Old GR Crystal Test Set

[J. Forster]

A friend wrote:

I have an ancient General Radio Type 275 Piezo -Electric Oscillator.  It's an
instrument with a one tube
circuit that ran on batteries in a wooded box about a foot square.  I  would
like to find a manual or at least the part number (tube number) of  the 4 pin
vacuum tube that went into it.  The unit was probably made in  the late 1920's
or early 1930's and was used to test crystals for early  radio equipment.  There
is a schematic in the lid of the box but no tube
number.  I remember that you may know something about  General Radio electronic
equipment so I'm trying to pick your brain on  this item.
Regards,
Edward

I'd guess that the tube was a triode, like a 30, but don't really know. Anyone
on the list have any ideas where such info might be found?

Thanks,
-John

BTW, here is an interesting GR history link:

http://www.ietlabs.com/pdf/history.pdf

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