Hi Jeff,
Here are some thoughts on short circuit current that others may be able to
expand upon.
Let me use my SB220 as an example.
The plate voltage is about 3,000V and the effective power supply output
capacitance is
25fuF. I have a 15 ohm glitch resistor in the plate circuit. Allowing about
5 more
ohms of circuit resistance and 10 ohms for the gridtoground short, the total
resistance
is about 30 ohms. This is just a rough guess. The initial current is about
3,000V/30ohms or approximately100 amps. The time constant with 25uF and 30
ohms is
~3/4millisecond. At the end of one time constant, the voltage is now 1,125
volts (37%
Vmax).
The AVERAGE current during the first time constant is C*(deltaV)/(deltaT)=
25e6*(3,0001125)/.00075= 62.5 amps. During the second time constant (37% to
13.5%
Vmax), the current will be 25e6*(1125V405V)/.00075 = 24 amps. I have ignored
the
transformer because I am assuming that the inductance is large enough so the
transformer
does not contribute much to the current flow during the first two time
constants.
If the inductance of transformer secondary is known, the transformer's
contribution to
the total current could be added in at the appropriate time.
This should provide a starting point. In general, I would expect that the
current will
be under 100 amps and decay towards zero in a few milliseconds.
Gl & 73
.... Bill K3HZP
 Original Message 
From: "hzp_electronics@juno.com" <hzp_electronics@juno.com>
To: wa1hco@gmail.com
Subject: Re: [Amps] Modern OpAmp shunt stabilizator power supply (for tetrodes
cr
Date: Thu, 3 Mar 2022 18:48:34 GMT
Hi Jeff,
Here are some thoughts on short circuit current that others may be able to
expand upon. Let me use my SB220 as an example.
The plate voltage is about 3,000V and the effective power supply output
capacitance is 25fuF. I have a 15 ohm glitch resistor in the plate circuit.
Allowing about 5 more ohms of circuit resistance and 10 ohms for the
gridtoground short, the total resistance is about 30 ohms. This is just a
rough guess. The initial current is about 3,000V/30ohms or approximately100
amps. The time constant with 25uF and 30 ohms is ~3/4millisecond. At the end
of one time constant, the voltage is now 1,125 volts (37% Vmax).
The AVERAGE current during the first time constant is C*(deltaV)/(deltaT)=
25e6*(3,0001125)/.00075= 62.5 amps. During the second time constant (37% to
13.5% Vmax), the current will be 25e6*(1125V405V)/.00075 = 24 amps. I have
ignored the transformer because I am assuming that the inductance is large
enough so the transformer does not contribute much to the current flow during
the first two time constants.
If the inductance of transformer secondary is known, the transformer's
contribution to the total current could be added in at the appropriate time.
This should provide a starting point. In general, I would expect that the
current will be under 100 amps and decay towards zero in a few milliseconds.
Gl & 73
.... Bill K3HZP
 Original Message 
From: jeff millar <wa1hco@gmail.com>
To: Steve Thompson <g8gsq72@gmail.com>
Cc: Amplifier Mailing List <amps@contesting.com>
Subject: Re: [Amps] Modern OpAmp shunt stabilizator power supply (for
tetrodescr
Date: Thu, 3 Mar 2022 10:14:46 0500
Consider the arc as a zero ohm resistor. The current peak is limited by
the glitch resistor and the impedance of the HV caps
jeff, wa1hco
On Thu, Mar 3, 2022 at 8:35 AM Steve Thompson <g8gsq72@gmail.com> wrote:
> A while ago I breadboarded a screen regulator supply with some
> similarities to Jim's design. One of these was that the 'pull down'
> transistor could act as a clamp if there was a flashover to the screen.
>
> I was fortunate enough not to test that function but I had a couple of
> questions which I never found answers for:
>
> How much current flows during a flashover to the screen (assuming the
> amp has a sensible glitch resistor in the HV supply) and
>
> How long does a flashover last?
>
> Worst case guesses are simple enough but I don't know how the physics
> of the arc affects things in practice.
>
> Steve G8GSQ
>
> On Mon, 28 Feb 2022, 21:37 Jim Garland, <4cx250b@miamioh.edu> wrote:
>
> > Attached is the screen voltage regulator I designed for my 160m/80m
> > duoband amplifier using three GU74B tetrodes. From your msg, I suspect
> > it’s similar in some ways to the circuit you’ve described. The circuitry
> is
> > deceptively subtle, however, in that it provides protection against
> damage
> > from gridplate flashovers in the tubes, which can be a problem,
> especially
> > with new (NOS) Russian tetrodes. I’ve used this circuit for more than
> five
> > years and found it to be always stable, with tight regulation. The
> > circuitry mounts on a small file cardsized PCB. The screen voltage is
> > adjustable and I set it at a nominal 340V.
> >
> >
> >
> > If the attachment doesn’t come through (I’m away from my home QTH and
> have
> > limited internet coverage, here’s a link to the complete schematic
> package
> > for the amplifier. Pse email me if you can’t get any of the diagrams to
> > load!
> >
> >
> >
> >
> http://www.w8zr.net/160amplifier/images/Amplifier%20Schematic%20Package.pdf
> >
> > 73,
> >
> > Jim W8ZR
> _______________________________________________
> Amps mailing list
> Amps@contesting.com
> http://lists.contesting.com/mailman/listinfo/amps
>
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