[Milsurplus] Tube Electronics Question

[J. Forster]

> Precision expectations.
>
> The long and short of it is: (1) Are you talking date of manufacture or
60 years later?

Now.

(2) When was the gear produced?

During WW II

(3) Who made it? (4) For what
> purpose was it made and how good did it have to be?

It's a Test Set for LORAN-A. It produces accurately spaced RF pulses.

> First of all, amplitude is largely irrelevant to Loran A (and indeed to
> its
> successors) except for the pedestal level inside the display unit. The
> transmitters were pulse modulated. You identified the transmitter by its
> carrier frequency and the pulse repetition rate.

True, but if you are working on a receiver (DAS-1) that has problems of
gain variation, you want the test source to be known good.

> You determined the line of
> position by the delay between receipt of the master pulse and receipt of
> the slave pulse.
>
> So the key specification is time.

For navigation, yes. Not for testing hardware.

> At the transmitter end time was very accurately determined by crystal
> oscillators and frequency-dividing multivibrators locked to them.
>
> AM Broadcast stations in the 1960s had no trouble maintaining a frequency
> accuracy well within the ±20 Hz FCC limit using tubes. (The transmitter
> crystals weren’t even oven controlled, although the frequency monitors
> were).That translates to about 10 ppm, so I would guess that the
> state-of-the-art limit for that era might be at least a factor of 10
> better than that.

My interest is not the carrier frequency. It's amplitude stability. In the
TS, the RF is generated by a keyed tube oscillator- no crystal. The Q of a
crystal is far too high to work in a keyed oscillator.

> Loran A, of course, operated just north of the AM broadcast band.
>
> Loran A receivers, which until the mid-1960s had no digital circuitry,
> basically used the oscilloscope technology of the era in which they were
> made.  In other words, they depended on values of resistance and
> capacitance
> to set the timing in the display unit (So did radar displays, for that
> matter.).

The rep rates were calibrated against an internal crystal reference.

> Using high quality components, trimming adjustments, and good circuit
> design
> it was possible to get timing accuracies down to 50 ppm or better. Hence,
> Loran A was good to plus or minus a couple of miles using ground wave.

Yup, but my interest is in the pulse amplitude, not the precision rep rate.

> Bay and large, the determinative factor was not whether tubes were used
> instead of solid-state electionics, but whether the circuits were analog
> or digital.

It's all analog. The circuit is on page 383 of the MIT Rad Labs book on
LORAN, BTW.

Actually, the problem turns out to be entirely different. I'm using a Tek
TDS1002 digital scope. Digital scopes sample. I was looking at the
envelope of two pulses on screen, and it was varying all over the place.
The display variation was actually related to an aliasing effect, because
the number of samples per cycle was small.

Oh for my trusty 7854 scope!

Take care,

-John

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