[Amps] SB-220/221 filament pin issue

[Jim Monahan]

Years ago, I found that the rear filament pins solder was melting with my
Eimac 3-500Z's in my SB-221.

This happened on more than one occasion with non-contest use of the amp.

The pins, of course, are in line to the air flow. And, to my amazement,
with the solder evacuated, there was no contact between the lead wire from
the tube to the pin!

I did four things to resolve the issue:

1. After evacuating the solder, I stuffed thin hook wire in between the lead wire
and the pin so that the filament current was not going through solder. 

While silver solder would have been a better idea, I wanted to use regular 60/40
solder so that the pins would not get hot enough to need it.

2. I rotated the tubes 90 degrees so that the pins were perpendicular to the air
flow rather than in line.

3. I replaced the fan motor with a larger CFM model ( I don't recall now that rating )
to blow more air and installed a small muffin fan in the top of the cover to assist
air flow.

4. Eventually, I installed Amperex 3-500Z tubes which have a different lead to pin
construction that doesn't go through solder.

Needless to say, the combination resolved the problem.

Jim, K1PX

K1PX at msn.com<mailto:K1PX at msn.com>