> Why do some designers use a triode versus a tetrode for
> linear amplifier
> finals? A perfect example is the use of a pair of
> 3CX800's compared to a
> pair of 4CX800's.
Price.
I was involved in amplifier design years ago when the
Russian tubes were available cheap. I decided to never use
the Russian tubes because:
1.) While the tubes were cheap (~$50-100) they required a
screen supply that included good regulation, a matching
system for the grid (if you wanted to have 100W drive be
full output), screen metering, and grid protection. The net
result was the manufacturing cost, IF the amp was properly
regulated and protected on the grids, was the same.
2.) The tetrodes, when we cut through the advertising
hyperbole, had worse IM performance than Eimac triodes. The
trick the manufacturers used was Eimac spec'd the tubes for
worse case IM using the commercial dB below one tone of two
equal tones, while the tetrode people used dB below PEP. You
can see this difference show up when you look at IM
performance of a 3CX800 amplifier like the Ameritron AL800H
compared to a 4CX800 amplifier like the Alpha. The Ameritron
is considerably better for IM performance than the QRO or
the Alpha using 4CX800's despite the fact the tetrode was
claimed to be cleaner.
3.) There was no way to reasonably confirm the tubes would
be available in the future. Look at all the 4CX1600 amps out
there without a tube source! Add to that the fact it is
almost impossible to substitute another tetrode if the
source for a tetrode being used goes away, and it would be a
big risk.
73 Tom
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