Now, here is something that has really puzzled me.
Plate, and grid dissipation is due to kinetic energy of the incident electrons
striking the plate or grid and not
the secondary electrons emitted. So, if you have -10ma screen current at 300
volts, what is the screen dissipation?
It will not be -3 Watts or +3 Watts. Is the screen producing one secondary per
incident electron or more? And how
many are be recaptured by the screen? When we have a negative current all we
can determine is the net sum of the
secondary electrons that are captured by the plate and incident electrons
striking the screen grid.
The plate is easy, unless the screen grid is more positive than the plate all
the plate's secondaries are recaptured.
Secondary electrons are just about always being produced by the plate.
Screen emission is most likely due to secondaries and not due to thermal
emission.
There is some interesting physics going on in these tubes and I am not sure
if there is a clear answer.
If currents and power inputs and outputs neatly match up with the data sheet we
can assume things are just fine.
That is all we can really know for sure.
73
Bill wa4lav
________________________________________
From: Amps [amps-bounces@contesting.com] on behalf of Carl [km1h@jeremy.mv.com]
Sent: Thursday, November 07, 2013 8:26 PM
To: Vic K2VCO; Amps reflector
Subject: Re: [Amps] Question for the tetrode experts
> I've finally applied full drive to my 4CX1000A amplifier. Here is what
> I'm
> seeing:
> I have tried two tubes, both pulls. In one case I can run about 1kW out,
> and
> with the other, maybe 1200. What happens when I try to push past this
> point
> is this: when I hold the key down for 10 seconds or so, the screen
> current
> starts to drop, then goes negative, and when it hits -35 mA the screen
> trip
> circuit kicks in. I haven't seen even a flicker of grid current.
> The screen circuit is well-regulated at 325v, so this isn't case of
> runaway
> caused by increasing voltage. What it seems like to me is that the
> screen is
> heating up and the secondary emission is rapidly increasing. Screen
> current
> before this happens is reasonable, 20-25 mA, so I am not exceeding its
> rating.
> I have a couple more tubes to try, but I have a bad feeling that I am
> going
> to end up buying a new tube if I want 1500 watts from this.
> Do the experts think my diagnosis is correct?
> --
> Vic, K2VCO
It could also be a damaged screen from overcurrent in the past.
Both are rather common tetrode problems where screen current isnt constantly
monitored as a seemingly simple QSY can drastically affect it.
Also running the screen voltage at or close to maximum rated is not a good
idea and I would also give up the idea of 1500W from one
tube...reliably...even Alpha couldnt do it (-;
Carl
KM1H
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