[Amps] Grid to Cathode Short

Robert B. Bonner rbonner at qro.com
Mon Jan 8 12:04:30 EST 2007


I used to own one of those things, but don't have a schematic left around
when it went away.

Yes I agree it sounds more power supply oriented to me too.  Those things
are real toasters...  The PS even in standby cooked.  Something has had to
have let go.

BOB DD

-----Original Message-----
From: amps-bounces at contesting.com [mailto:amps-bounces at contesting.com] On
Behalf Of Tom W8JI
Sent: Monday, January 08, 2007 8:33 AM
To: Bruce Osterberg; amps at contesting.com
Subject: Re: [Amps] Grid to Cathode Short

> 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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