Hi All
I've had a Pioneer SX-83 in for restoration, and it has been quite a task to get it to shine again.
Here is the top view after all the work.
Firstly I had to replace the output transformers with a matching pair made by myself. Opting for Ultra-linear operation.
All power supply caps were replaced, as well as most signal caps. This took quite a few hours.
Next I ran into problems with the 390V on the 7189 anodes. I could not bias the power tubes, there was always one that had current runaway. I tried cathode bias, but to no avail.
Desperate for a reliable solution, I remembered the Philips PA 6411 used EL81 with fixed bias for quite a decent performance.
The datapage was encouraging, noting that the max anode voltage should not exceed 550V. I installed the EL81's, but struggled again to bias them safely. Not even -40 volt could tame them. I was extremely frustrated, and had a look again at the data page, noting that recommended operating voltage was 250V DC.
Looking at the power supply, I realised that the voltage doubler has a half voltage node in between top B+ and earth. This was sitting at 220V DC.
I moved the transformer center taps to the 220V point. This turned out to be the magic solution. The EL81's all biased perfectly (after adjusting bias pot configuration.)
The sound was powerful and dynamic.
NOTE: the original bias pot arced whenever I switched the amplifier on. Replacing it with a wire wound 5k pot made the difference. Be warned: regular carbon track pot burns out very quickly.
I've had a Pioneer SX-83 in for restoration, and it has been quite a task to get it to shine again.
Here is the top view after all the work.
Firstly I had to replace the output transformers with a matching pair made by myself. Opting for Ultra-linear operation.
All power supply caps were replaced, as well as most signal caps. This took quite a few hours.
Next I ran into problems with the 390V on the 7189 anodes. I could not bias the power tubes, there was always one that had current runaway. I tried cathode bias, but to no avail.
Desperate for a reliable solution, I remembered the Philips PA 6411 used EL81 with fixed bias for quite a decent performance.
The datapage was encouraging, noting that the max anode voltage should not exceed 550V. I installed the EL81's, but struggled again to bias them safely. Not even -40 volt could tame them. I was extremely frustrated, and had a look again at the data page, noting that recommended operating voltage was 250V DC.
Looking at the power supply, I realised that the voltage doubler has a half voltage node in between top B+ and earth. This was sitting at 220V DC.
I moved the transformer center taps to the 220V point. This turned out to be the magic solution. The EL81's all biased perfectly (after adjusting bias pot configuration.)
The sound was powerful and dynamic.
NOTE: the original bias pot arced whenever I switched the amplifier on. Replacing it with a wire wound 5k pot made the difference. Be warned: regular carbon track pot burns out very quickly.
