>
Steve -
>
>Just bought an old dog sb-220 which has some Harbach mods including the
>parasitic kit---smooth as silk 80-10 including 12 and 17. Never owned a
>sb-220 before--guess it's going to be ok.
/\ To avoid the future possibility of a filament to grid short in a
3-500Z:
1. the stock 1mH grid to ground chokes should have been replaced with
1/4w or 1/2w, 24 - 39 ohm carbon film resistors. Also, the grid choke
bypass caps. should be padded with c. 1000 added pF to protect these grid
{fusing} resistors from 10m RF. .
3. there should be a 10 - 15 ohm, 10w vitreous-enamel (glass) coated
resistor in series with the HV+. A ceramic block type power resistor
should not be used.
>The Eimacs in it date 1971 and 72.
/\ One of the Amperex 8163/3-400Zs in my SB-220 was mfg. in 1967, and
it still puts out normal PEP. The reason is that unlike the TL-922, and
four models of Henry Radio amplifiers, the SB-220 does not use excessive
filament voltage. An extra 12% in fil, V reduces emissive life to c.
1/16 of normal.
cheers, Steve
- R. L. Measures, 805.386.3734,AG6K, www.vcnet.com/measures.
end
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