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

Fri Dec 23 16:22:11 EST 2016

Hi,

As far as I know that cathode poisoning problem applies to certain 
transmitting tubes with exotic materials in the heater or filament 
(directly heated cathode). The exotic materials may have been in the 
cathode (indirectly heated). I didn't bother to memorize the details 
when I saw that it applies to tubes I will never be using (can you spell 
QRP?). I never go over 100 watts with double 6146 rigs or <horrors> 
transistors. Most of the time power is considerably lower than that. 
There is nothing exotic in any of those nor in HF receiving tubes.

In another post somebody suggested that there should be bench tests to 
evaluate what really happens to receivers on reduced B+ and/or reduced 
heater power. I agree. If somebody would send me a benchful of 
laboratory gear I will be happy to run a lot of those tests and report 
the results. Meanwhile maybe I can get some kind of jig setup to do at 
least some measurements on some radios. I am having enough trouble just 
keeping the radios I have on the air. It's always something. At the 
moment I think it is just incredible that I don't have a two-circuit 
(stereo) 1/4 inch phone plug anywhere in the shack to get the Atlas back 
on the air. Several are in the order I placed a couple of days ago. 
Setup a test jig..yeah - right. What have I been smoking? Maybe some of 
those sound card applications can help. Maybe not.

Merry Christmas and...

73,

Bill  KU8H

On 12/23/2016 11:02 AM, Mike Feher wrote:
> 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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-- 
bark less - wag more