What's an easy way these days with limited audio xfrmrs around to get
the 600 ohm spkr output to 4 or 8 ohms?

--
Thanks & 73
Hank WD5JFR
"Richard Knoppow" wrote in message
...

"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