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

"Henry Kolesnik" wrote in message
...
What's an easy way these days with limited audio xfrmrs around to get the
600 ohm spkr output to 4 or 8 ohms?

120 Volt to 12 Volt power transformer. Good at least down to 50 Hz or
better.

W4ZCB

Harold E. Johnson wrote:
"Henry Kolesnik" wrote in message
...
What's an easy way these days with limited audio xfrmrs around to get the
600 ohm spkr output to 4 or 8 ohms?

120 Volt to 12 Volt power transformer. Good at least down to 50 Hz or
better.

The problem isn't the low end, the problem is the high end. But the ratio
is about right, and if you pick a good transformer you'll be okay.

Problem is that a lot of cheap transformers have poor core material that
leads to a lot of distortion, and a lot of the better transformers are wound
to specifically limit the frequency response. For example, the Talema toroids
from Digi-Key are great at 60 Hz, but down 6 dB at 500 Hz and keep falling
above that. This is a great thing for a power transformer where you don't
want to leak noise into the power line or the other way around, but it's a
bad thing for audio.

But there are a lot of filament transformers out there that are quite
reasonable for audio, and they are cheap enough that you can try a whole
bunch of them and see which work best.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

On Thu, 30 Oct 2008, Scott Dorsey wrote:

Harold E. Johnson wrote:
"Henry Kolesnik" wrote in message
...
What's an easy way these days with limited audio xfrmrs around to get the
600 ohm spkr output to 4 or 8 ohms?

120 Volt to 12 Volt power transformer. Good at least down to 50 Hz or
better.

The problem isn't the low end, the problem is the high end. But the ratio
is about right, and if you pick a good transformer you'll be okay.

And of course, one commonly written about trick in the old days was to
use a "useless" 400Hz transformer as an audio transformer, the operating
frequency being about where you'd want frequency response to start for
communication use. Since it moves the frequency up by six, the frequency
response is probably better at the higher frequencies.

That's only a useful trick, though, when 400Hz power transformers are
commonly available, and cheap because nobody can find a use for them.

Michael VE2BVW

Michael Black wrote:
And of course, one commonly written about trick in the old days was to
use a "useless" 400Hz transformer as an audio transformer, the operating
frequency being about where you'd want frequency response to start for
communication use. Since it moves the frequency up by six, the frequency
response is probably better at the higher frequencies.

That's only a useful trick, though, when 400Hz power transformers are
commonly available, and cheap because nobody can find a use for them.

I still see PLENTY of 400 Hz transformers coming out of the surplus
pipeline today, not to mention 400 Hz motors and brick power supplies.
You want some? I think I even have a box of 400 Hz fluorescent light
ballasts somewhere. Not even a little bit useful.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

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

70V PA distribution transformers.

A 70V distribution transformer with an eight-watt input tap will give you
600 ohms input.

Old 70V equipment is usually available free for the asking from your local
installed sound company.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."

Getting old is not recalling the alternatives offered, so I'm a dummy. I
found the filament xfmr first and tried it but it reduced volume so I
assume my radio which is non-military doesn't have 600 ohms like the
military manual I have. So I guess mine is commercial, either a
SP-200-SX or more likely a SP-210-SX and I think it has a 3.2 ohm
output transformer.
I can't find any info and I wonder if anyone knows for sure on the
commercial models output transformer's impedance.

--
Thanks & 73
Hank WD5JFR









"Scott Dorsey" wrote in message
...
Henry Kolesnik wrote:
What's an easy way these days with limited audio xfrmrs around to get
the 600 ohm spkr output to 4 or 8 ohms?

70V PA distribution transformers.

A 70V distribution transformer with an eight-watt input tap will give
you
600 ohms input.

Old 70V equipment is usually available free for the asking from your
local
installed sound company.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."

"Henry Kolesnik" wrote in
message ...
Getting old is not recalling the alternatives offered, so
I'm a dummy. I found the filament xfmr first and tried it
but it reduced volume so I assume my radio which is
non-military doesn't have 600 ohms like the military
manual I have. So I guess mine is commercial, either a
SP-200-SX or more likely a SP-210-SX and I think it has a
3.2 ohm output transformer.
I can't find any info and I wonder if anyone knows for
sure on the commercial models output transformer's
impedance.

--
Thanks & 73
Hank WD5JFR


All of the Super-Pro models from the SP-100 on had 500
ohm outputs for the loudspeaker and a loosly coupled 8K
winding for headphones. The 500 ohm output will drive a
loudspeaker at much reduced volume. A 12.6V filiment
transformer has about the right impedance ratio (100:1) to
drive a 4 to 8 ohm speaker from this tap. These tramsformers
have enough core material to work down to low audio
frequencies and surprizingly good high frequency response.
70V public address transformers also work very well as Scott
Dorsey pointed out. The original Hammarlund speakers for the
Sp-200, 400, 600, had matching transformers built-in.
The SP-100 has a front panel switch for the loudspeaker
but the later versions do not. You need to mount a switch
(maybe in the loudspeaker cabinet) to cut it off. Use a DPDT
switch with a 500 or 600 ohm resistor switched in when the
LS is turned off to provide a load to the audio transformer.
It should handle a couple of watts.
The headphone winding is not critical of load impedance
and will work with phones from 50ohms to high impedance.
The military and commercial versions of the SP-200 and
210 were identical except for the use of hermetically sealed
transformers and chokes and oil filled filter capacitors in
the military versions, and some variations of panel finish.
There is an owner's manual for the SP-210 somewhere on
the web, maybe on BAMA, I have a copy but don't remember
where I downloaded it.
Most of the communications receivers built up to
perhaps the 1950s had 500 ohm or 600 ohm outputs. A few even
had the output transformer in the loudspeaker cabinet.
--

--
Richard Knoppow
Los Angeles
WB6KBL

Richard
The volume decreased with the filament transformer so I need to open it
up and see if someone made some mods. I downloaded the military manual.
It seems to me that someone at Ham went nuts changing model numbers for
very insignificant reasons.
"Richard Knoppow" wrote in message
m...

"Henry Kolesnik" wrote in message
...
Getting old is not recalling the alternatives offered, so I'm a
dummy. I found the filament xfmr first and tried it but it reduced
volume so I assume my radio which is non-military doesn't have 600
ohms like the military manual I have. So I guess mine is commercial,
either a SP-200-SX or more likely a SP-210-SX and I think it has a
3.2 ohm output transformer.
I can't find any info and I wonder if anyone knows for sure on the
commercial models output transformer's impedance.

--
Thanks & 73
Hank WD5JFR


All of the Super-Pro models from the SP-100 on had 500 ohm outputs
for the loudspeaker and a loosly coupled 8K winding for headphones.
The 500 ohm output will drive a loudspeaker at much reduced volume. A
12.6V filiment transformer has about the right impedance ratio (100:1)
to drive a 4 to 8 ohm speaker from this tap. These tramsformers have
enough core material to work down to low audio frequencies and
surprizingly good high frequency response. 70V public address
transformers also work very well as Scott Dorsey pointed out. The
original Hammarlund speakers for the Sp-200, 400, 600, had matching
transformers built-in.
The SP-100 has a front panel switch for the loudspeaker but the
later versions do not. You need to mount a switch (maybe in the
loudspeaker cabinet) to cut it off. Use a DPDT switch with a 500 or
600 ohm resistor switched in when the LS is turned off to provide a
load to the audio transformer. It should handle a couple of watts.
The headphone winding is not critical of load impedance and will
work with phones from 50ohms to high impedance.
The military and commercial versions of the SP-200 and 210 were
identical except for the use of hermetically sealed transformers and
chokes and oil filled filter capacitors in the military versions, and
some variations of panel finish.
There is an owner's manual for the SP-210 somewhere on the web,
maybe on BAMA, I have a copy but don't remember where I downloaded it.
Most of the communications receivers built up to perhaps the 1950s
had 500 ohm or 600 ohm outputs. A few even had the output transformer
in the loudspeaker cabinet.
--

--
Richard Knoppow
Los Angeles
WB6KBL

"Henry Kolesnik" wrote in
message ...
Richard
The volume decreased with the filament transformer so I
need to open it up and see if someone made some mods. I
downloaded the military manual.
It seems to me that someone at Ham went nuts changing
model numbers for very insignificant reasons.
"Richard Knoppow" wrote in
message
m...

Before you do any surgery make sure the matching
transformer is connected the right way around, the 120V
winding goes to the 500 Ohm winding on the RX, the filiment
winding goes to the loudspeaker.
The output transformer would have to be changed to
provide a low impedance output, that would be pretty
obvious. The headphone output is also brought out the back
on the terminal board.


--

--
Richard Knoppow
Los Angeles
WB6KBL