ST-70 Upgrade boards

[TomMcNally]

The complete docs and schematic are in the first message in this forum topic. I can't comment on the VTA, but Shannon's sounds sweet !

[EWBrown]

The VTA uses three 12AT7s and has no CCS regulator. it also has the -V bias circuitry and B+ dropping resistors and filter caps on board, hence it's a bit more complex, and it's longer than the standard ST70 chassis driver board cutout, so it has to be mounted below the chassis, and many of the components must be mounted on the bottom of the board. Otherwise, the audio circuit topology is fairly similar.

I would guess that the following little tidbit would apply to the DIY70 board, as I found it while experimenting with two of the Poseidon (built for MKIV use, PI plate resistors are 43K, 2W) boards:

I tried various 12AU7s and 12BH7As and there was always a 5 to 6 VDC difference between the two plates (254 and 248-2499 VDC, with a 350VDC B+), but when I tried out two GE 6189s, the two plate DC voltages were within half a volt of each other (252VDC, with a B+ of 350VDC).

The CCS cathode voltage was between 13.1 and 13.6VDC, depending on which type tube was being tried.

HTH

/ed B in NH