[ARC5] Drift in BC-453 - more

[Bill Cromwell]

Hi,

The capacitances in the tubes are absorbed into the external circuit. 
The physical size of the elements changes when heat is applied. The more 
heat the larger they get. The spacing may change a small amount too. 
Those two things affect the interelectrode capacitance. Those changes 
result in a change of resonant frequency. One way to control the changes 
(after a warmup) is to limit further changes in temperature. Regulated 
heater supplies will do that.

Changes in the voltages applied to the other electrodes will also 
influence the frequency of operation. The variations from those sources 
can be hard to account for and can vary a lot. In some applications the 
resulting 'drift' is simply ignored. Other applications fail (WSPR). It 
is easier to control the temperature changes than to control the 
undesirable results of temperature changes.

I started using battery supplies to improve isolation between my 
receivers and the power line crud. I noticed the side effect of no drift 
when the blower on my shack heater kicked in. First was a wobble at 
blower startup when the B+ suddenly changed. Next there was a slower 
drift as the heater voltage sagged a little and the tube cooled just a 
bit. The batteries may be viewed as a regulated DC supply. Probably over 
the course of the hours that the heater batteries last before recharge 
the tubes gradually cool and the frequency moves slightly. The same 
applies to the B+. The batteries I use for B+ last several weeks 
typically. I haven't been using the 453 much and the B+ stack is about 
four months along. It still provides full voltage under load (yes I 
checked). Even if the idea of batteries scares you, try regulated DC on 
both the heaters and the B+ for major improvements. By the way, going to 
batteries did greatly reduce the power line crud. The conducted RFI on 
the power line is no longer piped directly into the radios.

I hope this helps.

73,

Bill  KU8H

On 12/13/2017 12:02 PM, John Hutchins wrote:
> Watching the conversation take shape over time:
>
> If the tube is the culprit in MO drift,  I may have miss remembered,  I
> read some where that regulating the filament voltage to the M.O. 6J7 ...
> , and the Detector (12,6)K8 may help in subduing the tube drift, do to
> changes in temperature do to voltage swing.   True or not true?
>
> Thanks
> Hutch
>
> On 12/13/2017 9:54 AM, J Mcvey via ARC5 wrote:
>> I have been musing about the "why" of VT oscillator instability. OK,
>> there's definitely a thermal element, but what parameters of the tube
>> are most affected?
>> I checked the spec on the resonating cap for the BFO and it is +/- 2
>> pf ! So the tube must be the culprit.
>> Not being from the vacuum circuit design era, I took a look at some
>> texts on the subject. It looks like the solution can become quite
>> complex if one desires tight VT stability specs.
>>
>> I'm going to put a cheezy Cmos LC oscillator together and let it run a
>> while to see what kind of drift it produces.
>> There were some 1 Mhz clock modules in the junk box, but there is no
>> clean integer divide down that will arrive at 85Khz...dang!
>> Maybe a 455 Khz resonator oscillator can be padded down to 425 Khz so
>> it can be divided by 5? Or will that cause instability- on not work at
>> all?
>> Has anyone here tried heavy padding of a resonator? Did it work Ok?
>>
>>
>>
>> On Tuesday, December 12, 2017 8:56 PM, Kenneth G. Gordon
>> <kgordon2006 at frontier.com> wrote:
>>
>>
>> On 12 Dec 2017 at 0:00, spr at earthlink.net wrote:
>>
>> > Yes, but it doesn't have the truly horrible AVC LO frequency pulling that
>> 6A7and 6A8 have.
>>
>> True, but both of those were early tubes that the 'K8 was supposed to
>> supplant.
>>
>> > >
>> > >6/12K8 is the absolutely noisest "converter" tube in Terman's entire long list.
>> I hate the thing
>> > >for that, but it does work well enough in the "ARC-5" receivers....
>> > >
>> > >Ken W7EKB
>> >
>>
>>
>>
>> <https://www.avast.com/sig-email?utm_medium=email&utm_source=link&utm_campaign=sig-email&utm_content=emailclient&utm_term=icon>
>> 	Virus-free. www.avast.com
>> <https://www.avast.com/sig-email?utm_medium=email&utm_source=link&utm_campaign=sig-email&utm_content=emailclient&utm_term=link>
>>
>>
>> <https://mail.yahoo.com/?soc_src=mail&soc_trk=ma#DAB4FAD8-2DD7-40BB-A1B8-4E2AA1F9FDF2>
>> ______________________________________________________________
>> ARC5 mailing list
>> Home: http://mailman.qth.net/mailman/listinfo/arc5
>> Help: http://mailman.qth.net/mmfaq.htm
>> Post: mailto:ARC5 at mailman.qth.net <mailto:ARC5 at mailman.qth.net>
>>
>> This list hosted by: http://www.qsl.net <http://www.qsl.net/>
>> Please help support this email list: http://www.qsl.net/donate.html
>>
>>
>>
>>
>> ______________________________________________________________
>> ARC5 mailing list
>> Home: http://mailman.qth.net/mailman/listinfo/arc5
>> Help: http://mailman.qth.net/mmfaq.htm
>> Post: mailto:ARC5 at mailman.qth.net
>>
>> This list hosted by: http://www.qsl.net
>> Please help support this email list: http://www.qsl.net/donate.html
>
>
>
> ______________________________________________________________
> ARC5 mailing list
> Home: http://mailman.qth.net/mailman/listinfo/arc5
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:ARC5 at mailman.qth.net
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
>

-- 
bark less - wag more