[time-nuts] Oscillator temperature compensation

Joseph Gray jgray at zianet.com
Fri Jun 21 09:21:41 EDT 2013


Charles, you obviously are familiar with this transmitter. You don't happen
to know where I can get a schematic, do you?

Joe


On Fri, Jun 21, 2013 at 5:07 AM, Charles P. Steinmetz <
charles_steinmetz at lavabit.com> wrote:

> Joe wrote:
>
>  I have an old AM transmitter that has three 6AL11 compactrons. The crystal
>> is a fundamental, cut for 660 KHz. I don't have a schematic for this
>> thing,
>> but I believe that one half of one 6AL11 is used for the oscillator.
>>
>
>  The problem is, the frequency decreases as the rig warms up. It will
>> eventually stabilize, but the final frequency is over 200 Hz low.
>>
>
>  It is an LPB RC-6A carrier current AM transmitter.
>>
>
> The LPB 6AL11 transmitters use the pentode section of the tube as an
> oscillator.  There is a capacitive divider to ground from the screen, and
> the crystal is connected from the midpoint of the divider to the grid.
>  There is a small capacitor from the grid to ground that tunes the crystal.
>  On some models, it is a selected mica cap in the neighborhood of 40-50pF.
>  On others, it is a ~33pF fixed mica in parallel with a ~30pf (max) mica
> trimmer.
>
> You should be able to get it more or less on frequency unless the crystal
> has aged too far, but it is always going to drift badly unless you redesign
> it.
>
> Best regards,
>
> Charles
>
>
>
>
>
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