The am plifier is designed around a6U8 tube with which it is possible to achieve a gain of approxi
mately 1000. This is adequate for the use for which this computer is designed. This high gain
is achieved by operating the pentode section of the 6U8 with a large plate load and with low volt
age on the screen grid. * This gives a gain of approxim ately 700. The gain is increased to ap
proxim ately 1000 by adding a small amount of positive feedback. The 2.2 megohm re sisto r p ro
vides this feedback. A high frequency filter consisting of a 1KS7 re sis to r and a 5600 pjuf capac
itor in se rie s is used to prevent oscillation of the am plifier.
The 3 K£2 control is used for adjusting the output of the am plifier to z ero volts when there is no
input signal. One way of doing this (used in the E C -1) is to use a 1 megohm feedback resisto r
and a 1 megohm resistor between input and ground, as shown in Figure 13.
IM
With the am plifier set up as in Figure 13, the 3 Kf2 ze ro adjust control is set for z ero reading
on the m eter. In the E C -1, this is accom plished by m erely operating a switch to its BALANCE
position, which disconnects the am plifier from the problem board and connects the proper feed
back and input re sis to rs, thus sim plifying the balancing operation.
The triode section of the 6U8 is connected as a cathode follow er output stage. The two NE-2H
neon lamps are used as a referen ce voltage dropping element by which the output signal level
can be dropped from approximately 112 volts to ze ro volts without lo s s in gain, as would occu r
if a purely resistive element were used.
The input and output term inals of the am plifier are on the panel or problem board with two of
each provided. These are so arranged that plug-in units may be easily and rapidly used to
change impedance values for various problem s.
AM PLIFIER POWER SUPPLY
The +300 volt power supply is a conventional electron ica lly regulated unit which provides the
+300 volts for operation of the am plifiers. The 250 K£7 control adjusts the output voltage over
a range of approxim ately +250 to +350 volts. The circu it is shown in Figure 14. P rovision is
made for using the m eter on the panel for setting the voltage to +300 volts. The -150 volts DC
required by the am plifier is supplied by a half-wave rectifie r, the output of which is controlled
by an OA2 regulator tube.
* George E. Kaufer, "How to Design Starved A m p lifie rs", T ele -T e ch and E lectronic Industries,
Vol. 14, No. 1, January 1955, Page 68.
Walter K. V olkers, "U ltra-H igh-G ain D irect-C oupled Am plifier C ircu its", - paper read before
1950 IRE National Convention in New York.
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