Interstage Audio Transformer Recommendations

[MT70B]

I am working on an Amrad Symphony (Model 81) TRF radio from the late 20's early 30's. It was passed down to my father-in-law and now to me. It needs a replacement interstage audio transformer that feeds the grids of the push-pull 45 tubes from the first audio stage's 27 tube. The transformer in the radio has both an open primary and one half of the secondary. So I think it is toast (think materials used to pot it inside the metal box may have gotten the best of it over the years). I have been looking around for a suitable substitution and thought I would ask for recommendations from the forum.

What you you recommend, and why so I can learn from your experience.

Thanks!

Mike

PS if you want to see the schematic it can be found here: http://www.nostalgiaair.org/resources/065/M0001065.htm

[polyphase]

I have successfully used an interstage transformer from Antique Electronic Supply, part P-T156, in 20s vintage radios. It should work fine with the 27-45 tube combo. It is quite small, so it could well fit into the original enclosure.
I have a couple extras, happy to send you one for a tryout.

[MT70B]

Originally Posted by polyphase

I have successfully used an interstage transformer from Antique Electronic Supply, part P-T156, in 20s vintage radios. It should work fine with the 27-45 tube combo. It is quite small, so it could well fit into the original enclosure.
I have a couple extras, happy to send you one for a tryout.

Polyphase, Thanks for your input! I looked at the P-T156 on the Antique Electronic Supply web site but they did not show a spec sheet for that part like they do for some of the other transformers. By chance do you know what the primary and secondary resistance is for the P-T156. They do say the primary can handle 10 ma which is good. Looking at the RCA tube manual looks like the 27 tube in my radio who's plate current is supplied through the transformer draws up to 5.2 ma which is in range.

Thanks again for your input.

Mike

[polyphase]

The DC resistance of a new transformer will be much less than a period original, thanks to modern materials (a good thing). The important parameter, besides current rating, is the input impedance which is 10k Ohms. The 27 specs plate resistance at 9k to 11k Ohms depending on applied voltage, so this is an excellent match

[dieseljeep]

Originally Posted by polyphase

The DC resistance of a new transformer will be much less than a period original, thanks to modern materials (a good thing). The important parameter, besides current rating, is the input impedance which is 10k Ohms. The 27 specs plate resistance at 9k to 11k Ohms depending on applied voltage, so this is an excellent match

IIRC, I used one of that type transformer in my Wells Gardner chassis model ODM. It was kind of a different design, using some metal tubes and a 76 driving PP 45's, built in late 1935. Naturally, the interstage transformer was open.
I couldn't get enough volume, using the AES transformer, plus the tone control, would have little effect. The schematic is in Rider's seven. The AES transformer is a 3:1 ratio, where the W-G needs a 2:1 ratio.
A 3:1 ratio transformer might work OK in your radio. Everyone I talked to, said a 3:1 was fine.

[MT70B]

Polyphase, Diesieljeep,

Thanks for the additional information. I guess what I was getting at from a backwards perspective was the transformer ratio when I asked about the resistance. Probably making a big assumption that the wire size between primary and secondary are the same. However, the (dissected) transformer in the radio appears to have the same size magnet wire coming out to all of the leads. So using the same backwards logic, the primary resistance is supposed to be 1,600 and the total secondary resistance was 10,600 ohms in the defective transformer. So using the resistance as a loose way to simulate the impedance and plugging the numbers into the turns ratio calculator found here: http://www.maxmcarter.com/classecalcs/tratiocalc.html I get a ratio of @ 2.57 to 1. Since you guys have indicated the suggested replacement is 3 to 1, the primary impedance matches a 27 tube, and the current carrying capability is within range, plus the small size, it sounds like it would be worth a try.

Thanks again for the assistance,

Mike

[MT70B]

Just a note to let anyone following the thread know that I installed the P-T156 interstage audio transformer in place of the defective one in my Amrad Symphony (Model 81) today. So for I have about an hour of playing time with the new transformer installed and it seems to be working quite well. I have plenty of volume and the audio sounds just fine with no distortion detectable by ear. Will let it cook for a while and if all stays well I will install the old transformer's metal cover over top of the replacement, which is small enough to easily fit undetectable under the cover.

Thanks for the recommendations!

Mike