Todd
To expand on Rich's reply. Amp builders in the 70s and 80s faced a similar
question when deciding what tube to use in that era. There were two choices in
the big bottle arena, the 3-1000Z triode and the 4-1000A tetrode. The 4-1000A
was around since the late 50s being developed in part for radar use. Ham Amps
initial configured them in grid driven service and boy did they work good.
Eimac, for a number of reasons, was encouraged by the government in the late
60s to develop and push triodes on to the market. One of their projects was to
chop the screen grid out of the 4-1000A from which the 3-1000Z sprang. Hams saw
the simplicity of the G-G design and decided to employ the 4-1000A in that
service. And it did work although the 4-1000A required substantially more
drive, 110 watts to the 3-1000Z's 65 watts for the legal .
The 3CX800A7 and the 4CX800 are not in the same family of tubes. Rich has
already pointed out the grid in the 4CX800 is not capable of dissipating the
power developed in G-G service. Something else to consider the filament
voltages for those two tubes are different. The 3CX800 uses 13.5 volt filaments
the 4CX800 uses 12.6 Volt Filaments. .9 Volts may not seem like a big
difference but Thoriated Tungsten cathodes lose 50 % of their lifetime for an
increase of 3% over their rated voltage. Oxide Cathodes are not much better. So
unless your friends are ready to add or change out the filament transformer in
their Amps they had better be prepared to stock up on tubes.
As to your original dilemma about what to do when your 3CX800s give up the
ghost I would not worry. Eimac's product line will shrink over the next 5 to 10
years but the less popular models will go first. eg the 3CX1200 series,
4CX1500A, 4CX600J and the like. As long as there is money to be made you can
bet Eimac will keep manufacturing those tubes. The 3CX800 is used in at least a
dozen widely distributed amps. The 3CX800 is also manufactured by Svetlana so a
second source is available. Todd if I were you I would not even consider
modifying your amp to take a different tube.
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