Valve/Bulb type ref 10AB/4604 what is it?

[Pete_kaye]

I have bought this recently as it was described as a valve but I don't think it is. It has 7 pins. Can anyone throw some light on the subject? I haven't picked it up yet. The picture is very small so I will post a better one later. The original pic is on Ebay.

[Jeremy M0RVB]

It's a thermocouple, I have one similar
http://www.tubecollector.org/unknown-thermocouple.htm - but no idea what it was for.

[GMB]

Looks like a true-RMS device.

Thermocouple with resistor.

[Pete_kaye]

Here are some close ups. it looks the same as the one above under a different number. There are 2 very fine cross wires near the top. What was its use?

[Radio Wrangler]

There are several sorts of devices it could be. It needs someone who can decode the military type number to be sure.

Some thermocouple devices consist of a resistor in an evacuated bulb with a thermocouple grafted onto it. RF heats the resistor, the thermocouple makes a tiny DC voltage, and a sensitive moving coil meters a suitable scale to show either RF power or RF amps. And, yes, it's true RMS pretty much by definition.

Some Marconi signal generators use these sorts of things to monitor the output level.

Another possibility is a 'thermistor mount' where a matched pair of thermistors are wired in series and together form one arm of a Wheatstone bridge. RF is injected through a capacitor to the mid-point of the two thermistors. One end of the pair is ground, the other end (one outer corner of the bridge) is RF decoupled, so the RF sees the thermistors in parallel. But the bridge sees them in series. If we want it to be 50 Ohms load on the RF (Other impedances have been used) then each thermistor must be 100 Ohms, and each arm of the bridge must be 200 Ohms. If we adjust the DC voltage to the top of the bridge, the thermistor R will change following a change in their temperature due to the change in the current in each arm. The other resistors need to be stable ones, so the bridge balance changes.

Now the trick is that we can take the bridge unbalance differential voltage and amplify it to control the bridge drive voltage. It's feedback if we get the sense the right way, and the bridge becomes self-balancing. Neat or what?

If we run the thing and measure and note the bridge drive voltage with it sitting happily balanced, and then we apply some RF, the RF heats the thermistors and the amplifier adjusts the bridge drive voltage to get back to balance .... IE to get the thermistors back to their original temperature before the RF.

THEREFORE the DC power subtracted from what the thermistors had before, is equal to the RF power. If the other bridge resistors are 200 Ohms each, then the DC power into the bridge changes by four times the applied RF power.

So we have a very accurate way to convert an RF power measurement into a DC power measurement without having to worry about rectifier efficiencies and errors.

It wasn't always done with DC, for a while audio frequency power substitution was used because it was easier to amplify and avoided DC offset errors.

Look up 'Bolometer' (good for scrabble) and DC substitution RF power measurement. HP 478 thermistor mount (a later generation beastie) HP 432A power meter to go with that head. The HP apps notes make good reading (now moved to Agilent and probably Keysight)

David

[Pete_kaye]

thanks for that reply.

[Alan Stepney]

I too have one of those and have been trying to find out what it was used in / for.

[Jeremy M0RVB]

The 10AB series of number is radar stuff. One thing that was suggested to me a while ago is the four bent bits in the metal arms are resonant somehow.

[Radio Wrangler]

Yes!

It's a little dipole antenna in a glass bulb, and the parallel section back to the pinch is a resonant line choke.

Estimating the dipole length will show what band it's on. I expect there's a bead thermistor between the dipole halves. That's one way to get microwave operation with an octal base

So it's a bolometric field sensor.

A gentle ohms check will see if there's still a thermistor at home.

David

[Pete_kaye]

It reads 4 ohms on pins 2 to 7.

[G6Tanuki]

I'm guessing you poke it into a waveguide...

The odd 'hairpin' bits on the support wires - yes they look precisely like resonant stubs, spaced at the right wavelength apart so as to present a high impedance to any microwaves wanting to make a bid for freedom down the "DC" connections to the base.

[Pete_kaye]

Can anyone tell me about it? About 6 “ tall.

[barrymagrec]

Been here before:-

https://www.vintage-radio.net/forum/...d.php?t=120133

[Station X]

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