In the 90's my dad bought a Dynaco Heathkit stereo system from the late 60's comprised of the PAS3 preamplifier and ST70 Power amp. By 2020 the units were almost 60 years old and in need of some care. Vacuum tubes being a thing of intrigue due to their presence in guitar amplifiers and Hi-Fi gear being another interest of mine, the Dynaco gear - originally sold as a do-it-yourself kit - presented an intersection of two things I wanted to learn more about in a well documented and workable form factor. I started with the PAS 3 preamp, which comes first in the signal chain and has an easier to understand circuit.
The first step in restoring the preamplifier was to repair the power supply. There was a loud 120Hz hum not attenuated by the volume knob, which is characteristic of faulty filter capacitors (120Hz because the rectification doubles the frequency of 60Hz Wall AC). Instead of replacing the multi section can capacitor that was the culprit, I decided to get rid of the selenium rectifier which strays from its original performance with age and loads the transformer unnecessarily. After some research online I found a PCB design by Sheldon Stokes of SDS Labs that fulfilled the exact purpose I needed. The board is designed to add additional voltage overhead and capacitance to the stock capacitors that would have been in the can and parallel chassis mounted caps. The board also has two extra 1N4001 Diodes on it in case I want to replace the 6X4 rectifier tube with silicon when the tube goes out. For safety's sake I rewired in a three prong power cord with proper grounding and got rid of the death cap connected to the chassis.