The author originating this discussion said that they had one 3-500Z
that was the vertical thin fin plate, Eimac original, and one 3-500G
that has the graphite anode presumably with the horizontal heat
radiating structure. This would look like the Amperex/Covimag tubes that
are no longer made, but are cloned in China. For that matter, the
original 3-500Z from Eimac is long out of production as well.
I think it would be more risky to pair these two tubes together as they
come from different manufacture with different plate physical design.
There is no guarantee that the cutoff bias voltage is identical and that
the mu is the same or within a tolerance. Operating this way might cause
one tube to hog more current, have higher dissipation due to the idling
current (Zsac), if they both share the same cathode bias setting.
It would be optimal to put the older style NOS tube into the amplifier
with one of the original (weak?) tubes to see if it works better. Then
try it with the other original tube. If the two tests show large
difference in power capability, then I would suspect just one of the old
tubes. At least start with this plan, and see how it works, report back
here.
If anyone is interested in Amperex/Covimag 3-400Z or chimneys for either
triode, get in touch.
73 and Happy Holidays
John
K5PRO
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