----- Original Message -----
From: "John Boyle" <jmbo...@ripnet.com>
To: <drakelist@zerobeat.net>
Sent: Tuesday, April 26, 2011 11:55 AM
Subject: [Drakelist] SW-4A amp
The SW-4A amp is not push pull, neither is the amp in the
R-4A or R-4B. However, the SW-4 does have a push pull amp
with two 6AQ5s and 1/2 of a 12AX7 as a phase inverter.
Most audio amps consist of two stages. The first is a
voltage amp, usually a triode such as one unit of a 12AX7
but sometimes pentodes are used such as the 6GX6 used in the
R-4. The second audio stage is a power amp, usually either
a power pentode such as the 6EH5 used in the R-4, R-4A and
R-4B, or a beam pentode (6AQ5) as used in the SW-4. There
are a wide variety of triodes and pentodes used in audio
circuits. There is little point in using a push pull audio
stage in a communications receiver due to the restricted
audio frequencies used. A push pull audio stage requires
two more tubes - a triode phase inverter and a second output
tube - plus additional capacitors and resistors so it was
not cost effective for most applications.
John, VE3PMA
I think audio amplfier quality can have a significant
effect in a communications receiver despite the narrow band
of audio frequencies handled. This is because distortion,
both harmonic and intermodulation, can exagerate the effect
of noise. Single-ended amplifiers can be made to have
reasonably low distortion by carful design and the use of
some negative feedback. The advantages of push-pull are that
even harmonics are automatically supressed and that core
saturation from unbalanced DC in the transformer windings is
eliminated. Core saturation reduceds low frequency responce
but can also cause distortion. Intermodulation distortion is
often overlooked but it can have a significant effect on
narrow band information even including CW. For the most
part push-pull amplifiers are not used in tube
communications receivers because of the added complication,
as you describe, plus the lack of necessity for either high
audio power or amplifier efficiency. Feedback also is seldom
found in single-ended amps because it requires more gain and
a higher quality output transformer, both of which drive
cost up. Its an interesting experiment to pick off the audio
from the detector stage of a receiver and feed to a good
quality amplfier driving the usual receiver loudspeaker.
Sometimes the difference is quite amazing.
--
Richard Knoppow
Los Angeles
WB6KBL
dickb...@ix.netcom.com
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