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

[Dennis Monticelli]

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
> >
> >
>
>
> ______________________________________________________________
> ARC5 mailing list
> Home: http://mailman.qth.net/mailman/listinfo/arc5
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:ARC5 at mailman.qth.net
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
>