Are the screens getting grounded when you remove the screen voltage?
If not that will cause runaway plate current.
73
Gary K4FMX
> -----Original Message-----
> From: Amps [mailto:amps-bounces@contesting.com] On Behalf Of Roger
> Sent: Saturday, December 28, 2013 12:16 AM
> To: amps@contesting.com
> Subject: [Amps] Runaway Plate Current
>
> Trying to finish up my 6m Junk Box Amp. Quad 4x150a's, 1700v anode,
> ~355vdc
> screen, ~ -55v bias. Input terminated in 50ohms, output terminated in
> 50ohms. Keying the amp with no excitation to adjust idle current all
> goes
> well. Idle current adjustable from ~150ma to 400ma +. Screen current is
> ~
> -8ma depending on idle current. Grid Current is 0. As soon as I unkey,
> which removes the screen voltage, plate current goes through the roof
> forcing an instant shutdown. Anybody have an idea what might be causing
> this
>
> 73, Roger
> AI7RR
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