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

Fri Dec 23 18:08:17 EST 2016

I think we are talking about two different problems with slightly different names.
I can see the transmitting tube problem due to very high plate voltages causing bombardment of the negative cathode with positive ions that get knocked off of the plate.  Makes sense for power triodes with kilo-volts on the plate and this is exactly how silicon wafers are doped to make transistors, IC's and such.  
The cathode interface issue is a low power tube problem which essentially makes an unwanted capacitor between the conducting nickle cathode material and the cathode's oxide coating.  The silicon contamination of the nickle combined with elements in the cathode oxide coating to form this capacitor when the tube was in cutoff for long periods.  This condition affects gm at DC and at low frequencies but at high frequencies, the "parasitic capacitor" allows for normal gm.  So the affected tube would work fine in some circuits and not at all in others.
Fun stuff,Jim

      From: Richard <brunneraa1p at comcast.net>
 To: milsurplus at mailman.qth.net 
 Sent: Friday, December 23, 2016 3:32 PM
 Subject: Re: [Milsurplus] Fwd: Re: Lowering Receiver B+, Again.

Memory is that the cathode problem came about in early computers using 
many 6SN7's.  When sitting long times in the non-conducting state there 
was a resistive build-up in the cathode.  The low voltage problem is 
with low heater voltage the electron cloud around the cathode is meager 
and can be stripped away leaving the cathode unprotected from ion 
back-bombardment which is destructive.  How's that for memory?

Richard, AA1P

On 12/23/2016 04:22 PM, Bill Cromwell wrote:
> 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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