[Milsurplus] Lowering Receiver B+, Again.

[Ray Fantini]

I use autotransformers on the AC primary side of the house for feeding the old stuff. Of more concern then the high B+ I was observing that old radios feed today’s 125 VAC line tend to also beat the filament strings hard. On my old HRO RAS I was seeing around 7.5 volts on the 6.3 volt filament string along with the increased B+ and using a autotransformer found that if I run the radio around 115 VAC input all the numbers look good. In practice set the input to around 100 VAC that results in a 5.6 volt filament but still around 220 on the B+
Think the only possible issue may be in disconnecting one side of a full wave is that you still have the original load on one side of the windings, that along with the need for an increased size requirement for the input filter of the power supply but if you don’t hear any hum there you go. Perhaps a better test would be to look at the AC ripple voltage on the second filter capacitor of the power supply before and after with an oscilloscope and see what that reveals.
There has been much published in the past about running the B+ at a fraction of original voltage at things like 24 volts and I have to think that with only a fraction of the original voltage on things like IF transformers and interstage or output transformers the reduced magnetic field would be such to reduce coupling to an almost useless extent, but not having done this myself cannot say that as a definite. But have not seen much if any decrease in efficiency or gain with reductions within twenty five percent or a third of original voltage in receivers.
Attached is a picture of the autotransformer and meter attached to the HRO, having a local AC meter on the output is always useful.

Ray F/KA3EKH
________________________________________
From: Milsurplus <milsurplus-bounces at mailman.qth.net> on behalf of David Stinson <arc5 at ix.netcom.com>
Sent: Thursday, December 22, 2016 9:16 AM
To: milsurplus at mailman.qth.net; boatanchors at mailman.qth.net
Subject: [Milsurplus] Lowering Receiver B+, Again.

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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