RAT gm/mu Tester Application Note 1

Using the Tester to test the linearity of a Valve

Because the gm/mu tester can check the operation of a vacuum tube
(valve) under many conditions, it is possible to gain a measure of
the linearity (or conversely, distortion) of your proposed tube in
a specific circuit. Similarly, you can check different tubes, or
tube types, to establish how linear these might be in your circuit.

The way to do this is to establish a trial loadline for your tube,
then check the MU of the device at several spots along this load
line.

Some examples will clarify this procedure:

Suppose you have an amplifier with a 6SN7 operating at 350 volt
supply and it has a 15k ohm plate load resistor. Using ohms law,
when there is 300 volts on the plate, there will be 50 volts
across this load resistor, or 3.33 mA. The point to be checked
using the tester is 300V, and 3.33 mA. Likewise, with 200 volts
on the plate, there is 150V across the load resistor, or 10 mA.
The point to be checked using the tester is 200V, and 10 mA.
Continuing like this we can establish a table like:

Vp	Ik	MU
300	 3.33	18.9
280	 4.66	19.3
260	 6.00	19.7
240	 7.33	20.1
220	 8.66	20.4
200	11.33	20.7
180	12.66	20.9
160	14.00	21.1
140	15.33	21.3
120	16.66	20.4	(essentially 0 volts grid-cathode)

Note how linear the tube is, even with this less-than-ideal
load line.

For comparison purposes, here's data for a couple other
tube types:

417A with 200 volt supply, and 6.66k plate load resistor.
Vp	Ik	MU
180	 3	38.0
160	 6	39.2
140	 9	40.2
120	12	41.3
100	15	42.0
 80	18	42.5
 60	21	42.6
 40	24	42.3	(essentially 0 volts grid-cathode)

6DJ8 with 200 volt supply, and 10k plate load resistor.
Vp	Ik	MU
180	 2	27.0
160	 4	28.3
140	 6	29.2
120	 8	30.2
100	10	31.5
 80	12	32.9
 60	14	33.9
 40	16	25.1	(essentially 0 volts grid-cathode)

In these examples, the 417A is not quite as good (percentage wise)
as the 6SN7, which ought to be no suprise to anyone. Notice,
however, how much the mu of the 6DJ8 changes over the load line.
This might explain why this tube gets a bad rap in some circles.

Regards,
Steve