Hi gang,
I am getting ready to rebuild a school club's SB-220 amplifier and could use
some advice. So far we plan to replace the power supply capacitors, diode
stack and metering circuit, add a soft-start and keying interface (using
essentially all of the Harbach SB-220-related products). The amplifier
itself was a gift from an alumni who built it in the 70s, used it for a few
years, and placed it in a closet for the next roughly 30 years. It looks
like it is brand new and has the original Eimac tubes.
Questions:
1. Is there a procedure to out-gas, or otherwise prep these 'dormant'
3-500Zs for operation? Is it reasonable to hope that they will work after
all of these years (they were good when put in storage).
2. Is replacement of other stock parts recommended for preventative
maintenance? In particular, I am wondering if the 5.1V/10W zener, the
carbon comp. resistor in the parasitic suppressors, and some of the HV
ceramic bypass caps are good candidates.
3. Is it advisable to directly ground the 3-500Z grids, instead of the stock
arrangement of grounding through a parallel RFC and capacitors?
4. Are there other modifications to the stock circuit, such as relay timing,
parasitic suppression, etc. that anyone would recommend?
5. Finally, does anyone have a favorite HV DC probe (model, manufacturer)?
I am going to buy one for this project and future amp work (under 10 kV OK).
Many thanks for any advice,
73, Chris KA8WFC
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