[Amps] Question for R. Measures

[ROBERT MARSTON]

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.