General Radio 1931

[KA3EKH]

I have just obtained a General Radio model 1931 modulation monitor. I got it from a CC sister station up in Delaware, saw it in their junk pile and decided it would work great with my RCA BTA1MX I did download the manual but the question now is when was the G/R 1931 in production? Mostly octal tubes but one or two seven pin tubes and the two meters look like something from the late thirties but almost all General Radio stuff looks like that . Any ideas?
Ray Fantini KA3EKH

[Pete, WA2CWA]

The copyright date in the manual should give you some clue.

[Todd, KA1KAQ]

Interestingly Ray, the earlier versions of the 1931-A used round meters. The old-buzzard fan meters were added to later versions (post war onward, I think). Despite their dated looks, they were made into the 1970s or later, IIRC. Clearly a case of 'if it ain't broke, don't fix it'.

Love mine, hope to finally have it hooked up soon now that I have the proper T adapter for the transmitter. The meters give a really nice, warm glow as does the peak flasher.

I may have a .pdf of the manual if you need it. Can't remember if I downloaded or just copied it.

[KA3EKH]

The pdf manual I have has no dates, parts of the manual and some schematics look almost pre WW2 other parts like the component list are much newer appearing maybe fonts and such from the fifties. The tube line up uses primitive stuff like the 6X5 in the power supply but there is also a 6AL5 used as the detector and that’s at least mid forties production. Having used G/R bridges before know that their appearance has very little to do with their age. The monitor I have has the old style looking meters and not the round ones so guess that makes it a newer one.
Ray Fantini

[Pete, WA2CWA]

I have three original 1931-A manuals. One is tagged FORM 639-D (no date), another FORM 639-G (no date) and the other is tagged FORM 639-H (March 1953)

[steve_qix]

Hmmmmm.. I've got a couple of those things around here, but I can't say I remember a 6AL5, but then again, I didn't pay a lot of attention.  I would have expected either a 6H6 or the loktal equiv, but maybe the 6AL5 goes back that far.  I'll take a look in the back of one of mine and if there is not a 6AL5 there - will report back!!

Used to fix lots of them back about 35 years ago as they were quite common in small broadcast stations.  Of course, they were all phased out when stations needed to read accurately to at least 125% modulation (there is a simple modification that will make the 1931A read to 125%, but it requires changing the meter scale).

The hardest tube to replace is probably the gas "flash" tube, used in the negative peak flasher.

Regards,

Steve

[Pete, WA2CWA]

All three schematics in the three different manuals (FORM 639-X) show a 6AL5.

[Todd, KA1KAQ]

Yep, the design was pretty much unchanged electrically through the run. Though I've never had the opportunity to look over the early round meter version in person.

Got mine from Steve 'TOW a few years back sans manual and power cord. Was able to pick up the 639-D manual free at NEAR-Fest last spring (thanks G) but as you say Pete, there's no date in that manual indicating a timeframe of manufacture.

I like the fact that, with a simple change, you can use the same monitor across a wide range of bands/frequencies. Not a big thing today, but considering the age and simplicity of the unit, it's a handy feature.