Alternate methods of Transconductance Measuring?

[Shannon Parks]

Howdy Folks,

Let me just share my test setup as that might be the best way to explain what I'm doing. I have a 6DJ8 with the halves paralleled in a grounded cathode config - trying to get a ton of gm for a phono project. I decided to go this route instead of using the more expensive 6C45pi tubes.

So I have it rigged up with:
- 110V B+,
- one of the IXYS CCS's set at 8.25mA (see Pete Millett's page)
- 200 ohm cathode resistor bypassed with a 470uF cap

I have a 100 ohm grid stopper (I still saw oscillations - need to make this MUCH bigger) with a 150K grid resistor. The output is a .22uF cap with another 150K resistor to ground.

How can I measure gm on this setup? The CCS makes this a little more difficult. I have both my audio analyzer and o'scope hooked up to it - certainly there's an easy method. Many thanks!

Shannon

[Shannon Parks]

Finally found what I was looking for here:
http://www.triodeel.com/tester.htm

This is from Steve Bench's RAT tester project:

Transconductance:

This is defined as the incremental change in plate current for an incremental change in grid voltage, with all other parameters (plate voltage, for example) held constant. The way this is done is to place a small AC voltage (lets say 100 mV) on the grid and measure the output AC current on the plate. In practice, this is done by measuring the voltage across a small resistor, (lets say 100 ohms) connected from plate to a constant DC voltage source. The current can be controlled by placing a constant current source in the cathode circuit of the tube under test, and bypass the cathode for AC purposes. For the example given (100 mV AC on the grid, and a 100 ohm plate "current sensing" resistor), a transconductance of 1 mS (1000 micro mhos) would be indicated as a 10 mV signal across the 100 ohm resistor.

Very straight-forward and practical. RDH4 basically just explained how to build a full blown tester (ala Hickok). Thanks, Steve! Ned, too, for the web archive.

Furthermore, thanks to Steve for this archived post.

Shannon