[Tubesforme]

Tubes, you might try exchanging the 1.5 uF film cap you already have with the input 10 uf cap after the rectifier. Connect the 1.5 uF to the side of the choke connected to the 80 filament, the 10 uF to the other side of the choke as it is now, and the remaining 10 uF to the output of the field coil where you had the 1.5 uF. The last 10 uf would then be from the B+ side of the audio transformer primary to B-. This would give you additional filtering at the 27 audio amp stage, where you need it the most.

This should act to reduce the B+ voltage and bring it down close to the level it was originally. Then you can proceed to track down the hum problem.

This sounds like the best solution to me and would quickly let me know if the problem of loading is being caused by the first 10uf capacitor and I agree, it would add needed filtering to the audio amp. Other people have told me that they have always used 4.7uf caps for the first two filters and never had a problem with a model 20 doing that. Another old timer told me that the audio bias always needs more filtering than originally supplied by Philco. Your solution would verify this. Thanks