[Milsurplus] Fwd: Re: Lowering Receiver B+, Again.

Fri Dec 23 14:17:09 EST 2016

MikeDon't know off hand what contamination is caused by low heater power.  
If you are thinking of the cathode interface or sleeping sickness, I believe that was caused by high cathode temperature which caused silicon impurities to leave the nickle cathode base metal and migrate to the junction of the nickle and oxides used in the cathode coating.  This caused problems at low frequencies but not so much at high frequency because of a leaky parasitic capacitor formed at the cathode interface.  So the same tubes would work in certain sockets but not in others.
I believe this problem was solved in the early 1960's so if you are using tubes made after this date, there should be no issues.  The cure was better quantity control of the impurities in the metals used in tubes and the adding of cirtain impurities that aided cathode activation.  Then RCA developed the "Dark Heater" at about the same time and was able to lower filament temperature by a few hundred degrees which also helped tube life.  Heat always seems to be the enemy of component life.
There are examples of tubes run at low heater voltages such as the 8BN8 used in the Drake 2B receiver.  This tube is run at 75% of rated voltage to minimize the effects of a parasitic diode between the heater and cathode.  Hallicrafters also did something similar by adding a resistor in series with some tube heaters to accomplish the same thing according to: http://www.wb4hfn.com/DRAKE/DrakeArticles/Why_8BN8-2B.htm.
I have also seen the electrolytic capacitor issue that you mention.  It sometimes takes much longer to reform caps that are in circuits with lower then rated voltages.  Experience indicates that the only capacitors worth reforming are in high end electronics where better quality caps were specified.  So military and upper tier radios get the treatment, not consumer grade radios.  They weren't made to survive the test of time.

Here is hoping that everyone finds a treasure under their tree!Jim
      From: Mike Feher <n4fs at eozinc.com>
 To: 'Kenneth G. Gordon' <kgordon2006 at frontier.com>; Milsurplus at mailman.qth.net 
 Sent: Friday, December 23, 2016 10:02 AM
 Subject: Re: [Milsurplus] Fwd: Re: Lowering Receiver B+, Again.

As I recall, lowering filament voltage on a tube is not such a good idea.
While the filament may last longer, some sort of contamination takes place,
maybe between filament and cathode, I do not recall what or the physics,
rendering the tube useless much quicker. Also, reducing the B+ substantially
could have an effect on the electrolytics in the radio which do have a
memory associated with them. This would only be a problem if the radio was
ever attempted to run again on its rated voltage. Merry Christmas & HNY & 73
- Mike  

Mike B. Feher, N4FS
89 Arnold Blvd.
Howell, NJ, 07731
848-245-9115

-----Original Message-----
From: Milsurplus [mailto:milsurplus-bounces at mailman.qth.net] On Behalf Of
Kenneth G. Gordon
Sent: Thursday, December 22, 2016 11:33 AM
To: Milsurplus at mailman.qth.net >> Milsurplus
Subject: Re: [Milsurplus] Fwd: Re: Lowering Receiver B+, Again.

I'm with Bruce on this one, David:

1) Full-wave rectification.

2) CHOKE input (get rid of the input capacitor). You MAY have to increase
the capacitance of the remaining capacitor(s) to keep ripple at a low level,
though.

3) Bucking transformer on the input.

Reducing the filament voltage is not going to effect operation and will
contribute to longevity of the tubes.

You might also consider inserting a resistor in the B+ feed, since the
current draw in receivers is normally pretty constant.

On another note: reducing receiver operating voltages appears to have almost
no effect on operation of the receiver at all.

I have mentioned this a time or two before, but I think repeating it may
contribute to the ongoing discussion:

Many years ago, I read an article in one of our ham magazines in which the
author recounted his experiences with reducing the operating voltages in a
Drake 2B receiver. To make a long story shorter, he reduced the operating
voltages in steps to something like 12VDC from the normal 250 VDC, and found
that other than it taking a bit longer for his receiver to warm up, and the
reduction in audio output power,  nothing else was effected.

He finally raised it back up to about 50 volts, mainly in the interests of
adequate audio output power, and left it there.

The main effect in the operations which he noted was very noticeable
reduction in noise, so much so that signals seemed to "pop out" at him when
he tuned across them.

In my own experiments, I have determined that the optimum (in my opinion)
for operating voltages for our ARC-5 receivers is about 170 VDC, although at
that voltage, one must move the screen voltage feed to the "hot" end of the
divider string.

BTW, the HFO in the ARC-5s at the normal 250 VDC input is operating at a
voltage of 35 VDC. It seems to have plenty of injection to the mixer at a
voltage of as low as 12 VDC, so I really don't think reduction in operating
voltage is going to effect your RME in that regard either.

In any case, in my opinion, 340 VDC is MUCH too high for a receiver!!! It is
not necessary.

Ken W7EKB

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