Next up, Ian's Stereo 20
It's a slightly later "chocolate" coloured version (earlier ones where gold) and develops 20 watts from a pair of EL84's (6BQ5) driven from an all ECC-83 (12AX7) layout of preamp and phase-splitters. Power is provided from a GZ32 rectifier.
Dating from 1962, it's nicely laid out, and easy to work on. Looks like it's had some repairs in it's life, especially to the left hand channel (top in this picture) ...
Every capacitor is changed out for new, even though the two cathode bypass capacitors in the pre-amp had been changed some time in '97 accroding to the date codes. The main coupling capacitors were dated 1979.
A good number of resistors were outside of tolerance, or not to the schematic, and were replaced. If something was out on the left channel (of which there were many), the corresponding part was replaced on the right, so as to avoid upsetting the balance between channels.
The main cans are re-stuffed with modern 32uF components, and are mounted back using a 3D printed plastic cap.
Whilst the actual foil and (dried out) electrolytic matting were very easy to remove, not so the black sealing goo, which I presume is some kind of bitumen. Leaving the empty cans in the freezer for a few hours (whilst Mrs. Doz was absent!) hardened it, and it was easy to chip off.
The modern components were a tight fit.
You can get the files from https://www.thingiverse.com/thing:4872632
Once everything is done, the unit is powered up, and gives reasonable results, albeit down on power. HT is good, but one side has mismatched valves, and the other side, well, they are just tired. Ian told me that he'd borrowed a valve out of his other amp, as one had gone to air. It really does deserve a new set of output valves.
The guilty parties...
Nice job and thanks for the information on how to keep the original caps and stuff them with modern ones.
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