[Milsurplus] Lowering Receiver B+, Again.

Thu Dec 22 16:50:36 EST 2016

All good suggestions but none address the initial issue, preventing B+ from shorting to ground and I assume protect the power transformer and other hard to replace components such as the previously mentioned band switch.
First off, fuse it!  Put a fuse in series with the center tap of the power transformer secondary.  Fusing the power transformer primary is a good idea too.  I think reducing the B+ to the absolute minimum is just overkill.  Isn't this changing the radio design which is frowned upon on this and other list servers?
The RME-45 seems to be a late 1930's design.  Some use an 80 rectifier and others use a 5Y3.  In either case, the filter capacitors are 10-15-15 uF.  10uF is the recommended input capacitor value for the 5Y3 to keep the initial charging current below the 2.2 amps maximum hot switching current.  Any capacitor value higher then 10 uF will cause a higher capacitor charging current that is above what the 5Y3 can safely tolerate.  
B+ in the high 300 volt range was common in mid 1930's line voltage powered radios, both consumer and commercial.  The Atwater Kent model 55 uses a 2 uF input filter capacitor and has a B+ of 230 volts on the audio PA plates.  230 volts / 0.707 gives  a peak voltage of 325 volts not including the rectifier voltage drop.
What to do to preserve the radio?  As mentioned earlier, reduce the applied voltage by auto-transformer enough to keep the tube heaters at the low end of the range.  6.3 volts minus 10% gives about 5.7 volts which is high enough for the tubes to meet specs.  Put 100 or so ohm resistors in series with the rectifier plates to further limit the hot switching current.  If you size this resistor power level low enough, they will fail before the power transformer and offer further protection in lieu of the B+ fuse mentioned above.  The Radiotron Designer's Handbook, #4 on page 99 gives the overview of this technique. This of course will make the power supply less stiff or regulated from no load to full load but I don't think voltage regulation is of concern if the original idea was to reduce B+ to the absolute minimum!
Putting a 10 volt zener diode in series with the high voltage center tap to ground will reduce the B+ by 10 volts so this is another way to reduce B+ in 10 volt increments.Comments?Jim

      From: Bill Cromwell <wrcromwell at gmail.com>
 To: milsurplus at mailman.qth.net 
 Sent: Thursday, December 22, 2016 10:38 AM
 Subject: Re: [Milsurplus] Lowering Receiver B+, Again.

Hi David,

There are already some good suggestions. I like the idea of changing to 
choke input AND the bucking transformer. I have used 'reduced' B+ on 
several radios down to about 45 volts. At 45 volts the performance 
decrease is noticeable and would require some circuit changes for better 
performance but is still acceptable (to me). 90 volts seems to be near 
the 'breakover' point and I did not notice any decreases in the radios I 
use at 90 volts B+. Some others have reported that things are better at 
around 180 volts. My main use of 90 volt B+ is for my 'command 
receivers' and I find no difference 'on the air' between 90 and 180 
volts. The 90 volts sags to under 80 volts before I finally toss the 
batteries (after several weeks). I didn't measure anything with NIST lab 
quality measurement gear. Just my model 1A ears.

I haven't done this yet but it might be worthwhile using a high voltage 
pass transistor to regulate the higher B+ down to something around 200 
or a little less for B+. It would not require any serious, permanent 
changes - easily reversible.

Getting that voltage down a ways is probably a very good idea. I have 
used the 45 and 90 volt battery packs on radios with 70 year old caps 
and they wont work on the original B+ but they do at the reduced 
voltages. That tells us something useful.

Good luck with your project. Merry Christmas and..

73,

Bill  KU8H

-waves toward Texas-

On 12/22/2016 09:16 AM, David Stinson wrote:
> We've probably covered this particular issue in discussions about 
> lowering B+ in receivers to help preserve them, but I can't find it in 
> my archives
> and don't remember, so I beg your patience with my aged and leaky head.
>
> I've recently restored a nice RME-45.  Here is a photo of it next to 
> the WWII "Liberty Ship" MacKay:
> https://goo.gl/photos/zwz8CeYmGb9EVR1k8
>
> As originally designed, the output of the full-wave
> B+ rectifier (type 80 tube) at the capacitor-input
> filter is a needless 340V and better than 320V gets distributed. I've 
> already had an insulaton failure at an RCA "accessory" jack which 
> shorted B+ to ground:
> https://goo.gl/photos/So8HJtFZKEeY7mQ29
> (I will be Q-doping the old, oxidizing wire insulation in the future.)
>
> There are nearly irreplaceable band switches which
> could suffer the same fate.  So lowering the B+
> to this set is a priority.  While trouble-shooting,
> I determined that the radio would play
> acceptably on AM with as little as 90V for B+.
> I did not test for BFO operation or higher-band LO
> operation at that level- need to do more testing.
> I don't "chase DX" with boatanchors and hold  "preservation over 
> performance," so reasonable
> reductions in performance are acceptable to me.
>
> I know several non-destructive techniques to reduce the B+.
> A "bucking" transformer will not do the job as one which took the B+ 
> down enough would also take the filaments below an acceptable voltage. 
> As a temporary measure, I removed one leg of the
> HV transformer winding from a plate of the 80
> rectifier, converting it into a half-wave rectifier.
> This reduced the B+ at the filter input to 260V,
> giving some temporary "breathing room."
> Performance did not suffer, nor is there any
> notable increase in AC hum in the audio.
>
> Here is my question:
> What, if any, are the pitfalls of converting the
> full-wave rectifier into a half-wave rectifier?
> Overall current draw is reduced in proportion
> to the voltage reduction.  No evident increase
> in transformer heat dissipation.  What are the
> possible long-term issues with this?
>
> 73 DE Dave AB5S
>
>
>
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-- 
bark less - wag more

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