----- Original Message -----
From: Joe Subich, K4IK <w8ik@subich.com>
To: <amps@contesting.com>
Cc: Conrad Farlow <conrad.farlow@virgin.net>
Sent: Tuesday, October 17, 2000 12:28 AM
Subject: RE: [AMPS] Screen protection WHAAAATTTTTT!!!!
>
> Conrad Writes ...
>
> >
> >
> > Rich wrote
> >
> > >>Transistors that C/E short offer zero protection.
> >
> > Ok so sense the voltage drop across a resistor across the screen supply,
> > feed the transistor via an opto isolator,make the transistor trip a fast
> > relay in the primary of the supply transformer.This is better and more
> > reliable than a fuse surely.
> >
>
> Make that trip a fast zero crossing solid state relay in the primary
> of the screen supply. The SSR should "open" at the zero voltage point
> and, with the proper MOV to limit the inductive kick-back spike, should
> be the quickest, most reliable protection. Frankly, I would sense screen
> current, screen voltage, as well as plate current and trip both the
> plate and screen supplies in the event of a fault. I might even use
> a big SCR (screen) or triggered spark gap (plate) to discharge the
> supplies to limit the fault (if I could justify the CO$T).
Yes good point about the cost, good ideas none the less.I have a TH347 that
I will be putting on 1296 eme.I think that the cost and scarcity of the tube
in this case is justifiable.I wouldn't go this far for a 4cx250b
however.Still you have me thinking.
Tom I usely have an HT transformer and then a seperate transformer for
grid,screen and control (aux) voltages.I have started to get transformers
custom wound locally as I have found it quite cost effective.I still need to
get big HT caps from the US.How much will an 8kV 40 mfd HT cap cost me
surplus these days?(Winter project).
73 de Conrad G0RUZ
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