PHILCO PHORUM

Notes on Models 20, 21, 70 & 90: The Speaker Problem in early Philco Cathedrals (Models 20, 21, 70 & 90)

As these particular Philco cathedrals continue to increase in popularity, the availability of speakers for these sets continues to decrease. Original speakers for Philco Models 70 and 90 are in very high demand, and when offered for sale, often bring high prices.

At the same time, the large number of Model 20 sets made can provide a good, original-looking substitute for the 70 and 90 speakers.

NOTE: I am not advocating the disassembly of restorable Model 20 sets just to get the speakers. I am merely pointing out that, since Philco made over 300,000 Model 20 cathedrals and more consolette models, all of which used the Type K speaker, for those of you who happen to come across an unrestorable Model 20 parts set, it will provide a speaker you can use.

Therefore, I have assembled the following cross-reference table, showing the specifications of the speakers used in Philco's early cathedral models.

Using this table and the information which follows, you will be able to determine how to best use a Model 20 speaker.

Looking at the table above, you will notice that all of the speakers listed have the same field coil and voice coil ratings. Incidentally, all of the speakers in the table above have field coils rated at 40 MA.

NOTE 2: As these speakers are getting older, often you may find a speaker from a junked Model 20 with a bad field coil. This is a problem all its own. What makes it easier to cope with is the fact that the "pot" is held together with a large nut in back, rather than the center of the "pot" being pressed in as on the Model 70 and 90 (K-2, K-3 & K-5) speakers. This makes it easy to remove the field coil for repair and/or rewinding. Before proceeding, check the value of the speaker field - it should be 3100 ohms.

If you must remove the field coil of a Model 20 (Type K) speaker, THE SPEAKER CONE MUST BE REMOVED FIRST before you loosen the nut on the "pot." Otherwise, the spider and cone will be destroyed when you pull the pole piece loose from the "pot!" See below for details on removing the cone.

Notice that I did NOT include Models 50 and 51 in the table above. The lowboy versions of these two models use the same size speaker (8 inches), and the speaker LOOKS like the 20/70/90 speaker - but it is a Type S, with an 1120 ohm field! (Model 50 and 51 cathedrals use a smaller speaker than the 20, 70 and 90 cathedrals.)

Now the next question - how to convert a push-pull 20 speaker to a single-ended application such as a Model 70, or a 90 with a single Type 47 output:

The first thing that needs to be done is to change the audio output transformer. To avoid damaging the speaker cone (if it is still good), unsolder the leads which run to the voice coil from the terminals on the frame. Then remove the screws holding the cone to the frame. There are several around the perimeter of the cone, plus one in the center (in the middle of the spider). Finally, remove the cone (carefully) and set it aside.

Carefully drill out the rivets holding the old audio transformer to the frame. (Some Philco audio output transformers are held in place by bolts instead of rivets, which makes the job much easier.) Remove the old transformer.

Mount the new transformer to the frame in place of the old. If possible, try to mount the new transformer inside the metal shell of the old transformer for the sake of appearance.

Now use the schematic below which gives the proper pinout for the speaker plug and shows how to connect the wires to the speaker field and the new audio transformer.

While you are doing all of this work, you should replace the old speaker wires with new cloth-covered wires. These are available, in various colors, from Antique Electronic Supply or Radio Daze.

Once the new single-ended transformer has been mounted and the wires hooked up properly, reattach the speaker cone to the frame. Reconnect the voice coil leads. Your 20 speaker has now been converted for use in a Model 70 or single-ended 90!