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[Amps] 3-500Z filaments

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Subject: [Amps] 3-500Z filaments
From: "Jim Monahan" <>
Date: Sun, 23 May 2010 13:01:04 -0400
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I would like to relate a particular experience I had with my SB-221 several
years ago relating to the filament pin melting issue that Lou mentioned.

At that time, the tubes were the Eimac versions. Shortly after building
it, I found, one day, that a filament had gone out.

In this amp, the filament pins are in line to the air flow. The pin farthest
from the fan had its solder melted.

To my amazement, I discovered that there was no pin to wire lead contact
when the solder was evacuated. In other words, part of the connection 
for the filaments was through solder.

I checked both pins on the pair and a pair of backup tubes and found that
this was consistent in every case.

If you removed the solder, there was no physical contact from the pin to
the filament down lead out of the tube envelope.

As a short term cure, I did three things.

I found solid bare hookup wire just large enough to "stuff" the gap between
the pin and the filament lead, checked for continuity and then resoldered them 
for all four tubes.

I used conventional 60/40 solder.

Since cooling is some kind of issue in many ham amps, I decided to spin the
tubes (sockets) 90 degrees so that the filaments are now both facing the
air flow rather than having one pin in front of the other somewhat
obstructing air flow.

I obtained a larger fan that fit perfectly with a higher CFM rating and 
installed it.
I don't recall the specs right now.

There was no further problem with filament pin solder melting.

A few years later, I replaced the Eimac tubes with Amperex 8802's which have a
solid filament connection.

Jim, K1PX

K1PX at

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