[AMPS] advice on and old SB-220

[Ian White, G3SEK]

Rich Measures wrote:

>>4.  The tubes test good, but could they be gassy from sitting all of these
>>years? I am not very familiar with 3-500Z's, will letting the filaments
>>heat, and just sit there for a few hours, cooking, de-gas the tubes if they
>>are gassy?
>
>In my experiences, gassy tubes do not miraculously de-gas.  

Quite right, no miracles are involved.

If a transmitting tube like a 3-500Z that has been in store for a long
time is found to be "gassy", the possibilities for recovery depend on
where the gas came from.

1. If there has been a leak from the atmosphere outside, the tube is
dead. As Rich says, it will not miraculously de-gas.

2. If the gas has been released by outgassing of the internal surfaces
of the tube, then there is a good chance of cleaning up the vacuum by
letting the anode heat up. This is how the tube is designed to operate. 

Cooking the tubes for a while on filaments seems like a good way to
start. As pointed out in an earlier thread on this topic, the next step
is to cautiously apply HV and run the tubes in idling current so that
the anodes get some direct heat. With the SB-220 this could be done in
two stages, first on "CW" (low HV) and then on "SSB" with more HV and
more idling current.

Unless you can reduce the bias and increase the idling current, the
final bake-out would have to be done with RF drive applied so that the
anodes come up to full heat.

Just a thought... since some amps are less stable when not terminated at
RF, it would make sense to keep 50 ohm loads on both input and output
while the amp is sitting at idle current.

Again, no miracles involved.

73 from Ian G3SEK          Editor, 'The VHF/UHF DX Book'
                          'In Practice' columnist for RadCom (RSGB)
                           http://www.ifwtech.com/g3sek

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