[Guest]

I want to replace the jury rigged permenant speaker on my model 118 with one that is more authentic. The schematic shows the filed coil with a resistance of 1,140 ohms and the secondary on the audio transformer of 1.1 ohmn. It does not show the resistance for the primary on the audio transformer. What is the appropriate resistance for the primary? and How close do I need to be on rersistances for the primary and the field coil? For example is a 1500 ohm coil close enoough to 1140 or will it cause the plate voltages on the tubes in the set to run too low. Thanks

[Chuck Schwark]

The audio output transformer is Philco Part No. 32-7078.

Secondary impedance (NOT DC resistance) = 1.25 ohms

Primary impedance (NOT DC resistance) = 5,580 ohms

(From Philco parts catalogue)

[Ron Ramirez]

Addendum:

Primary D.C. resistance of the 32-7078 output transformer is 310 ohms (Philco Service Bulletin 180B).

But you need an output transformer that will match the load of your push-pull 42 tubes to whatever speaker you end up using; so you need to use the info Chuck has given you (5580 ohms primary impedance to voice coil) in order to obtain a suitable replacement transformer.

Oddly enough, Bulletin 180B gives the primary impedance as 5,900 ohms! But the two figures are close enough that either will be fine. Don't deviate too much from the 5580 to 5900 ohm range for the primary impedance to get a good match.

As for the field coil, yes, 1500 ohms will drop B+ a bit, but not a lot (it's only an extra 360 ohms); but you should try to obtain a genuine Philco speaker for the best match here. You didn't mention whether this 118 is a cathedral, tombstone or console? The console models will use the 10-1/2 inch round speaker, while the cathedral and tombstone 118 uses an 8 inch pin cushion speaker.