I am restoring a Philco model 40-201 and the audio transformer was bad. The speaker was blown to shreds etc. I have replaced numerous components including installing a Hammond 125E and I installed a speaker I had laying around, which gave me very low volume. Turns out that the transformer primary was open. I initially tried this on the bench and there was no sign of heat etc, so I am not sure if I burned it up somehow or if it was bad when installed. What's the best universal transformer for this radio, and do I need to add resistors in parallel to the primary of the transformer to get the 350 ohms on each side of the center tap? I am guessing and it could get expensive.

No. The audio output is push-pull and you need a xfmr with a center tap on the primary. The 350 number is volts, not ohms.

Look at RadioDaze and tubesandmore for xfmrs.

The Hammond 125E is center-tapped, suitable for push-pull. After looking at the schematic, it does seem that it indicates 350 ohms from center tap to each side - - very low resistance. However, impedance and resistance are differently measured, as you know. With a VTVM in 1940, maybe 350 measured ohms is correct. Question - - you said you replaced the speaker. Is the field coil the same as what you removed? If the impedance/resistance is higher, your b+ voltage might be significantly lower than it should be. Lots of variables here. . . .

I think that the ohms value given on the Rider schematic is for DC resistance so a service tech could use a simple ohmmeter to get a rough idea of the condition of the transformer.

Impedance of an audio transformer is usually given for a standard value of 400 Hz. The 400 Hz impedance of a Type 42 output transformer primary should be around 7,000 ohms.

You might first want to replace the bypass caps hanging off the output tubes' plates. If either of these shorts out it will burn open one side of your output transformer in no time.

It's difficult to sub in a "random" speaker and have it work well on these sets, because the speaker field winding serves double-duty as the B+ filter choke. Using a different speaker with different field coil impedance could result in poor/no operation due to low B+ voltage or overheating/damage due to excessively high B+ voltage. Philco used pretty much the same 12 inch speaker ((860 ohm field) for its big consoles from 1938 through 1942 and they are easy to come by on eBay

Beyond that, any push-pull output transformer that is designed to work with 6F6 output tubes will work. Output winding impedance should be 4 to 8 ohms. The 42 tube that your set uses is electrically identical to the 6F6 tube, it is just packaged differently. Zillions of radios were made with dual 6F6 (or type 42)output tubes before the more efficient 6V6 became popular, so finding a junk donor set should not be a challenge.

If the output tube is a 42 or a 6F6 ...According to this data sheet the output transformer impedance for PP should be about 10k ohms.
http://www.datasheetarchive.com/dl/Scans-008/Scans-00176138.pdf

Here is a wonderful short explanation about understanding output transformer impedance.
Please read it ... it is easy to follow and a great article.
At the bottom they also have a chart which indicates 10k impedance for 42 in PP mode.
http://www.radioremembered.org/outimp.htm

You may be able to find a transformer used at "Play Things of The Past":
http://www.oldradioparts.com/pg2a23d.htm

Yes, I agree that the primary impedance should be 10K for 42 or 6F6 tubes operated in push-pull. 7K is the appropriate impedance for single-ended operation.

Give up. Hardly anyone on these forms will try to learn anything on their own. It's much easier to ask someone who has done their home work and then go on their merry way until the next time.

.............:Here is a wonderful short explanation about understanding output transformer impedance.
:Please read it ... it is easy to follow and a great article...............

Geez, Old Man - - if everyone knew everything about their "hobby" - not profession, mind you - then there would be no need for forums at all. In fact, to follow your logic, perhaps we should all study medicine, carpentry, plumbing, etc. so we wouldn't have to bother the pros at all! It's like asking a chef a question about the conversion temperature of a gnache, and he becomes indignant that you didn't go to a culinary institute. While I agree that many folks venture into projects that are potentially unsafe (like buying your first chainsaw at Walmart to cut your own firewood), those of us who cut our teeth on vintage electronic theory are a dying breed - literally. If we can pass on our knowledge to someone else, why not? When we die, that knowledge will pass with us; knowledge (AND experience) which is not easily obtained these days. I'm reminded of the cliche "reinventing the wheel". . . . . . . Personally, I'll help anyone I can with the resources I've been grateful to have learned. I am never resentful of those who choose to learn . . .

I take it you are one of those who will not read the reference provided in the post.

:I take it you are one of those who will not read the reference provided in the post.

Umm - I wasn't the one asking the question, although I agree the reference is very clear and explanatory. Perhaps you're the one not following the post/author sequence. . . .

::I take it you are one of those who will not read the reference provided in the post.
:
:Umm - I wasn't the one asking the question, although I agree the reference is very clear and explanatory. Perhaps you're the one not following the post/author sequence. . . .

All, thanks for the input. I found the problem with the transformer, a broken lead on the center tap. I fixed the transformer and I am going to check some voltages when I have the time. Unfortunatly my wife needs me more than the radio does right now, she has some heart issues to resolve and thats where I need to be. You guys gave me lots to work with and I do appreciate the feedback.
Walt

Kindest regards. Wishing both of you all the best.

:Kindest regards. Wishing both of you all the best.

Good news on two fronts, most important the wife will be fine soon. On the learning side, I read the recommended article on impedance matching and it is great, it cleared up what has long been a mystery. My thanks to Doug for the initial answer, it started a dialog that actually taught me some useful tricks. I already have a Variac which I may actually use now. Most of the feedback was positive and useful, and even the negative comments challenged me to do the homework.
THANKS AGAIN - Walt

Congratulations. You are one of the few who actually took the time to learn. Keep it up. Google for things you don't understand. Look for authoritative sites such as those from universities. Hobby sites can be helpful also, just don't be too trustful of technical advice until you verify it from other sources. The more you learn the more fun the hobby will be.

Hi Walt:
I'm glad I could point you in the direction of some really great information about output transformers.

I'm glad you read and digested it. That's what really counts.

I learned so much from that one great article that I now take every opportunity to steer others to it so they can benefit as well.

:Hi Walt:
:I'm glad I could point you in the direction of some really great information about output transformers.
:
:I'm glad you read and digested it. That's what really counts.
:
:I learned so much from that one great article that I now take every opportunity to steer others to it so they can benefit as well.
:That radio is now a beautiful refinished Philco, sitting in my sons living room and working like new. I used to shy away fro radios with speaker issues because I knew it wasn't my strong suit. BRING em on!

:That radio is now a beautiful refinished Philco, sitting in my sons living room and working like new. I used to shy away fro radios with speaker issues because I knew it wasn't my strong suit. BRING em on!
--------

Atta boy!

Restoring these old sets is very rewarding... and a great sense of accomplishment too.
Right?