"PJR" wrote in message
news:XpmdncIb_cmzlJzUnZ2dnUVZ_rmdnZ2d@metrocastcab levision.com...
On Wed, 22 Oct 2008 11:33:14 -0500, Henry Kolesnik wrote:

I ended up with this & it's separate power supply in a
package deal,
couldn't buy what I wanted without taking it. It looks
better than
average but doesn't have the original S-meter. Both are
rack mount
units in cases with a heavy interconnecting cable, and
they are heavy!
It covers 1.25 to 40 Mc in 5 bands. Both units have
aluminum tags with
serial numbers and the Rx is stamped Type O with a
40,000 serial. I've
heard that they have terrific audio but with only a
portion of BCB it's
a bummer.
I'd like to know a bit more about it and if they have any
value as
collectors items or are they true boat anchors? Is it a
BC-794 A or B
or a SP-210-SX or what? Why did they make the power
supply so heavy?
Thanks

Yours should be a SP-210-SX or BC-794-B (1.25 - 40 MHz)

More info at:

http://www.roveroresearch.org/sp200/bc779.html

An interesting site. There were more military
variations on the Super-Pro than shown, for instance, there
was a modification that allowed the use of crystal control
for fixed frequency operation. The crystal unit was mounted
behind the main tuning dial with two knobs projecting above
the dial, one for crystal selection and the other for fine
tuning just as in the SP-600. These were evidently used in
diversity operation with two or three receivers being linked
together.
While most of the drift will be over in half an hour it
really takes many hours for the receiver to stablize,
perhaps 12 hours. They were meant for continuous operation
and will not drift much when so operated.
It has been a puzzle to me that Hammarlund did not use
voltage regulators for the later Super-Pro's not even the
SP-400, even though they did use them in the comtemporaneous
HQ-100 and had the knowledge. The HQ-and its later versions,
also had temperature compensation. Perhaps the temperature
compensation would have required too much redesign of the
very complex tuning unit but voltage regulation would have
required little and is easily applied to existing receivers.
Some of the war time receivers, notably the
Howard-built ones have some other variations, probably due
to shortage of parts. For instance toggle switches are used
for the BFO switch instead of a rotary switch. Makes no
difference to operation but doesn't look as nice.
One somewhat obscure effect of the high quality audio
circuit is that static and interference are less bothersom
because the audio circuit is not exagerating them with
distortion. Most tube communication receivers have very
simple, single-ended, pentode output stages with no
feedback. The are big time distortion producers. The
difference is easily heard in a direct comparison. For
instance, for a receiver like the SP-600 which has a
detector output listening to this output on a good quality
external amplifier in comparison to the receiver's own
amplifier is night and day. Its interesting that the
original ads for the SP-600, based on a developmental model,
indicate it had push-pull audio. I rather think that the
desire to put the entire receiver, including the power
supply, on a single chassis required some serious
compromises including dropping the push-pull circuit. The
two chassis arrangement of the older Super-Pro's allows room
for the quite large audio stage, comprising three tubes and
two rather large transformers.
Some other receivers of the time also had push-pull
amplifiers, notably the SX-28 and SX-32, SX-27, SX-36 and
the National NC-100, NC-200 and later versions, and the
HRO-50 and 60. Even though the National receivers have
rather narrow IF's the lack of distortion makes a difference
as noted above. This is not an exhaustive list, there were a
number of other receivers with high quality audio stages.
Many had a jack for a crystal phonograph pickup in order to
get double duty from the audio stage.
The Super Pro is a favorite of mine as you can probably
tell:-)




--
Richard Knoppow
Los Angeles
WB6KBL