[Test-Equipment] tube question

[k4pf at juno.com]

> Ron bonddaleena at aol.com wrote

<snip>
I have several tube testers including a nice Weston Transconductance tester. However, my Sencore TC-142 gets the most work. I realize this is pretty much a 'go - no go' tester, but here's my question:

Testing several of the tubes, the 'emissions' test was fine, but when switching to 'leakage', some of the tubes showed very high leakage, initially. However, the longer I left the tube in the tester, the leakage came down to an acceptable level.
When testing a NOS spare of the same #, switching from 'emission' to 'leakage', immediately, shows ZERO leakage.

Is this a situation where the grid is contaminated?
<snip>

Hi, it could very well be grid contamination.  That shows up
when the grid is hot.  The grid "contamination" is barium and strontium
that evaporated from the coated cathode during normal operation.
What heated up the grid is the "cathode emission" test.  In that test,
the control grid, screen and plate are connected together,
and the tube acts as a strange diode.  The tube tester puts
around 30 VAC across the series combination of the "strange diode"
a current limiting resistor in the plate circuit, and a 200 Ohm
"Load" pot in the cathode.  The panel meter is connected as a voltmeter
connected from the slider of the pot to one side of the pot.

Specifying the Load pot position was a handy way for the tube tester manufacturer to adjust the sensitivity of the meter 
for many different tubes.  Mid-scale is 80 percent of the typical
new tube deflection, this is a somewhat arbitrary "end of life"
point.

The "circuit" switch of the emissions tester selects the value
of the current limiting resistor, and also selects a series resistor
to adjust the scale of the meter.  

Just after emissions testing, the grid can be hot.  My Triplett
type 3413-A tube checker puts about 30mA (indicated average DC)
cathode current through a 6AQ5 during emission testing, Circuit "3". 
Most of that current is thru the control grid.  Result: grid emission
is possible right after emissions testing, but returns to normal
as the grid cools down.

That's why you shouldn't hold the "test" button down on an emissions
type tube tester. The tubes are being given a rough test,
especially the control grids, which are not designed to act
as plates.  Just hold it down long enough to take a reading.

By the way, the circuit "1" and "2" tests are easier on tubes,
due to higher value plate resistors.  A 12AX7 tested in circuit 1
may only have 2.4mA thru it, and a 12AT7 in circuit 2 may have
10mA.  These currents were found by connecting an analog DC voltmeter
across the 200 Ohm "load" pot of my Triplett tube checker.
Peak currents are much higher, of course, this is merely the average 
indicated current of the half wave rectified 30V a.c. supply.

Similar testing currents should be expected with those 
emission type tube testers which are clones of the Triplett 3413-A:
 Knightkit 600, Eico 625, and Heath TC-1 and TC-2.
I assume your Sencore cathode emission tester is similar.

73,
Ed Knobloch