[Amps] 0.82 Ohm Again

[John Finner]

I inherited an SB-221 from the estate of my best friend K4IPV. It was DOA with the R3 0.82 ohm grid shunt open. Joe had told me a number of years ago that it had died and he never got around to trouble shooting it. He did a great job of building it from a kit and it still has the original Eimac metal anode 3-500Z's. He used it quite a bit on 40 and 20 cw. I cleaned much dust out, washed the grim off the tubes and brought the power supply up slowly with a variac before putting the tubes back in. The grid shunt, which I replaced, was open and not burned.

I only use it several times a year in the summer to be heard over the thunder storm crashes on the 75 meter nets. I tune it in the CW position A0, adjusting the drive and peaking the tune and load for max at Ep x Ip = 1 KW. The Bird says 650 out. I normally operate SSB in the CW position, adjusting the mic gain for voice peaks to the 650 watt line on the scope. Under these conditions, I am not talking any color into the anodes. The operating bias is not changed when operating with the lower anode voltage of the CW position and I am not sure what this does to linearity on SSB. Zero bias with a slightly higher ZSAC might be better here than the 5.1 volt currently used for both positions. Then one would have to switch in the 5.1 volt when selecting the SSB position. I have not received any bad reports, but you can not always trust reports. I once very quickly retuned it in the SSB position A0 to Ep x Ip = 2 KW. Bringing the drive up slowly, the Bird with a 1 KW slug gets to full scale before I reach the 2 KW line on the scope. Nothing went bump in the night and everything seems to be working as it should. I realize the higher Ep of the SSB position means more gain and a greater likelihood of a "bump." I only briefly switch the meter to the grid current position to see what the grids are doing, never leaving it there. The band switch does not have a really solid click when changing bands, feels sort of soft. It otherwise looks alright.

Can I assume an arc, for what ever reason, took the 0.82 shunt out and did no other damage?

I have only done Rich's fan oil mod from his SB-220 improvements article and will do all but the QSK stuff.

Will minding my operating P's and Q's and Rich's mods be the best course of action to keep this great old amp in good health?

John, WA4EPI