The PHILCO Phorum
Model 91 cathedral with gain control switch - Printable Version

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Model 91 cathedral with gain control switch - Mikeym53 - 07-14-2025

I'm working on a model 91 Philco that has a switch incorporated with the on-off switch. You move the knob to a second position. It adds or removes a 160k ohm resistor between the cathode of the 44 RF amp tube and Bakelite block #9. I can't find any schematic that shows this. It looks to be all factory. The resistor is a dog bone that is out of range so I'm changing it. I'm going to put it all back like it is. I am rebuilding all the Bakelite blocks and the Can capacitor. I am replacing all resistors out of range. I guess my only question is. Is it wired correctly and is the resistor the correct value? Thanks all for any help with this. Mike McCrow

RE: Model 91 cathedral with gain control switch - Mikeym53 - 07-28-2025

Hi everyone. Thanks to all that had a look at this post. I found another change when I went to rebuild #27 Bakelite block It was numbered 3903 with no AF or any alpha designation on the side of the block. The parts list shows it as 3903 AF .01 single for single speaker, that is what I have. It is actually a .01 dual and the second capacitor is coupled in parallel with the #9 block .015 and 200 ohm resistor. I finished the restoration and left everything as it was. The radio plays very well. The shadow meter works good and the added gain control switch works good too. I hope this helps anyone that runs into this model 91 with these changes.
Mike

RE: Model 91 cathedral with gain control switch - RodB - 07-28-2025

I think that Philco found that bakelite blocks made it difficult to make production changes and keep track of them. As changes were made the suffix would change. Just a minor change like the voltage of a capacitor would have an effect. And as we now know, 80 years later, paper caps are made of 80 year old paper.