[ARC5] Drift in BC-453 - more

[J Mcvey]

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> 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   >  
     
 
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