[time-nuts] Vintage Frequency Measurement

[Alex Pummer]

just be careful, because if you under-heat the cathode you could kill it

73

Alex


On 2/13/2017 7:11 PM, Bob Camp wrote:
> Hi
>
>
>> On Feb 13, 2017, at 8:15 PM, Scott Stobbe <scott.j.stobbe at gmail.com> wrote:
>>
>> On Mon, Feb 13, 2017 at 6:41 PM, Bob Camp <kb8tq at n1k.org> wrote:
>>
>>>
>>> I think what you would find is that it *is* a fairly normal AT cut and the
>>> data book
>>> that came with the instrument plotted out the data for the specific
>>> crystal in
>>> the device. The usable temperature range was fairly small, so the plot will
>>> be pretty linear.
>>>
>> Attached is a plot of crystal calibrators temperature stability. Span is
>> roughly 65 degC.
> Which eyeballs out to be pretty close to an AT. Without knowing the PPM
> scale there isn’t much way to be sure.
>
>> One of the other aspects I think is intriguing is the DC PSRR of a vacuum
>> tube crystal oscillator. In the case of a bjt based oscillator you have the
>> C-V relation for depletion capacitance and the base-emitter dynamic
>> capacitance as a function of collector current. I would suspect that for a
>> one active device oscillator, tube vs bjt, a tube crystal oscillator would
>> be less sensitive to small power supply variations (+- 10% ).
> Except you *do* have miller effect which pretty much messes things up
> for a triode. A pentode is a bit less sensitive, but you still have issues.
>
>> Which is a
>> convenient attribute for a poorly/unregulated battery supply in the vacuum
>> tube case. Unless filament current has an appreciable impact on frequency,
>> I wouldn't think so…
> Umm… errrr …. check it out :)
> Oddly enough, I remember a high school physics lab where they had us plot
> the effect of filament voltage on plate current and gain. Seemed like a weird
> thing to do to me at the time. Turns out the teacher grew up with microwave tubes
> that were tuned by varying the filament. Who knew ???  Pretty strange stuff if
> you ask me.
>
> The bigger issue is the tubes get hot. The heat varies with supply voltage.
> Temperature change is the result. That temperature change messes up
> oscillator stability. You pretty much have to wait for things to hit equilibrium
> before you do useful stuff ( = let it warm up for an hour or four).
>
> Bob
>
>
>
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