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General Radio

grlogo.jpgThe General Radio Company (or GR) was most prominent from about 1930-1970 when they were dominant makers of instruments for impedance measurement, sound and vibration measurement, precision decade boxes, and standard resistors, capacitors and inductors. Their Strobo-Tac measuring strobe lights are also famous.

General Radio's History

In the 1970's GR invented computerized automated test equipment, which was hugely successful. The company name was changed to GenRad, and in 1991 the remaining standards and test equipment products were spun off as QuadTech. Around 2000 Quadtech sold the standards and decades line to IET labs. GenRad was bought by TeraDyne in 2002, so the General Radio name has disappeared, but IET labs is still selling the standards and decades which had been sold with only evolutionary improvements for over 70 years, and Quadtech does still make impedance measurement instruments.

Here's some additional history I've found:

History of GR up to 1965
This is an excellent 120 page pdf with photos, written by Arthur E. Thiessen.
Speech on GR history
Transcript of a speech on GR history up to 1965 by Donald B. Sinclair (who was the president of GR at that time.)

Quality and Unusual Creativity

Why is General Radio noteworthy? There are several different levels on which GR instruments are distinctive. Appearance: GR instruments were built in about three distinctive styles over the years, and all had some design appeal, rather than being purely utilitarian. At a superficial level, GR made its own knobs, but some of the deeper idiosyncrasy is also visible in the shapes and sizes of the packaging. Quality Construction: Although there were other quality instrument makers, GR definitely didn't cut corners, and GR got an extra quality edge by custom making many components that other companies bought from stock component manufacturers. Innovative Products: GR was the first to market with many different products, and although this was partly because GR was one of the first electronic instrument makers, innovation was a continuous process. Clever Design: GR instrument designers also exhibited great cleverness and engineering finesse in the execution of an instrument's function.

The Unit Instruments

GR had an interesting philosophy of making instruments that were more modular than HP offerings. On the left is the popular HP 608 signal generator, which covered from 10 to 480 MHz in one unit. It had a zillion tubes, weighed a ton, and cost thousands. On the right is a GR “Unit Oscillator”. GR had a series of these, each of which had one tube and one band, and was small and light and less expensive. However if you wanted modulation or leveling or buffering you had to add that on, and you would need three different unit oscillators to cover 10-480 MHz.

hp-608e.jpg unit-osc.jpg

In fairness, the unit oscillators were not mainly intended for duty as a “signal generator” for receiver testing like the HP 608. They were more often used as a local oscillator or signal source for an RF bridge, or often as a part of some special test setup where the features (and cost) of a signal generator were not needed. However, you could build a signal generator by adding on an attenuator, modulation source and special leveling power supply.

The GR-874 Coaxial System

Complementing the modular approach of the unit instruments was the GR-874 system of coaxial components. This was a sort of RF tinker-toy set that you could use to glue the oscillators and whatnot together in test setups, often in combination with some custom electronics. An interesting feature of the 874 connector is that it is Hermaphroditic (there is no male and female), so gender-changers are not needed. At the time it was designed, it also had superior microwave performance. This is a GR874 fixed attenuator:


This is another view of the end of an 874 connector, see how by rotating the connector 90 degrees, it will mate with itself:

Delta RF makes and sells adapters from GR-874 to other types.

Innovative Components

GR had a components business even before they got into instruments. GR made the variable capacitors that Edwin Armstrong used when he invented the first superhetrodyne receiver in 1917. Most noteworthy are GR's instrument-grade rotary switches, potentiometers and precision capacitors and resistors. GR made a good business out of putting their resistors and capacitors in boxes along with their switches (making decade boxes.) The switch is the heart of a precision resistor decade.


Another component that GR made in house was transformers. They are massive, efficient, and distinctive in appearance. GR was also creative in transformer design, inventing the Variac variable autotransformer, which is still widely used for power supply testing:


Engineering Finesse

The finesse is a little harder to explain, but part of it was due their ability to make custom components in-house. One of the most artful hacks I've seen is in the in the GR 1211-B unit oscillator which covers 500kHz to 50Mhz in two decade bands. Usually in a tuned LC tank, bands are narrower than 10x, since they are tuned by varying the capacitor, and a 10x range would require a 100x capacitance change, which is not practical. The 1211 does this by simultaneously varying both the inductance and the capacitance. The inductance is varied by rotating a carefully sickle-shaped composite of powdered iron and aluminum through the tank coil. Now, the use of a movable slug of either iron or copper to tune an inductor was common, but it was rarely done for a front-panel control. But I have never seen anywhere else either the use of a composite slug, or the peculiar mechanical arrangement for circular motion of the slug.

What really staggered me, though, was how the band switching was done. Recall that I said the unit oscillators usually had only one band. The 1211 does have two bands, and the switching is done by a funny lever on the front. This is indeed a switch, but if you look inside, it works more by rotating the instrument around the contacts than by moving contacts past each other. The effect is to install the HF coil in the same mechanical position on the tuning slug that the LF coil was previously in.

All this control weirdness was simply not how things were done at other instrument companies. They bought their variable capacitors from component manufacturers. They may have had them custom built, but the variation was just in terms of the capacity, number of gangs, etc. They didn't design weird controls and then wrap an instrument around them. GR instruments are anomalous engineering artifacts because they are often so highly optimized. They aren't just assemblages of standard components, the components were designed as integral parts of the instrument.

I am also impressed with the GR 1205-B Adjustable Regulated Power Supply. This supplies 200ma of regulated power from 0-300V, which is 60W. This a good bit of power to be controlling with a handful of tubes in a unit that small. It is pretty dense because about 1/4 of the volume is taken up by a massive GR transformer. The secret to the power handing is it is not a purely linear regulator. It uses a pair of thyratrons (a tube SCR analog) as a phase control pre-regulator. That way, the output tubes only have to drop 25V or so to maintain regulation, and don't melt. Also, the regulator uses various clever tricks for better line and load regulation. In addition to the classic hack of biasing the reference device from a regulated output derived from the reference, the error amplifier also makes direct use of positive feedback to boost the gain. With tubes, each additional tube had significant costs in size, price and reliability, so people really sweated to squeeze all they could out of a few active devices.


Information for this page was contributed by:

If you have any GR related info, I'd be happy to add it to this page.


Ronald Webb, 2010/07/22 20:35

Picked up a General Radio Type874-LBA “slotted line” US Patent No. 2,548,457. I guess I,m going to ebay. Would you think it would be of use or needed by someone

Robert MacLachlan, 2010/07/28 21:23

Ebay is a good market for 874 components, but don't expect to get a lot, maybe $5-10. You can search completed sales to see if any have sold recently, and for what.

H.P. Claasen, 2011/11/19 14:23

A.M.Detector unit type 1932 PI what is this

jeremy brown, 2012/08/04 15:13

I have a strobotac type 1531 and i would like to know what it is and what year it was made it is in mint condition?

jared, 2012/12/08 03:13

i have a 1960s general radio 1304 b beat frequency generator in perfect shape works great cant seem to find if its worth nothing or what hard to find this info.

Robert MacLachlan, 2012/12/08 19:08

Ebay is the main market for older used test equipment. There are not many GR items that sell for more than $100, and a lot will sell for $20 or less. Almost all GR items are highly obsolete, though they may still be usable for their original purpose. Some older units also have more antique looking wood cases that may be appealing to collectors. Calibration standard inductors, capacitors and decade boxes still hold some value because the technology hasn't changed that much. The strobotac strobes also seem to go for a fair amount (perhaps $100). I don't know if that is use value or collectibility. The later versions do make shorter pulses than typical industrial strobes.

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wiki/user/ram/electro/gr/0main.txt · Last modified: 2010/11/06 09:44 by ram