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PWK Amplifier Schematic


dBspl

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I thought you guys might enjoy this...

 

I've probably passed by this picture in the engineering lab in Indy a 1,000 times.  It finally dawned on me one day as I passed by it that it's an amplifier schematic drawn by PWK's own hand.  Hunter was able to find the original drawing the picture was based, but unfortunately the drawing isn't complete.  First, the power supply schematic is missing, and second, the amplifier schematic doesn't show an output transformer.  There's also a puzzling comment about "crossover output (to speakers)", which doesn't make a lot of sense. Anyway, I was thinking it would be cool to build this amplifier given the connection to PWK.

 

The 6A5G output tubes are very similar to the 2A3, and the output is in the range of 10 - 15 watts push-pull. I'm guessing the schematic was drawn in the early 50's, but going all the way back to the early 40's seems plausible, too. It's little surprising the schematic isn't dated since PWK rarely wrote anything down without putting a date on it. Maybe the date is with the part of the drawing with the power supply schematic.:)

 

I'm planning to take a shot at designing a power supply, but I've never really done that before and I may need some help. The C+ & D+ voltage supply isn't specified, but based on the 6A5G data sheet, it looks like the C+ voltage should be around 325.  The D+ supply looks like it needs 300 volts based on the 6J5 & 6SJ7 tubes. I'm not sure what the screen supply voltage on the 6SJ7 should be. It could be as much as 300 volts, but the little schematic shown on the lower left hand side of the drawing could be indicating the screen voltage should be 100 volts.  I don't know.  

 

From what I can tell, the output transformer primary needs to be 3000 ohms?

 

Kerry      

IMG_1504.JPG

PWK_Amplifier_Circuit.jpg

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Seti,

 

You might get there before I do. I have another project that I committed too before I saw this one.

 

I'm not very familiar with tube circuits, but it hit me as odd the phase inverter circuit is also the front end of the amplifier.

 

DrWho,

 

I understand what you're saying. I suspect it would also be a challenge for me to get up to speed on the software, but I may take a look at it. I did find a couple of 6J5 and 6SJ7 tubes (metal envelope) in a box of old tubes I have.  They tested good, too.

 

Thanks,

 

Kerry

 

 

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20 hours ago, dBspl said:

Seti,

 

You might get there before I do. I have another project that I committed too before I saw this one.

 

I'm not very familiar with tube circuits, but it hit me as odd the phase inverter circuit is also the front end of the amplifier.

 

DrWho,

 

I understand what you're saying. I suspect it would also be a challenge for me to get up to speed on the software, but I may take a look at it. I did find a couple of 6J5 and 6SJ7 tubes (metal envelope) in a box of old tubes I have.  They tested good, too.

 

Thanks,

 

Kerry

 

 

 

I am in the middle of Brook 12A clone build but the chassis build has gotten out of control. Also working on a butterworth tube based line level adjustable crossover for various projects. Not to mention a four chassis eimac 304tl 30wpc SET build but I am just the solder monkey on this project. So I think you'll make it before me....

 

 

 

 

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  • 1 year later...

So I was down in Hope earlier this week and Jim Hunter was showing me a few things in the museum.  We were down in the museum basement and I was just looking through some of the items in storage and noticed a piece of tube gear stuck in back of one the storage racks.  We pulled it out for a better look, and it turns out that it is the PWK 6A5G amplifier based on the schematic shown above.

 

It looks to be in pretty good shape.  It is missing the output transformer and the rectifier tube has separated from its base.  As you can see, the chassis is designed to be mounted in an equipment rack (typical PWK). I'm guessing, but this is probably his laboratory measurement (and probably listening) amplifier from the late 1940's to early 1950"s.

 

dBspl 

PWK_amplifier__1.jpg

PWK_amplifier__2.jpg

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Lot of clues or guesses.  Numbers or name off the power transformer might give you the specs on that, and I'm sure there must be a way to measure the choke. Then check the resistor and multitcap values against the partial schematic to see if they match up, and that might yield the likely output of a the OPT. The rectifier has got to tell someone a lot (hint, hint Maynard and Craig).

 

Looks like there are two rca terminals so it appears to be single-ended, and the four wires on top if they ran to the OPT tell us there were only two values, say16ohm, or 8ohm, or maybe 8ohm and 4ohm Or, if  the amp was designed for voice or broadcasting , like say the Bogen MO100's, the output values could be dramatically different.

 

Haven't tried it in a long time, but there is power supply modeling software where you can work back.

Could be a group build. Need pics of the underside.

 

Are you a shy guy and don't want to show us that baby's underside!

 

 

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I attached the only pictures I took of the underside.  It's not very pretty.  The #3 photo is the power supply and output tube side of the chassis.  The #4 photo is the pre-amp and driver side of the amp, although this side also includes the (missing) output transformer. There is an discrepancy that I need to figure out.  Jim confirmed the metal tubes are 6SJ7's as shown in the schematic (driver tubes), but the there is no 6J5.  This is odd given the 6J5 in the schematic serves as the phase splitter.  I'm guessing the front-end of the amplifier has a different topology than the schematic.  It could be an earlier version?   

 

I believe the rectifier is a standard 5U4G.  The 6A5G is rated for 10 watts in class AB.  A 10 or 15 watt output transformer with a 5000 ohm primary would probably be correct.

 

dBspl  

PWK_amplifier__3.jpg

PWK amplifier #4.jpg

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