Scott Export Set and "amp"

[Kent King]

Although it was built after the war, the Export set is a relatively scarce set. At the end of WW2, Scott was "stuck" with a large number of partially completed SLRM sets. They put a better face on them and the Export was born. They removed the BFO, but otherwise, the Export is an SLRM. The SLRM is an AC/DC (transformerless) set with AM and SW coverage. I have only ever seen 4 or 5 of these sets, however, there is an odd little thing going on. In the pics here, I provided the front and top view of an Export set I have. It came with the small "amp" unit which appears to be partly Scott components but is quite crude. Initially, I thought it was something someone jacked up and pretty much ignored it, in fact, I almost pitched it. But then I found another Export, with an <identical> ugly amp. And then I got wind of a 3rd one. So I pulled out the little amp and drew up a quick diagram. 

The amp includes an isolation transformer, which makes a lot of sense for the AC/DC set. The 8ohm speaker output becomes an input to a pair of 6V6s which can drive a 500ohm load. The set was modified with a 4-pin cable to connect to the amp, it includes the on/off switch line and the isolation power. A twin lead wire has speaker terminal connections. The two sets I have with the amp are both modified with the 4-pin wire (identical mods). I do have a theory: Cliff Coon.

Cliff Coon was a <long time> Scott employee, his signature is on many of the schematics (C L Coon). After the great schism in 1945, Cliff left Scott and advertised "Precision Radio Laboratories" in Chicago. I greatly suspect (but can't confirm) that Cliff might have built these units for Export owners. I'm not sure why anyone would want a 500ohm output off the 6V6s, but otherwise, the unit makes sense. I'm interested in any other thoughts or info about this...thanks!!

Kent

[Norman S Braithwaite]

More or less.  I suspect if one ordered just the Export receiver they received the chassis and supplied their own speaker.  If they ordered the console, the amplifier and a 12-inch PM speaker were included.

Norman

[David C. Poland]

so, the SLRM 25L6 output tubes have been repurposed without alteration, to now function as drivers for the 6V6 tubes.

[Kent King]

Given your research, I went back and looked at mine again. My set speaker terminals are carefully ground off to remove the impedance.There is an 8 ohm matching resistor on the input (grid side) of the transformer:

and like yours, my output says 600 ohm....I'll check the impedance tonight.

Kent

[Norman S Braithwaite]

Okay, the only thing I am finding that relates to a line impedance is the label on the speaker output terminal strip.  It is marked 600-ohm speaker.  However, measuring DC resistance across the terminals yields approximately 3-ohms.  Hence the actual output transformer impedance is approximately 8-ohms.  The original Scott labeled speaker has an 8-ohm voice coil matching the output transformer speaker impedance.  Made from spare parts.  The 600-ohm label is incorrect.

Norman

[Norman S Braithwaite]

Those audio transformers appear to be swapped!  The SLRM has a 500-ohm output which would match up with the 500-ohm winding of what you show as the output transformer.  I'll investigate my Export receiver with amp and original speaker when I have a chance.

Norman

[Russ Webb]

I think that I may have just found a purpose for my 510 amp. If I added a octal style plug to the SLR-H I could do away with the little accessory trans (pic upper left). That way I can still use it as a test fixture, it's current job. And  there would be plenty of audio out, something that is in short supply in a lot of vintage boat anchors.

[Kent King]

Yes, this unit just connects to the speaker output from the set, driven by the 25L6s.

[David C. Poland]

So, with the outboard amp, the radio still have/run the pair of 25L6s?