[St Tee]

Forum members,

I have a Philco model 38 Farm radio in restoration. I need to build a power supply for it to replace the batteries. I remember seeing some time ago a schematic that showed the circuit. I believe the circuit had connected several zeners to arrive at the various voltages needed. It was simple and clever. I started looking for the diodes, but lost the paper and with it the design. Has anybody in this forum built this before? I thought perhaps Ron Ramirez might remember. I think that it tapped +135vdc, +67.5vdc, +2vdc, -3vdc, and -7vdc. These are the listed source voltages for the 38, per the schematic. Otherwise, I'll need to breadboard something up and experiment.

St Tee

[morzh]

If using zeners I would use zeners with a wide range Iz. They should withstand the power equal Iz-max times Uz at about 14-15mA.

Then I would put two in them that are 68V in series, and in addition serialize a 3V and 7V zeners. Reference the point where the two connect to the lower 68V zener as a GND.

You will need a separate power source for 2V filaments. Could try AC, but filtered DC is cleaner.

Why 15mA?

Your B+ battery drain is 8-12mA and you need the zeners to withstand this current without load. The zeners should preferably still stabilize at about 2-3mA of current, which will be the case when the load is connected.

However Zeners are bad regulators to power up things - they are mostly used as references, so if you could make a clean DC power source, that would be better.

Again you need all voltages at low current except 2V (700mA).

[David]

        Hope this download thing works

[David]

    Sorry about last post ! David

[Arran]

One problem though, none of those designs supplies the 135 vdc you need. You don't really need Zener diodes in a B+ supply anyhow, those battery eliminators from the 1940s and 50s used a Pi filter like an AC/DC set and there was minimal hum or ripple with the right size electrolytics. If you used a filter choke, or a solid state substitute, there would be no ripple.
I have a schematic someplace from a Radio Craft magazine that uses an old style circuit where they supply the bias voltages through a resistor network in the B- through a resistor network. I may have it on file someplace as I sent it to Bob for his Philco 623 project. These days you can hook up diodes backwards to achieve the same result off of a cheap transformer.
Regards
Arran

[David]

I have never used the two design I posted placed here for your use. On the farm radio I have, use a D cell for the filments and 90 volts dc for the B+. The design was found on Phil's old radios look under building, what a nice site. To get 135 volts dc connect a full bridge rect to line voltage(120) add a 47UF cap and resistors to get the correct voltage for your current draw. Line voltage divided by .707 gives peak voltage which the Ecap will charge to without a bleeder resistor. I am sure the EEs will be alone to help explain what I am trying to say. David

[Ron Ramirez]

It's been many years since I built any power supplies such as what St Tee described, and like the ones David posted. I would have to search for my notes...and since I moved, I still don't know where some of my stuff is...

[St Tee]

many years ago I looked at spice to do software simulation of circuit behavior . It 's a bit of overkill for this but there is open source software available that can model a circuit to see what components would work. It just might be a fun experiment. I am 25 years removed from this work but it's good to get back into the flow. BTW, the circuit I saw some time ago belonged to Ron, so it went beyond the napkin note phase as it was online somewhere.

[morzh]

Guys,

you do not need a simulator to do this kind of jobs.
Using Spice for this is, as we say, like using a cannon to shoot sparrows.

As for free Spice, LTSpice (fom Linear Tech) is a good tool and is fully free.

[Ron Ramirez]

BTW, the circuit I saw some time ago belonged to Ron, so it went beyond the napkin note phase as it was online somewhere.

Yes, you did see it - and it was right here in the Phorum!

http://philcoradio.com/phorum/showthread.php?tid=469

(I had to do a Google search to find this. B-B-B-Bing didn't bring it up.)

[St Tee]

Yes that's the baby!

[St Tee]

To MORZH about using Spice: You're absolutely right. Started digging back in to it and realized that it was way too much. I then started drawing up a circuit when Ron gave us his reference. Thanks to all, and save the sparrows!