Topband: 8877 Tube

[Tom W8JI]

> The Spec Sheet for the 8877 tube lists the "Absolute Maximum Plate Voltage 
> of 4000 Volts" for the tube, and also says in "typical operation" the 
> plate voltage is between 2700 and 3500 volts. In my amp (Ameritron 
> AL-1500), the plate voltage is 3750.  My question is - should I be 
> concerned (it's clearly below the maximum" but above the range that is 
> considered "typical"?   I just want to be sure I'm not adversely affecting 
> the useful life of the tube.
>

If you are talking about Eimac, they generally obtained "typical operation" 
from a single test system under a variety of conditions. Typical operation 
is what Eimac found in a few test cases, and is not a limit or rigid 
specification. It might not even repeat out in the field.

High voltage does not affect life at all in thoriated tungsten tubes (it 
cannot strip the cathode), and in oxide cathodes (like the 3CX1500A7) HV 
only affects tube life when HV gets so high it strips the protective 
electron cloud away from cathode. This causes cathode poisoning. (This of 
course assumes the tube does not arc, and a typical good 8877/3CX1500A7 will 
hold off 15 kV or more peak voltage so arcing is unlikely in a good tube. )

http://www.w8ji.com/vacuum_tubes_and_vaccum_tube_failures.htm

73 Tom