jeremy-ca wrote:
>
>The cheap 4CX250 family may still show good power on HF but often
>suffer from secondary emission from the screen. This dramatically
>decreases IMD performance on SSB.
>
Agreed about the IMD; but those tubes will probably have to be pulled
anyway, due to thermal runaway. We learned far too much about this
subject in the Bad Old Days when 4CX250 pulls were the only cheap source
of VHF/UHF power.
Secondary emission makes the screen behave as a source of reverse
current, flowing backward from the tube into the screen supply. In an
amplifier that has poor screen voltage stabilization, the reverse
current will drive the output voltage upward, which increases the anode
current and increases the secondary emission from the screen, which
increases the reverse screen current, which increases the... until the
whole thing runs away.
Depending on the design of the power supply, the results could be
anything from a quiet controlled shutdown (shameless plug:-) to a loud
bang.
After this scenario has happened to a 4CX250 for the first time, the
flash heating seems to cause a permanent chemical change in the surface
of the screen grid wires, which increases the tendency towards secondary
emission and makes it even more likely to happen again.
The only solution is to pull the tubes... and try to sell them to
unsuspecting hams.
--
73 from Ian GM3SEK
http://www.ifwtech.co.uk/g3sek
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