CT207 Thermal Noise Generator

Mine is Serial Number 487.

Picture 1, picture 2.

From scatterpoint, May 2011:

Generator, Thermal Noise, 6625-99-943-3249, a.k.a CT 207. This is another noise factor
meter, comprising both the noise generator and the meter to measure receiver output. The identity
label also says '100 - 600 Mc/s' but it might be usable at higher frequencies if calibrated. The
noise generator is interesting in that it used a special diode whose geometry makes it a nominal 75
ohm coaxial line. The output impedance is 75 ohms but I may have an N-type 75 - 50 ohm adaptor
somewhere. To conserve the diode tube life, the instrument is controlled by a front panel timer
that switches it off after about 8 minutes. I also have a couple of spare noise diodes and the circuit
diagram. [I bought this item several years ago from AH Supplies of Sheffield.] Recent storage
history as above, believed working but untested. I have the circuit diagram and a photocopy of a
Marconi instrument that looks as though it is/was the 'civvie' version of the CT 207.

The problem is that the noise generating diode has a life of something like 20 hours (which is why there's a timer). So this thing is probably not useful for much more than a donor for a different project.

"The Noise Generator consists of a temperature-limited coaxial diode (CV2341) held in a special valveholder, the noise power being developed in a resistor which terminates the coaxial line. The noise output is controlled by altering the filament current of the diode. [...] The Audio Power Meter comprises a resistance network, a metal rectifier bridge and a D.C. meter."

They sold them for £95 surplus in 1975 / 1976.