McIntosh 50W 2 Service Manual

Page 2

INSTRUCTION BOOK FOR '

50W-2

McINTOSH AMPLIFIER

DESC RIPTION

The output circuit of the McINTOSH Amplifier is the first of its kind and pro-
vides 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 schematic diagram is used. This arrangement provides a "take-turn" primary
for the operation of the output tubes in contrast to the conventional push~pull" cir-
cuit 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 factors 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 McINTOSI-I 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. v

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 6L6-G tubes 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