The ST70 has always had a pretty great output section but the driver could use a little more attention. From the hard to find 7199 tube (only made by RCA) to the 60 year old PCB, there is room for real improvement for a small investment.
I had swapped out the original factory assembled driver card from 1962 about 20 years ago? I had used a board from SoundValves which used a 6GH8 but was designed to maintain the original specs.
It was time to replace the electrolytics in the old amp and give it a 100,000 mile tuneup and I was very interested in the approach that VTA took so I took a chance. it uses easy to source 12AU7s in a modern circuit using a schmitt phase inverter with a current source in the tail which is pretty ideal.
All in all, a great value that really made the amp punch way above its weight class. And it was fun to assemble and install.
"Before", the Sound Valves 6GH8A driver board has worked well.
The VTA board going together. Very nice quality, and individual bias pots for each tube.
All 3 driver boards, the original (rear), the Sound Valves (middle) and the new VTA.
New 1 ohm 5W precision resistors for setting the bias. The 50 year old 1.56 ohm resistors came out. They were originally specified so an average buyer could use a battery tester to check the bias! You would read 1.56 volts (like a C or D cel) when that pair was drawing 100mA or 50mA per tube.
That moment in every project where you hear David Byrne's voice asking, "my God, what have I done?"
New speaker terminals too. Those tiny terminal strips had to go.
Almost there!
Trying out some PIO capacitors. I didn't see colors, but they seem fine.
Back in action "after" shot.
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