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

Many transconductance testers have multiple switch decks to move each
element around. This represents a number of additional steps, over the
emission tester.

And, I often can get the radio back up and running in 20 minutes - and speed
is important since I do not charge for it.

Chuck no doubt is more thorough than I am - but I am only servicing an old
radio and not a 50 Mhz precision scope. Incidently, one of my close friends
designed many of the circuits in the 585, and single handedly designed the
519 and 130, and I have a row Tek scopes and other Tek equipment.

Colin K7FM

COLIN LAMB wrote:
Many transconductance testers have multiple switch decks to move each
element around. This represents a number of additional steps, over the
emission tester.

And, I often can get the radio back up and running in 20 minutes - and speed
is important since I do not charge for it.

Chuck no doubt is more thorough than I am - but I am only servicing an old
radio and not a 50 Mhz precision scope. Incidently, one of my close friends
designed many of the circuits in the 585, and single handedly designed the
519 and 130, and I have a row Tek scopes and other Tek equipment.

Colin K7FM

Hi Colin,

My first emission style tester had a bank of switch levers, one
for each of the 12 possible pins on a tube. Each switch had
several possible positions:

1) Heater +
2) Heater -
3) Plate
4) Cathode
5) Open

The Hickok testers arrange the pins by using a bank of rotary switches,
with a separate rotary switch for each of the following elements:

1) Filament
2) Filament
3) Cathode
4) Grid
5) Plate
6) Screen
7) Supressor

For setup, it becomes a 6 of one, half-dozen of the other situation.
Either way, you have to account for all of the elements in the tube.

As to fixing a radio in 20 minutes, or less... Sure, since most radios
have fewer than 20 tubes (KWM-2 has 18). A minute a tube is very realistic.

I've done it both ways, and for me it takes as long to use an emissions
tester as it does to test the tube correctly with a transconductance
tester. There isn't a significant difference in the number of steps
either way. There is a major difference in the quality of the test.

-Chuck

Hi,
Colin wrote:

Incidently, one of my close friends
designed many of the circuits in the 585, and single handedly designed the
519 and 130, and I have a row Tek scopes and other Tek equipment.

The 130 L-C meter? Great little instrument: I still have one on my
bench, and examples of both the old and newer models.

Alan

Yep, the 130 L-C meter. Cliff needed something like that for a project and
there was nothing available, so he whipped one up. It was so useful, the
other engineers soon wanted one. Then Product Design got ahold of it.

I have his personal 575, too.

Colin K7FM