COLIN LAMB wrote:
Some tube manufacturers specified a maximum grid leak resistance. It was
often about 2 million ohms. Drake found that much higher resistance was
great for avc performance.

However, over the long run, it was also asking for trouble.

In theory, what Chuck says is true - that transconductance testers are
better than emission testers. However, I can just report that in hundreds
of tests I have just found a couple of cases where there was any practical
difference.

I often end up getting sucked into helping others get their receiver going
again. I do not charge for it and I want to get back to my own projects.
The emission tester is much faster to use and I do not really care if the
tube is 79% good or whether it has 10% less gain. The radio does not work
when I start and I am looking for black or white, not shades of gray.
Often, it is not the tubes, but rather another component. The tube tester
simply eliminates the tube as the culprit for total failure.

I suppose if I were looking for a tube with the best performance for a 2
meter moon bounce preamp, I would use a transconductance tester. However,
in that case, I would not use a tube at all.

I could not imagine running all the tubes of an S-85 through the test
procedure for a transconductance tester - unless I had to.

Colin K7FM

Hi Colin,

The setup for a transconductance tester has only one extra step over the
emission tester: you have to set the grid bias. If you do the gas test,
you get two more steps: set plate resistor to specified plate current,
press button.

Emissions tester:

1) set heater voltage
2) set socket connections
3) plug in tube (start warming up)
4) set line voltage adjustment rheostat
5) set plate load resistance pot.
6) do shorts test
7) press emission test button

Transconductance tester:

1) set heater voltage
2) set socket connections (two rotary selector switches on Hickok)
3) plug in tube (start warming up)
4) set line voltage adjustment rheostat
5) set plate load resistance pot
6) set grid bias pot
7) do shorts test
8) press Gm test button

+ gas test:

1) press Gm test button and hold
2) adjust grid bias for 100umhos
3) press Gm and Gas test


Since most of the time you don't need to do the gas test, the additional
step is hardly a noticable increase in effort considering the increased
quality of the test.

I have tested every tube in every Tektronix scope that has come my way,
and it is no more burden than doing the test in an emission tester would
be. Since I have to pull all the tubes to wash the scope, I might as well
test them. The complete test for all tubes in a 585A scope takes maybe
20 minutes... most of the time is spent waiting for heaters to warm up.

-Chuck