>
>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.
>
RF Parts sells graphite-anode 3-500s at a reasonable price. Heating
causes a graphite anode to release stored gas.
>
end
- Rich..., 805.386.3734, www.vcnet.com/measures.
end
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