[ARC5] [Milsurplus] Screen Bias in ARC-5 Transmitters

[Ian Wilson]

Also a great excuse for building a crystal-controlled 1- or 2-tuber and
getting on the air with it :)

73, ian K3IMW


On Fri, Oct 25, 2013 at 1:39 PM, Dennis Monticelli <
dennis.monticelli at gmail.com> wrote:

> John,
>
> I think the single most important test you could run is peak cathode
> emission because this usually how power tubes degrade.  You could take the
> plate and screen to the B+ of choice.  You don't have to use the rated
> plate but you should use the rated screen voltage.  Then ground the cathode
> through a small resistor of 1 to 10ohm and connect that to your scope.
> Bias the grid for cutoff through a resistor of say 10K using another
> supply,   The generator cap couples into that grid and will have to produce
> enough of a low duty cycle pulse to push the grid positive.  Your scope
> will tell you if the tube is capable of meeting its peak current
> requirement by comparing the measured current to the published curves for
> amount of grid voltage you drove it with.
>
> One thing to check first is grid to cathode resistance.  The 1625's often
> have leakage but since the circuit grounds the cathode key-down that isn't
> a big deal unless it is awfully low.
>
> Finally, a great way to test 'em is to stick them into a functioning
> transmitter.
>
> Dennis AE6C
>
>
> On Fri, Oct 25, 2013 at 9:24 AM, J. Forster <jfor at quikus.com> wrote:
>
> > Hi,
> >
> > I am looking at the simplified Modulator print in AN 16-30ARC5-2, Fig 4.2
> >
> > > On 24 Oct 2013 at 17:33, J. Forster wrote:
> > >
> > >> In CW, the VR tube is switched out, so the screen voltage will be much
> > >> higher...  probably 450 to 500 VDC.
> > >
> > > I don't understand: do you mean in your present supply?
> >
> > I have no present supply. I'm looking at how the 1625s were used in the
> > ARC-5 Tzs, and intend to do a test setup roughly emulating the nominal
> > conditions.
> >
> > > There is no VR tube in the original supply, and 450 to 500 VDC screen
> > > voltage would be MUCH too high in every case !!!
> >
> > I never sais 400-500 on the screen.
> >
> > > Maximum rated screen voltage for 807/1625 is 300 VDC.
> > >
> > > What power supply are you talking about here?
> >
> > I have a few DC supplies.
> >
> > My intent is to set the tube up as follows:
> >
> >   Cathode to GND.
> >
> >   12.6 VAC or VDC on heater.
> >
> >   Control Grid to function generator w/ offset
> >
> >   Screen to DC supply, through roughly 1k resistor
> >
> >   Plate to 500 - 550 VDC through 1k current sampling resistor; AC coupled
> > DMM across current sampling resistor.
> >
> >
> >   'Scope on plate.
> >
> > The function gen amplitude and offset are adjusted to give a pretty good
> > current swing on plate.
> >
> > -John
> >
> > ================
> >
> >
> >
> >
> > >
> > > Ken W7EKB
> > >
> > >
> >
> >
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