First, I want to thank all the members of this group who provided me so much information while I browsed all the topics. Your discussions were invaluable to my success in this rebuild.
So, this is an ST70 that I acquired a couple decades ago for cheap. It was not in too bad of shape, but definitely not ready for prime time. I dug it out of my storage about 18 months ago and resolved to make it work (and look nice).
The only things I re-used were the three transformers and the chassis. I ordered the VTA board, the stepped attenuator, and triode/ultralinear mode switch kit from Bob and Roy's site. Also got new tube sockets hardware and misc. parts as needed.
The chassis had too much pitting to leave polished without sending it out for refinishing, so I just cleaned and painted it. I cleaned the transformers and gave them a coat of hammered black. I wanted to make it look a little more modern, and to get better spouse by-in :-). So I built a very simple drop in wooden enclosure out of African padouk I had lying around and made mount pounts for the inputs, attenuator, and a power indicator light. Then life got busy, and it sat for months.
This past week, I decided to finish the project, and started soldering. It took a while, since I am slow and methodical.
I did a couple unique things I have not seen elsewhere. Instead of the 4 ohm speaker connection, I connected the 16 ohm to post three to provide a connection point for my powered subwoofer's high level inputs.
And since I would not be using the front power takeoff sockets, I needed a different bias contact points. I added a test lead socket next to each tube. mounted in the existing vent slot (see the small red things in the picture), and a ground lead socket in between the triode/ultralinear swithces. They work great.
I got it done, and no smoke on first power up!!! Just a bad solder on one of the triode/ultralinear switches (easily fixed).
I do have a bias issue I will explain below.
Here is the final result.
Now to my problem.
I cannot get the tubes to properly bias. With the bias pot turned all the way down, I cannot get below 0.45 V with brand new EL34's (one would not go below .65 V). When I tried some 6L6GC's, I could not get below 1.2 V. House line voltage during the tests was at 121 VAC.
The B+ to the driver board is 424V with a brand new 5AR4. The bias voltage between D1 and R39 is -70 VDC.
It plays music with zero hum, and sounds great. But I do want to be able to have proper bias. Any suggestions would be welcome.