D&A pdx 400

[YardDog]

Have an old pdx. Looks good for the age and worked like a champ until it got hit with to much drive. It's a 10 tuber. First tube into 3 into 6, all 31lq6. It smoked the z14 choke under tube #1. My understanding is the first tube adds no power but stabilizes the voltage across the rest. Been looking up chokes on the inter web and only finding one z14. Some are orange, some are yellow, and some light blue. All seem to be equal in values stated other than color. Is there something I'm missing here? All say for d&a, palomar, and several old antique amps, basically any old tube burner. I know a lot of folks will say why bother with the choke, tubes cost to much, ect...But guys like me will always have a tube burner, kinda like the guy that wont give up on restoring a 1970 Roadrunner with 440 6 pack.

 

[nomadradio]

orange, some are yellow, and some light blue.

Actually the "blue" is more like gray. I have no idea why the color changed. Same part, just a different production year.

Since it was sold as a "Ham CW transmitter" out the factory door, it had to be modified to use for its original intended purpose.

And there is the wild card in the deck. Modified.

Mostly this means that your amplifier will be different from most any other PDX you look inside of, at least a little.

It was built with tubes that had 6.3 Volt heaters. Pairs of them in series were powered by a 12.6 Volt AC winding on the transformers.

Those tubes became rare and expensive decades ago. The version with the 31-Volt heater was used in cheap color TVs that all went to the landfill when they broke down. Lack of demand made that tube cheap, but only for a while.

Wiring two of the 12.6-Volt windings in series gets you 25.2 Volts. Connecting only the center-tap of the third winding adds 6.3 Volts to that. A good match for that tube.

That's how I would do it. But yours may or not be modified the same way.

And that's the challenge of the PDX. You pretty well have to come up with your own diagram, based on what's actually in your amplifier.

73

 

[YardDog]

Thanks nomad, great info.