[Amps] Grid to Cathode Short

Tom W8JI w8ji at w8ji.com
Mon Jan 8 09:33:29 EST 2007


> I have a Tempo 2002 that uses the 3cx400/8874 tubes.  All 
> of a sudden I
> have an extremely high swr when the amplifier is on line, 
> (in fact I can
> not put it on line.)  Plate current is around .4 amps and 
> voltage is
> 2400VDC.  Grid current is around 10 MA and never 
> increases.

I assume that is without the transmit relay closed?

I'm not familiar with that amp but the normal result of 
shorted tubes is the grid current goes negative while the 
plate current stays high even with the trasfer relay open. 
Thias is because the grid meter returns a portion of the 
cathode current to the B-. While the grid normally returns a 
negative voltage when there is a short it returns a 
positivevoltage from the cathode so the grid meter reads 
backwards.

A forward reading grid meter generally indicates another 
problem like an electrolyic (or power supply) negative 
terminal to chassis short.

 Is there an on-line schematic of that amplifier?

73 Tom 




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