I use and it is standard procedure to put a suitable MOV from the screen
terminal to ground on 4CX250 VHF amplifiers. In the event of a flashover
the MOV will protect the screen bypass capacitor built into the tube socket.
Otherwise expect to hear a pssfftt, a whiff of smoke followed by a BANG.
With screen operating voltage at 350VDC a 400V MOV at the base of each tube
does the job. I also include a MOV at the output of the screen voltage
A wide variety of MOV's and 50 watt Zeners are available from K2AW at very
----- Original Message -----
From: "Dr Gerald N Johnson electrical engineer" <firstname.lastname@example.org>
To: "Phil Chambley Sr" <email@example.com>
Sent: Wednesday, November 28, 2001 12:31 AM
Subject: Re: [TenTec] Re: QSK Problems
> Not likely that the NE-2 will do much protection. It will probably
> explode at less than 100 ma. current while the PA tube flash over is
> likely to be a few amps. A LARGE 50 volt zener diode may be more
> effective, but it may also protect the transistor by failing shorted. A
> 50 volt or so rated MOV will probably be better protection than the
> zener diode.
> 73, Jerry, K0CQ
> Entire content copyright Dr. Gerald N. Johnson. Reproduction by
> permission only.
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