From an idea related by John Grady; additional comments by Ron Ramirez

What do you do when you have an audio interstage transformer with a push-pull secondary, and it is bad? Hopefully, the following discussion will help.

To begin, let us take a look at a (slightly simplified) audio interstage circuit, as used in a Philco Model 20. Notice how the audio signal comes in to the grid of the Type 27 tube, is amplified, and sent to the audio interstage transformer, where it is stepped up a bit more and applied to the grids of the Type 71A output tubes, the two 71A grids receiving signals which are 180 degrees out of phase with one another.

The primary of the audio interstage transformer always has DC voltage across it, which feeds the plate of the Type 27 audio amplifier tube. It is the primary winding of these audio interstage transformers which usually goes bad.

While a Model 20 is used here as an example, it is applicable to several Philco models with push-pull output tubes made between 1929 and 1936.

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Modification A

If you have an otherwise good audio interstage transformer, its life can be lengthened by a slight modification which removes the DC current from the primary, as shown at left. In this circuit, B+ is fed through the added 27K, 2 watt resistor, to the plate of the Type 27 tube. This B+ is blocked by the added coupling capacitor between the 27 plate and the primary of the audio interstage transformer.

The circuit shown at left shows a 0.47 uF capacitor used for audio coupling; 0.22 uF may be used instead, and you might even try 0.1 uF if that is all you have on hand. It should be rated at 630 WVDC. I have found that the 27K value of the plate resistor is not critical; it can be between 27K and 33K and still work well. A 2 watt unit is recommended.

With this modification, you need to ground the primary lead which formerly went to B+, as shown at right. Do not allow it to remain tied to B+!

In this circuit, the DC current in the core is not only removed, but audio frequency response is enhanced; the 27K resistor also prevents a rise in gain with frequency.

To make another huge difference in the sound of a Model 20, the original speaker can be replaced with a speaker from a Philco 70, a Philco 90 with single-ended 47 output, or even a modern permanent magnet speaker. The original 20 speakers have very stiff, solid spiders (in the center of the speaker cone) which do not have a lot of “give,” reducing frequency response. It has been reported that a Model 20 using Modification A, and using a modern PM speaker, can produce a sound similar to a Bose!

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Modification B

If the audio interstage transformer does have a bad primary, but has a good secondary, don’t despair – it can still be used, utilizing the circuit shown at right.

Here, the primary is completely disconnected from the circuit. A coupling capacitor, which can be 0.47, 0.22 or even 0.1 uF, is placed between the plate of the Type 27 tube and one of the grids of the Type 71A output tubes, as shown. The 27K, 2 watt resistor is used here again; feeding B+ to the plate of the 27.
Phase inversion is still accomplished in the secondary of the interstage transformer, allowing the push-pull output tubes to operate normally.

I have used Modification A in Philco sets which still have good audio interstage transformers, and it does seem to increase frequency response. I have also used Modification B where necessary, and it works extremely well.

Thanks to John Grady for allowing me to publish his methods of protecting, and bypassing, the primaries of audio interstage transformers.

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Modification C

By Ron Ramirez

Many years ago, I came up with a way to bypass single-ended audio interstage transformers which were bad. This problem is prevalent in 1920s battery radios, as well as the Philco Model 511 series, many Atwater Kent AC metal box sets, etc.

If you have a bad single-ended audio interstage transformer, and wish to bypass it completely, take a look at the circuit at left. A 100K plate resistor, a 0.047 uF coupling capacitor, and a 1 megohm grid resistor works quite well. I have used this circuit in numerous 1920s battery radios such as Freshman Masterpieces and the like over the years, and was always rewarded with very good volume and fidelity, even though you lose the “step-up” characteristic of the transformer. It also works well in the Philco 511 series (which is the circuit shown above).