by Bob Latino Fri May 15, 2015 8:54 am
First of all do not worry about the slight voltage difference that you see on pins 1, 8, 5 and 6. These voltages are controlled by the bias pots. If you turn the bias pots one way or the other, these voltages will change.
The voltages that you get across pins 2 an 7 (6.8 VAC which should be about 6.3 VAC) are probably normal if you are using an original Dynaco ST-70 that was designed for voltages that they had in the 1960's of about 115 - 117 VAC and you are now operating the amp on 120 VAC.
The voltages that don't seem right are the B+ DC voltages on pins 3 and 4. Your 416 and 419 VDC readings are low. This is usually indicative of either a leaky quad cap or a weak rectifier. If you have an original Dynaco ST-70 with the original quad cap - just replace the quad cap. Electrolytic caps do not last forever and do deteriorate with age. A weak rectifier tube is less likely than a weak quad cap. Does your amp have a slight hum on both channels whose hum level does not change with the volume control of your preamp? This is another indication that the quad cap is bad.
Bob
