[Milsurplus] Lowering Receiver B+, Again.

Fri Dec 23 10:16:29 EST 2016

BillYou are right, every case is different.  
Some interesting claims have been made WRT running a receiver on low B+.  It would be great if someone did a quantified before and after test to see what actually happens to performance as B+ is lowered.  Otherwise the claims seem to me to be similar to what the the audio guys make and in the end it all depends on how well one's ears and memory still work!
I guess I am firmly in the "leave it stock condition camp" although I have offered some suggestions that would help protect the power transformer and other high value components from catastrophic failure.  Happy soldering!,Jim

      From: Bill Cromwell <wrcromwell at gmail.com>
 To: milsurplus at mailman.qth.net 
 Sent: Friday, December 23, 2016 6:46 AM
 Subject: Re: [Milsurplus] Lowering Receiver B+, Again.

Hi Jim,

Every case is different and we have to consider what is at hand. In that 
old transmitter the High B+ did arc. There was a carbon track but that 
did not lead all the way to ground - just the jumping off point. Maybe 
there had been a cobweb or something to get it started. There was an 
arc. I had opened the rig and was looking right at it when I applied B+! 
I did not lower B+. Instead I placed the new terminal strip a little 
higher off the deck. I could also have used a bucking transformer to get 
the line back down to what that transformer was designed to use - 110 
volts. My line gets close to 130 volts sometimes. As it is I have just 
over 900 volts DC on the 6146 plates:)

I have used the bucking transformer with other radios. That is not going 
take the B+ from 350 volts down 90 volts. A lot of our boat anchors were 
designed for around about 110 volt AC power lines and now some of us 
have line voltages approaching 130 volts at times. Taking the line down 
to around 100 volts will depress the voltage enough to take the biting 
edge off the B+ and it will reduce the stress on the heaters too. They 
will work fine at somewhat reduced voltage and will last much longer. 
They aren't making most of them any more.

Dave and the rest of us have a lot of options to consider. I'm sure Dave 
will use the best combination for his gear. By the way..I run my command 
receivers on battery power just about all the time. The heaters are on a 
pair of 12 volt rechargeable batteries since the heaters are the big 
power hogs and the B+ is from a series string of 9-volt alkaline 
batteries. Those last for several weeks (months). I buy the cheapest 
alkaline batteries I can find and the cost is not unbearable. That 
approach will not appeal to Dave and won't apply in every case. It does 
support the arguments for using reduced voltages. I recharge the heater 
batteries when they get down to 10 volts (for battery health) and I 
*must* monitor the voltage. The radios do not show any noticeable 
decrease in performance when the heater voltages on those '12-volt' 
tubes get as low as 8 volts. Not good for the batteries!

73,

Bill  KU8H

On 12/22/2016 11:02 PM, antqradio at sbcglobal.net wrote:
> Bill
> What you describe seems to me to be leakage, not arcing.  This happens 
> to phenolic wafer switches, terminal strips and the like when a carbon 
> tract develops on the surface and over time lowers in resistance and 
> then dissipates more and more power.  If it was arcing from poor 
> component placement, why wouldn't arcing have been there from the very 
> beginning?
>
> One question, how you you reduce the high voltage developed by a 
> transformer's secondary winding without affecting the filament 
> voltage?  The only thing that comes to mind is to replace the power 
> transformer with one with a lower HV winding.
>
> Operating an ARC-5 receiver at reduced B+ is quite easy since one 
> needs to build one anyway and I see the advantage but an RME-45 is a 
> whole different matter.  Adding a filter inductor ahead of the full 
> wave rectifier first filter capacitor would work if you can find the 
> room.  The lowest the output voltage will go is 0.90 times the RMS 
> secondary voltage.  Then there is the bleeder resistors that are 
> needed to keep the voltage from rising under light loading.
> By changing from full wave to half wave rectification, the peak 
> voltage is unchanged but is averaged over the the remaining half cycle 
> so the average voltage is much lower (0.45 times the transformer HV 
> secondary RMS voltage).  Depending on how well the transformer was 
> designed and built, it will remain at the same temperature or most 
> likely will get hotter under half wave duty then under full wave duty. 
>  This is due to the increased current needed to recharge the capacitor 
> which will be much higher under half wave operation so transformer 
> copper losses increase.
>
> So to recap (no pun intended!) going from full to half wave rectifying 
> may increase the power transformer temperature rise, certainly will 
> increase ripple voltage amplitude and do nothing to reduce peak 
> transformer HV secondary voltage arcing.  If one then increases the 
> filter capacitor value to reduce ripple voltage, the rectifier will be 
> stressed more then before the change was made to half wave.
>
> There are all kinds of unintended consequences when re-engineering any 
> design.  It has been said many times before, the original designer 
> made compromises for specific reasons.  You have to trust that the 
> compromises were not made to make the cost to manufacture lower but 
> were made for other design reasons.
> Jim
>
>
>
>
>
>
> ------------------------------------------------------------------------
> *From:* Bill Cromwell <wrcromwell at gmail.com>
> *To:* milsurplus at mailman.qth.net
> *Sent:* Thursday, December 22, 2016 5:40 PM
> *Subject:* Re: [Milsurplus] Lowering Receiver B+, Again.
>
> Hi Jim,
>
> Lowering B+ reduces the EMF below that required to arc over from a
> terminal to ground. Been there. The uint was fused just above the line
> current needed for normal operation. When the high B+ did finally arc
> over it sat there and sizzled merrily - stinking and smoking until I
> pulled the plug. I replaced some parts and relocated them farther away
> from ground. The only other choice is to reduce the voltage. Extending
> that concept just to emphasize the point - reducing B+ to zero would
> certainly prevent any chance of an arc over. Something that dramatic
> isn't practical obviously. But reducing B+ from 300 volts (or more) to
> 180 volts won't have any bad effects and will probably not have until we
> go somewhere south of 45 volts. No changes required in the radio. And a
> much reduced chance of arcing over. The tubes last longer too.
>
> You are not required to do anything like this to *your* radios. But Dave
> did ask specifically about reduced B+.
>
>
>
> On 12/22/2016 04:50 PM, antqradio at sbcglobal.net 
> <mailto:antqradio at sbcglobal.net> wrote:
> > 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 
> <mailto:wrcromwell at gmail.com>>
> > *To:* milsurplus at mailman.qth.net <mailto: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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