[ARC5] Dynamotor Voltage Reduction

[J Mcvey]

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


______________________________________________________________
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
  
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mailman.qth.net/pipermail/arc5/attachments/20171105/d0feca13/attachment.html>