McIntosh MC 20W 2 Owners Manual

Page 1

INSTRUCTION 300K

20W-2
MC 03 AMPLIFIER

Pat. No. 2,477,074

and patents pending

Page 2

INSTRUC TION BOOK FOR
20W-2

lVI CINTOSH AMPLIFIER

DESCRIPTION

The output circuit of the McIntosh Amplifier is the first of its kind and provides
100% coupling between the two primary windings of the output transformer. Almost
perfect magnetic coupling is achieved by winding the two primary windings together
or bifilarly as if they were one wire. In order to cancel the static magnetic fields due
to the flow of. direct current through the windings, and, at the same time, arrange
that the AC components from the tubes add in-phase, the circuit as shown in the sche-
matic diagram is used. This arrangement provides a "take-turnH primary for the
operation of the output tubes in contrast to the conventional |'push-pull" circuit where
one half of a series winding is used for one tube and the other half of the winding for
the other tube. The McINTOSH Circuit, because of the high mutual coupling between
windings, from an AC standpoint, utilizes essentially the same coils. This circuit
provides a large frequency response advantage over conventional output circuits for
the following reason. The impedance between the two tubes has been reduced over
the conventional circuit by a factor of 4 to 1 since the turns ratio is reduced by a
factor of 2 to 1; a reduction of leakage inductance between primary and secondary of
4 to l is achieved because of the relative turn ratio reduction of 2 to 1. These fac-
tors in part account for the large advantage over the conventional "push-pull" circuit
and are a major contribution to the wide band and low phase shift of the high efficiency
McINTOSH Amplifier. It will be further observed that the load is 1/2 in the cathode
and 1/2 in the plate for each of the tubes. This arrangement provides directly a
feedback factor of approximately 12 db. The remaining feedback is obtained through
a balanced loop to the input of the phase inverter.

Considerable care has been given the problem of impulse distortion, and one of
the basic reasons for using the bifilar choke input to the final stage, as indicated in
the schematic diagram, is to provide a low grid circuit resistance path to DC which
circumvents the usual difficulty of excessive bias resulting from transient or impulse
program material.

The circuit used permits the maximum possible efficiency of wide band amplifiers,
and yet operates at full output with substantially less than 1% harmonic distortion
measured at any single frequency from 20 cycles to 20, 000 cycles.

ELECTRICAL DESIGN

The circuit consists of two 6V6-GTtubes in the final stage, driven by two 6J5 tubes
operated in push pull - one 12AX7 tube consisting of two triodes for the inverter
amplifier stage. This is the basic amplifier and it has a power gain of approximately
40 db requiring a maximum of 2 volts input at the grid of the phase inverter. CAUTION:
Avoid excessive grid drive voltages as prolonged excessive voltages will injure the