I just finished building my ST-70 out of an old unbuilt kit and the low-gain VTA board. Bias adjustments went fine as well as the AC balance. I fed the amp with a sine wave generator and, on my scope, everything looked fine.
I gave the amp a music test and the sound seems very good. There is, however a problem that bugs me: the voltages do not come at all to the same values as shown on the schematic:
- B+ to the center-tap of the output transformers' primaries is 450V which is fine.
- The B+ going to the VTA board is 412V which is 20 v above the specified 390V on the schematic but still acceptable.
- Between R13 and R11, I measure 320V against the 310V; again very satisfactory.
- It is when I measure the voltages on V1 plates that I see a huge difference: 227V (pin1) and 224V(pin 6) when the schematic specifies 150V !
- Also the schematic shows 100V DC at the junction of R27 and R17 and I measure 68V !
- The phase splitter tubes show 272V on their plates against 200V on the schematic !
- Bias voltages are normal at -38V.
Is it possible that the tubes supplied with the VTA board are out of spec? They are ECC81 from JJ Electronic? Those measures are identical on both channels. Could someone help me here with a few suggestions?