp warms.

So after a month of checking everything and rechecking everything, resoldering connections, pulling my hair out etc... I had almost given up hope. I also noticed my Transcendent Sound preamp was starting to sound a bit flat as it has a lot of hours on the tubes and the VTA st-120 was lacking in the bottom end. I decided to swap the JJ's from the 120 with the Jan Phillips from the pre to see if things livened up a bit. On the bench I turned the pre on with the JJ's and I get pops, clicks and hum. Just touching the JJ's made them all micro harmonic. I replaced both the pre and st-120 with fresh Jan Phillips and all is good. No hum, clicks or buzzes from the vta-st-120 or the pre. A good lesson learned is to trust your work unless you are very unsure of your work, which I wasn't. There's something to be said of the tubes from yester year compared to the poor quality of todays tube manufacturing. It's nice to put this thread to rest! Cheers...Leigh

Glad you found your problem and enjoying the amp. Great job on analysis and a big pat on the back for not giving up.