An essential mod for the SB-220 and SB-221

[wy6k]

The 3-500Z tubes in the SB-220 and SB-221 have a habit of developing grid to cathode shorts that destroy the filament transformer and the grid choke.

There is a simple mod to the bias circuit that prevents such a tube short from doing any damage to the amp.

I've posted an explanation and some pictures on my web site, at www.wy6k.com. I sincerely recommend you mod your amp before it melts down. Alas, I did not and now I'm repairing it. Sigh.

http://wy6k.com/homebrew_stuff/essential_mod_sb220.htm

[ka5row]

Would this be true for any amp that used the 3-500Z. I have a Kenwood 922-A and see very few mods offered for it, most are for Heathkit. I assume that all 3-500 amps are basically the same.

[wy6k]

Would this be true for any amp that used the 3-500Z. I have a Kenwood 922-A and see very few mods offered for it, most are for Heathkit. I assume that all 3-500 amps are basically the same.

Not necessarily for all 3-500Z amps. But it does appear to be true for the 922. The bias scheme on the 922 is exactly like that of the SB220, so it will be affected essentially the same way by a grid to cathode short in the 3-500Z.

However, the grid chokes and bias transformer may not be quite as prone to melting down. I rather doubt it, but I don't have any experience with the 922. Maybe someone who does can comment on that.

If I had a 922 I would implement the mod on it.

[KE5HSW]

first of all the SB-200 doesnt work with 3-500 tubes,,, they use 572b

[wy6k]

first of all the SB-200 doesnt work with 3-500 tubes,,, they use 572b

Where is there a reference to the SB-200?