The PHILCO Phorum
37-116 (122) - Audio Interstage Transformer - Modification - Printable Version

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37-116 (122) - Audio Interstage Transformer - Modification - spfradios2 - 05-26-2010

I'm new here, and not an electrical engineer. I've been to the Antique Radio Forum regarding my issues, and they sent me here. This is my second Philco (normally I'm a Zenith guy... sorry).

The primary is dead on my audio interstage transformer (part 109 - 32-7057). It uses a 6F6 to drive two 6B4's. I was lucky enough to find a good one at Play things of the Past.

Questions:
1) Should I take the time to protect the primary winding of my new transformer from the 6F6 plate per the Modification A idea on Philco Radio.com?
2) If that is a "yes", I need advice on the size of the plate resistor? By what principle do you size it? I can learn something here!

As my restoration proceeds, I'm sure I'll be back with more questions. But one issue at a time for now.

Thanks in advance
John Shaffer
Honor, MI

PS. I like your Philco sites, and the Schwark schematic stuff.

Re: 37-116 (122) - Audio Interstage Transformer - Modificati - hotrod - 05-27-2010

It good to see you made the trip over here from ARF. In addition, I'm glad you were able to find a good transformer at Play things of the Past. After reading your post talking about plan A, on ARF, I am wondering if I too should protect the transformer on my 38-116. Hopefully some one will answer you soon.

Rod

Re: 37-116 (122) - Audio Interstage Transformer - Modificati - spfradios2 - 05-27-2010

Thanks for the advice to come here.

Yes, I could not believe he had one. I looked on several pages, inquired as advised, but nothing came up. As it turns out, he (Gary) had one on the "Driver" page. I about fell over trying to get my wallet out! I asked him to check the DC resistance on all sides... he said it is within spec of the schematic values I gave him.

ARF people have countless times come between me and disaster. Its going to take me a long time to get the skills & knowledge you and others have. I can replace or replicate a radio... anybody can do that... but I can't modify a circuit. That's where these forums come in.

I'll be back.
John

Re: 37-116 (122) - Audio Interstage Transformer - Ron Ramirez - 05-27-2010

Hello John, and welcome.

The article on my website about modifying the interstage circuit of Philco radios came mostly from posts made by John Grady, who is (or was) a regular poster on ARF. With his permission, I used his ideas to make the page on my site on this subject; all of his ideas and text is credited to him on my page.

http://www.philcoradio.com/tech/audio.htm

It is up to you whether or not you wish to use one of these mods. If I had any question about the audio transformer, I would not hesitate to use one of these mods. In fact I recently completed an early five-band 16B cathedral. Initially, I could only barely hear one local station - and then, after I carefully checked the circuitry, took voltage measurements, and had the thing set up to do circuit tracing with my signal tracer, it decided to suddenly start working on its own. Upon further checking, I noticed the screen grid of the 42 driver tube was glowing cherry red. The screen is tied to the plate in this circuit. So I subbed another 42, and the screen no longer glowed red. I then performed Mod B on this radio as I did not want to take any chances on it quitting again.

John Grady suggested using a 27K to 33K, 2 watt resistor. I did not have a 2 watt resistor in that range in my parts stock so I used a 30K, 1 watt resistor. The radio worked fine with the subbed resistor. I have to say though, I did notice a very small drop in volume after I performed the mod to the radio. But since the 16 has so much power anyway, I would say this is noticeable only to the very discriminating ear. Your 37-116 will also have power to spare, so a very slight loss of volume can be lived with, I think.

Re: 37-116 (122) - Audio Interstage Transformer - Modificati - spfradios2 - 05-27-2010

Thanks for the info.

The thing I worry about is burning out the primary as it carries the plate load. I thought such reasoning at your web site made sense. And by my experience, first having my transformer dead, and then trying to find a replacement, it makes sense to protect it. These transformers are getting hard to find. So I think I'd like to try the modification. I may first hook things up as per the schematic. Then tune and experience how the radio works according to original design. Then implement the modification and see how it changes. I am concerned about the dynamic range and how it may/may not change.

Regarding the plate resistor to install, there is plate resistance and load resistance for the 6F6. RC 15 has them at 80K for fixed bias, and 7k. What part do these values play in selecting a replacement resistor for the primary winding?

Also, I'm wondering what happened such that the primary burned out. Did a tube short out? or what?

Further comments are welcome.
John