[codefox1]

I'm working on a Sears model 1850(Riders 5-42) Farm Radio. It is a broadcast and S.W. set with a "Skyscraper" Cabinet.
I'll not be posting any pictures just yet, as the cabinet still needs work. This winter project will probably linger well into Spring.

The volume control is merely a 35 ohm potentiometer that "starves" the filaments of the "'translator" and IF filaments. Never seen that before.

It uses 2 volt tubes, 135 VDC B+, and also needs -3.0 and -4.5 bias supply (relative to chassis.) So I found a little transformer that has 110 VAC input, a 6.3 filament winding, and another secondary for about 125 VAC. It will fit on the chassis where the wax box electrolytic used to reside. Perfect! I used the well known LM317 to get the filaments going, and used a bridge of silicon diodes to get my B+. Still OK. "B+" a little high, but I can fix this. This set has a #30 ballast tube so anything from 2 to 3 volts would be OK. The ballast evens the load. Cool.

But what about the "C" supplies. I'm out of windings to use!

Well I see that in this set the "B-" does not go to the chassis ground, but uses an 810 ohm resistor and a "backwards" electolytic to provide a negative bias to the output tube, in this case a 233. So the rest of the set sees a B+ 15 volts or so less than the supply.

Turns out it develops about - 16 volts on the 233 first grid, so I'm wondering why I couldn't just make a little voltage divider that would steal about 1 ma from this source, and provide my voltages. So I did, I used a 3000 ohm, a 1500 ohm and a 12500 ohm resistor to split it out, and was pretty much on the money.

I'm going to tinker around with the values a little bit to get everything right.

I'm a long way from getting the set working properly, but everything is now biased correctly.

Now, my question is just why didn't they do something like this this all those years ago?

Or am I going over to the dark side? Somebody's got to have gone there before.

[Ron Ramirez]

I have a schematic for a universal A-B-C supply around here somewhere. Let me see if I can find it.

It uses zener diodes to provide several B and C voltages from the same 120 volt source, plus a separate, regulated A supply.

I built it back when I used to collect 1920s battery radios. I no longer have the supply, nor do I have any of the 1920s battery sets from that period of my collecting hobby. I do have one three dialer that I acquired a year or so ago, an AK 20 Big Box with an AK model L horn. The radio needs complete restoration, and the horn has no driver.

[codefox1]

Thanks, Ron. I think I was just wondering why the "B-" developed by the audio stage was never tapped in these early designs to provide tiny current supplies for the bias circuits.

Of course I could take a small capacitor off the secondary of the the transformer, and make a bias supply much the same as you can do with any amplifier to produce a "C" supply, then divide it into the values I need, but I am still in wonderment why this was not done before. The answer may well be the cost of a couple of electrolytics at that time. I understand that bias batteries would last about as long as they would whether on the shelf or in use; about the same as the ones that rust and corrode our ancient VTVM's without warning.

Thanks

[philcoradio1234]

I have a schematic for a universal A-B-C supply around here somewhere.

I know a website that has a universal supply here it is http://antiqueradio.org/art/PowerSupplySchematic.jpg