[ARC5] Drift in BC-453 - more

[Bob kb8tq]

Hi

The R-390A has regulated heaters if the “ballast tube” is still in there ….

Bob

> On Dec 13, 2017, at 12:43 PM, J Mcvey via ARC5 <arc5 at mailman.qth.net> wrote:
> 
> Yes that was mentioned and may indeed contribute to a more constant cathode temp.
> However the old time designers textbook devised ways to make the circuit stable in a way that was independent of the tube characteristics.
> Unfortunately, it ain't an easy fix!
> Heater regulation wasn't mentioned , but it does make sense as a possible help.
> In fact , I have never seen an original design WITH regulated heaters. MAybe the cost vs benefit wasn't there?
> 
> 
> 
> On Wednesday, December 13, 2017 12:25 PM, John Hutchins <jphutch60bj at gmail.com> 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> <mailto:kgordon2006 at frontier.com> wrote:
>> 
>> 
> 
> On 12 Dec 2017 at 0:00, spr at earthlink.net <mailto: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
> >
> 
>   
> 
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