Pushbutton S302E and potentiometers R401 and R405 (fig. 2-2) are used for balancing the
transconductance (Gm) bridge circuit under actual tube operating current. Pressing S302E removes the
grid signal and allows a zero balance to be made with one potentiometer or the other, depending upon
whether the tube under test is passing high or low plate current. Lamp DS108 on the front panel lights
when S302E is pressed. Pushbutton S302C is used for checking grid-to-cathode shorts at a sensitivity
much higher than the normal tests. Results of this test are indicated by the short test lamps on the front
Certain special tests require the use of a continuously adjustable auxiliary power supply. By pressing
pushbutton S302B, you may use meter M301 to read the voltage of the auxiliary power supply on meter
M301. This voltage may be adjusted by the use of the potentiometer R142. The rest of the potentiometer
controls are calibration controls and are adjusted by the use of special calibration cards and a calibration
All circuits in the tester, except the filament supply, are electronically regulated to compensate for
line voltage fluctuations. The filament supply voltage is adjusted by pressing pushbutton S302A and
rotating the filament standardization adjustment switch S106 until meter M301 reads midscale.
The circuits to be used in testing are selected by a prepunched card. These cards are made of tough
vinyl plastic material. The tube numbers are printed in color on the tabs of the cards and also at the edge
of the card for convenience in filing. A special card is provided to use as a marker when a card is removed
for use. Blank cards are provided so that additional test cards may be punched for new tubes that are
developed or to replace cards that have become unserviceable.
Before operating the tester for the first time, and periodically thereafter, you should calibrate it using
the calibration test cards as described in the equipment technical manual.
NORMAL TESTS.The tester is equipped with a three-conductor power cord, one wire of which
is chassis ground. It should be plugged into a grounded 105- to 125-volt, 50- to 400-hertz outlet.
Before operating the tester, open the auxiliary compartment (fig. 2-2) and ensure that the
FILAMENT STD ADJ and the Gm BAL knobs are in the NOM position. The GRID SIG and CATH
ACT buttons (S302E and S302D) should be up and lamps DS108 and DS106 on the front panel should be
Turn on the tester and allow it to warm up for 5 to 10 minutes, then press the CARD REJECT
KNOB (fig. 2-1) down until it locks. If a nontest card is installed in the card switch, remove it. This card
is used to keep the switch pins in place during shipment and should be inserted before transporting the
Plug the tube to be tested into its proper socket. (Use the pin straighteners before plugging in 7- and
9-pin miniature tubes.) Select the proper card or cards for the tube to be tested. Insert the card selected
into the slot in the card switch until the CARD REJECT KNOB pops up. The card will operate the tester
only if it is fully inserted and the printing is up and toward the operator. Do not put paper or objects other
than program cards into the card switch, because they will jam the switch contacts. If the overload shuts
off the tester when the card is inserted in the switch, check to see that the proper card is being used for the
tube under test and that the tube under test has a direct interelement short.