The documented value for B+ for the VTA MK3 board is +400VDC. This is the value of the voltage from the power supply. The plate voltage for the first 12AT7 is 150VDC and 300VDC for the phase inverter(second 12AT7). Hope this helps.
On the Mark III with a VTA driver board anything between about 400 VDC to maybe 430 VDC is normal for the B+ line coming into the driver board at the driver board's "B+" eyelet. Solid state rectifiers and/or a higher incoming AC line voltage will give a B+ near the upper end of this range.
Hi, just wanted to say thanks for your help. The Mark 3 twins have been up and running for a couple of hours and I really like the result. I've had a ST 70 with the VTA high gain board for several months and I was so impressed the twins got the same treatment. Many thanks Gary