[jmac3ky]

Does anyone know the ratio and watts of the Philco audio driver transformer 32-7057 ? Thanks, John.

[Radioroslyn]

Ok I give up what's from????

[jmac3ky]

You got me Terry. A Philco model 116.  John

[Jim Koehler]

That interstage transformer looks to be a 3:1. Wattage would be very small. The output transformer would be handling the wattage.

[jmac3ky]

Thanks Jim,

Would the smaller 5W audio driver transformers that they sell handle the wattage?

John

[Mondial]

John, you will need better than a usual interstage transformer. 

The output stage of this radio runs in class AB mode, so you need a driver transformer with a step down ratio and the ability to handle the current of the 42 driver tube. The normal type transformer has a step up ratio and can't carry the high primary plate current of the 42.

Your best bet would be to find a used transformer from another Philco with the same "super class A" output arrangement of the 42 driving push pull 42's.

There is a Hammond driver transformer that can be made to work, but I will have to look it up.

Which winding is open on your transformer?

[jmac3ky]

Hi Jim,

I need to explain what the audio driver transformer post was all about. It's spiraling into space.

I'm working on a 1935 Stromberg Carlson 83 (a big 10 tube console). The primary of the audio driver transformer, part 22803 is open. By the schematic it steps up from one 6F6 tube to a pair of 6F6s (2) in a push-pull output setup. Then there is an output transformer and so on. This is actually where I need the repair. Because I had a 1935 audio-transformer from a Philco 116 (step down it turns out) I thought I would ask the Philco Phorum what kind of wattage these carry, getting a ball-park for the Stromberg. I guess Murphy's plan kicked in quick.

The Stromberg trf does appear to be a step up, between two/three to one. Still the question is, will the 5w ones they sell on the market handle the heat? Is all of this clear as mud? Thanks Jim.

[Mondial]

I think the Philco transformer should work fine. The 42 and the 6F6 are electrically identical, just the base is different.

Both radios use the same basic circuit with the tubes connected as triodes. The turns ratio difference is really not a problem, as with the step down ratio you have better low impedance drive to the grids of the 6F6 output tubes. 

The problem with the little 5 Watt transformers they sell is that it can't handle the plate current of the 6F6 driver. They are meant to be driven by something like a 27 or a 6J5 and the core will saturate. Those transformers also can't pass the low bass freqs as well as the Philco driver, and they can't supply the drive necessary for class AB operation of the output tubes.

If you have a good driver transformer from a 116, I would definitely use it. Its just about the best replacement you will find.

[jmac3ky]

Thank you. That's sweet music. I appreciate your patience.

Great people on the Phorum. John

[Radioroslyn]

Hey Mike,
Just a hypothetical question. What if you used the too small 5w transformer but on the plate of the 6F6 driver tube you use a 5k 10w resistor to supply the HV. Then use a few mfd coupling cap from the plate to the primary and ground the other primary lead. Sort of resistance coupling to the primary. Does this help to lesson the saturation issue. Have seen it done some 100w level AM transmitters to drive the 807's in the modulator.
Sorry A little off topic.

[Mondial]

Yes Terry, that would work but there are a few problems. 

A triode connected 6F6 as a driver draws about 25-30 mA of plate current. Through the 5K, that would give a 125 V drop, so you now have only about 125 V left on the plate, limiting your swing. A better choice would be an audio choke, which would pass the full B+voltage and allow full output. 

In fact John could use a resistor or choke to feed B+ to the 6F6 driver plate and bypass the open primary of his bad transformer, and then use the cap from the plate to feed signal to one side of the PP secondary. This would effectively give you a 1:1 ratio to the grids of the output 6F6's.

In any case, you really want a step down ratio to get full output from the PP 6F6's. As soon as your peak drive signal is greater than the negative grid bias, the output 6F6 grids begin to go positive and draw current in class B mode. The whole purpose of using a big tube like a 6F6 as a driver is that it can supply the current necessary to drive the grids positive. With a step down driver transformer, it is easy to supply the low impedance current drive necessary for the transition from class A to class B without distortion. The little 5W driver trans could not do this and your output would be limited to class A only.

Why Stromberg Carlson used a step up ratio is a mystery as it makes no sense. Maybe they already had this transformer design from another model,  but it certainly would not be the best choice for low distortion and max power output from triode connected class AB 6F6's.

[jmac3ky]

Update:  Installed the Philco 16 Audio Driver Transformer in the Stromberg-Carlson 83. It works fine. And the unit blends right in.

Thanks so much for the advice.  John