[Milsurplus] [ARC5] Dynamotor Voltage Reduction

[AKLDGUY .]

I don't agree with your analysis that the total of the screen currents is
insignificant
compared to the divider current. Using your figure of 80V at the junction,
it's
apparent that 80/7000 = 11 or 12 mA is flowing in the lower resistor.

The voltage dropped across the top resistor is 220-80 = 140V, so the
current in this
resistor is 140/7000 = 20 mA.

Therefore the screen current is 20 - 12 = 8 mA. This is significant
compared to the
divider current (12 mA).

Neil ZL1ANM

On Mon, Nov 6, 2017 at 10:08 AM, J Mcvey via ARC5 <arc5 at mailman.qth.net>
wrote:

> You have probably had those test sets a very long time!
> Never seen one at a Ham fest nor do they show up on Ebay very often.
> I think I saw one for sale on Ebay a couple of years ago.
> Very cool!
>
> Anyway, it shows one of the design mysteries ( at least to me) of the
> ARC-5 radios.
> Notice that the screen shows about 80 volts.( last pic)
> That is measured at the center of two 7K resistors with an input of 220V.
> You might expect 110V not 80V.
> Even re-capped sets draw it down.
> A well designed bias supply should have enough current not to be so
> affected by the screen current branch.
> -or was there some other design philosophy?
> The tubes actually spec 100V screen on the data sheets
>
>
> On Sunday, November 5, 2017, 2:16:43 PM EST, David Stinson <
> arc5 at ix.netcom.com> wrote:
>
>
> Fair Radio sold a ton of these dynamotors.
>
> https://photos.app.goo.gl/VK5Tvb74vaOGUiR12
>
> Sold in both 27 and 14 Volt versions, they were
> originally designed for a Magnavox interphone
> amp that serviced the TCS.  They fit on the back
> of
> Command Set receivers, but they output 285VDC;
> much
> too "hot" for this purpose
>
> https://photos.app.goo.gl/vI5KZ3y961HsTbnr1
>
> A simple way "calm it down" is to open the base
> and insert a 5-watt Zener in the B+ lead,
> cathode toward the dyno and anode to the output.
> Cover it will heat-shrink tubing.
> A 1N5371 will take 285V down to 220V.
> A 1N5378 will drop it 100V, but that might
> be pushing the dissipation of the diode.
> You can select a drop with any of the 1N53**
> series.
>
> https://photos.app.goo.gl/FaydfesDmarD5fQB2
>
> I've run it this way for hours.  No problems.
>
> 73 OM DE Dave AB5S
>
>
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