Philco 38-4 questions

[maxhifi]

Yeah, any pp transformer designed for 6V6 tubes will work... I would say cabnibalize a console stereo but I don't endorse such behaviour

[bandersen]

235 volts on the plates and the Philco service info specifies 5 watts undistorted output.

[rca2000]

That is sorta defeating the purpose of P-P isn't it?

A SINGLE 6F6 can do 5 watts or so, if driven the right way !!

[bandersen]

Might have been a marketing thing so they could charge more for a higher tube count ?

[DavGoodlin]

Two Philco consoles, with PP 41 tubes, I restored had defective output transformers. I took one of them apart and the wire is so fine, you can't move the leads without breaking them off.

I bought a spare PP output trans from Tubesandmore, so Im ready for the next one. At least its not your power trans, more of an issue in Zenith radios, which I see few of in comparison.

[darklife]

You guys may not believe this but I just found a power supply transformer that has exactly the same ohms on its secondary winding and the 6.3v taps come out to be around 3 ohms so it matches the blown audio transformer.
I wired it in place of the broken transformer and IT WORKS!
Not only does it work but it sounds good too!
It is a slightly larger transformer but it mounted to the original holes on the speaker where I removed the old audio transformer and rivets.
I know it is not meant for audio usage but heck it sounds better than the original and all the voltages check out.
Sometimes luck does exist.

[DavGoodlin]

Congratulations, you have discovered a solution we can use.

What was the secondary voltage on the power transformer you found and did you use the primary winding for anything?

[drdave3]

What did you do to get that grime off the chassis?

[old_coot88]

Originally Posted by darklife

You guys may not believe this but I just found a power supply transformer that has exactly the same ohms on its secondary winding and the 6.3v taps come out to be around 3 ohms so it matches the blown audio transformer.
I wired it in place of the broken transformer and IT WORKS!
Not only does it work but it sounds good too!
It is a slightly larger transformer but it mounted to the original holes on the speaker where I removed the old audio transformer and rivets.
I know it is not meant for audio usage but heck it sounds better than the original and all the voltages check out.
Sometimes luck does exist.

Yessir, a power tranny will work. Response rolls off dramatically above about 5khz, but that's still plenty adequate broadcast quality.

When i was a dumb kid and didn't know any better, i used two TV power xfmrs hooked back-to-back to plate-modulate a CW transmitter for voice. The primaries, 6.3V, and 5V windings were all in series and in-phase as shown.
It worked great.
(I didn't understand about core saturation then. The xfmrs were 25 cycle type with lots of iron, and that's probably what prevented any core saturation issues.)

[bandersen]

Originally Posted by DavGoodlin

Congratulations, you have discovered a solution we can use.

What was the secondary voltage on the power transformer you found and did you use the primary winding for anything?

I'm curious how it was wired up too. The schematic shows about 340 ohms on either side of the primary center tap of the output transformer.

Was the primary center tapped on the power transformer you used ? I would expect the primary of a power transformer to have a much lower resistance than 340 x 2 ohms

[Reece]

I believe he used the secondary (B+) winding of the xfrmr as his audio primary.

[darklife]

Originally Posted by DavGoodlin

Congratulations, you have discovered a solution we can use.

What was the secondary voltage on the power transformer you found and did you use the primary winding for anything?

The transformer came out of a gutted Heathkit AG-9a audio generator. I think the HV output was probably around 300v, and the fil. voltage at 6.3.
I used the HV B+ winding of the transformer for the audio primary (red, yellow-red, red) and the fil. voltage for the speaker output (greed, green).
117v wires (black, black) are unused but they do come out to around 15 ohms I believe, so theoretically they could have been used for another speaker supporting a higher impedance.

Measuring with a multimeter I got around 320ohms from Common to each red wire. and 3ohms on the filament wires.

Originally Posted by old_coot88

Yessir, a power tranny will work. Response rolls off dramatically above about 5khz, but that's still plenty adequate broadcast quality.

I haven't swept the transformer yet on a scope but it sounds like the highs are there just fine. It may roll off some but with the design of the radio I doubt the speaker itself would reproduce much above 5kHz.

Originally Posted by drdave3

What did you do to get that grime off the chassis?

Just a good dusting, then scrubbed with a cloth and some warm water. Owners do not care how shiny the metal looks when finished so just went for a basic clean.

Oddly the transformer sounds really good and distortion free.
Trying this transformer was a shot in the dark but it works wonderfully!