Heathkit TT-1 - Page 6

Gm TEST CIRCUIT
Of the many measurable characteristics of
vacuum tubes, mutual conductance is the one
most closely associated with actual operating
performance. This is the method used for test-
ing amplifier tubes in the HEATHKIT Model
TT.1, This test uses the circuit shown, and
24449 SOM UNSUS st Vat vue He. ema t
is performed in the following manner:
All element voltages on the tube under test are
filtered DC potentiais to give a more accurate
and indicative test. The plate circuit consists
of a variable DC plate supply and high-pass filter
(L-1, C-1 and R-1) which is connected to the
meter circuit. This filter passes only the 5000
cycle component of plate current and blocks the
DC plate current and any 60 cycle ripple that
may be present.
The grid circuit consists ofthe variable DC bias
supply and the 5000 cycle oscillator. Whena
signal voltage (eg) is impressed on the grid of
the tube under test, a signal current (ip) is pro-
duced in the plate circuit which gives a meter
reading proportional to the value of grid signal
and thus to the mutual conductance of the tube.
The useful life of a diode or rectifier type tube
ends when the emission capability of its cathode
falls below some given level, the exact value of
which may differ for each tube type. The circuit
used to test these tubes is shown below:
-VUFD
TUBE UNDER
TEST ale | |
METER
TSK
do + all See
i Reaseccl | |
DC =
BIAS hed
PLATE
SUPPLY | SUPBLy
L_ — J
For a given signal, ve meter can be calibrated
oe he waing tha frallaunnge
to
read
in
te1
iis
of
Sm
OY
USING
Wie
10s
waislg
relationship: ip = €g X Em where gm is the
mutual conductance. Rearrangement of the
above gives: gm = ip/eg.
Thawnofaunna af +h
2i1eL01yl &, pO
plate current is
deflection of the m
0
milliamperes, a full s
eter corresponds to .003 mho.
Since 2 mho = 1,000,000 micromhos, .003 mho=
3 003 X
,000,000 000 micromhos.
The term "proportional mutual conductance" is
derived from the fact that proportional values of
element voltages are applied to the tube under
load resistor are selected by the plate switch
and connected to the proper elements. Each
rectifier and diode type is rated in terms of
emissive capacity and must read above the
appropriate scale marking to be considered good.
fR2] _. {r3] fc 1
Lopes UR La | a |
620
n
100
n
TUBE
UNDER
| | ZI TEST
Gm ADJ
20, 45 OR |
177 VOLTS
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Page 6
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