Need some advice on something weird

[style0]

Going through an old box of tube stuff I came across two home made devices that go between a tube and it's socket. I'm assuming these were to test mA the tube is drawing to so called "match" the tube. I'm sure I built these from some online thing I found 20 or so years ago.

Anyone familiar with where I saw this and how to use them? I'm old and have no memory of how these worked.

 

[DonH50]

You have a picture?

20 or 30 years ago I would guess you read it in something like The Audio Amateur...

mA = current (milli-Amperes)

 

[style0]

You have a picture?

20 or 30 years ago I would guess you read it in something like The Audio Amateur...

mA = current (milli-Amperes)

OK, so here is the preamp tube version. It puts a 1 ohm resistor in series with pin 3 on the tube if that helps.

 

[amirm]

Looks like you figured out what the part is . Formally it is called a ceramic wirewound resistor. Its value is 1 ohm and has 10% accuracy error in its value. It is made by Dale which is probably the largest manufacturer of resistors.

I am not a tube guy so can't explain its function there. In transistor amplifiers, its job is to create a form of feedback loop to keep the transistor from destroying itself. Transistors work harder when they get warmer. And they get warmer if they work harder and so you have a vicious cycle that can lead to "thermal runaway." A resistor on the emitter of the transistor works inverse of this. The more current is pulled through it, the more power it dissipates so it stops the self destruction. Here is a schematic of that:

Re is the "degenerative" resistor used for this purpose.

Pin 3 on a tube is usually the plate so I don't think the resistor is used for that. Its use there as with transistors does impact the gain so maybe you are right that it is used for matching.

 

[DonH50]

What tube, do you know? It's the plate of an EL34, one cathode of a 12AX7, etc. How many pins on the socket (I can't quite tell, looks like 9?)

My guess would be it is to measure bias current as you surmised.