[Amps] Triode vs Tetrode Amp

[Tom W8JI]

> 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