The NCL 2000 used a pair of 8122s, I believe. The RCA data sheet says a
maximum screen supply impedance of 10kohm for them and does not say that
you can get negative screen grid current. The 4X150 can get negative
screen grid current, so it does need a relatively low impedance even with
the volts off the screen.
Strangely, the RCA data sheet for both the 4X150 and the 4CX250 make no
mention of negative screen current.
Now things like the 4-125, 4-250, 4-400 etc don't have those problems....
Happy New Year
73
Peter G3RZP
========================================
Message Received: Dec 29 2013, 03:59 PM
From: "Carl" <km1h@jeremy.mv.com>
To: g8on@fsmail.net, garyschafer@comcast.net, "'Roger'"
<ai7rogerroger@gmail.com>, amps@contesting.com
Cc:
Subject: Re: [Amps] Runaway Plate Current
OTOH, the NCL-2000 simply removes the screen voltage and hasnt run wild
since its 1963 inception and thousands sold.
Im sure many other tetrode amps do the same.
If the screen supply is switched at the wrong point and/or doesnt have a
low
enough bleeder value it may remain charged long enough to cause problems.
Carl
KM1H
----- Original Message -----
From: "peter chadwick" <g8on@fsmail.net>
To: <garyschafer@comcast.net>; "'Roger'" <ai7rogerroger@gmail.com>;
<amps@contesting.com>
Sent: Saturday, December 28, 2013 5:27 PM
Subject: Re: [Amps] Runaway Plate Current
>I sent basically the same comment four hours ago, but such are the
>marvels
>of the internet, it has disappeared. probably resurface ina couple of
>months.....
>
> But, yes, I totally agree with Gary.
>
> 73
>
> Peter G3RZP
>
>
> ========================================
> Message Received: Dec 28 2013, 09:14 PM
> From: "Gary Schafer" <garyschafer@comcast.net>
> To: "'Roger'" <ai7rogerroger@gmail.com>, amps@contesting.com
> Cc:
> Subject: Re: [Amps] Runaway Plate Current
>
> 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
> > _______________________________________________
> > Amps mailing list
> > Amps@contesting.com
> > http://lists.contesting.com/mailman/listinfo/amps
>
> _______________________________________________
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>
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