I have an old Magnavox Dynamic 80 speaker but the mice have gotten to part of it. There appears to be a pie filter C-L-C just after the audio input connection. the L seems to be okay but the caps are old and eaten up by the mice. I can read one of the caps (I think) It appears to be a .015 microfarad but I can't read the other. both are sprague "midgets" and I think one says 1500 volts (I don't understand why such a high voltage rating)...anyway can anyone tell me what the true value of the caps are so I can complete my restoration? thanks, Joe Miller

Hi,

Because they are on the audio input, they should be small value, and the .015 mfd is correct for potential RF energy that is still potentially present at the audio output of some radio designs.

There are 3 probable reasons for the high voltage ratings, first- the manufacturer had no control over what set their speaker would be attached to, so they needed to protect themselves for at least the warranty period. Second, the dielectric is essentially an oil impregnated paper at that time, and therefore an inconsistent dieletric which may break down at a significantly lower voltage if there is any RF energy left in the signal. Third, it was what was available that allowed Magnavox to achieve their design specifications.

: I have an old Magnavox Dynamic 80 speaker but the mice have gotten to part of it. There appears to be a pie filter C-L-C just after the audio input connection. the L seems to be okay but the caps are old and eaten up by the mice. I can read one of the caps (I think) It appears to be a .015 microfarad but I can't read the other. both are sprague "midgets" and I think one says 1500 volts (I don't understand why such a high voltage rating)...anyway can anyone tell me what the true value of the caps are so I can complete my restoration? thanks, Joe Miller

I suspect that's a 10 kc whistle filter, for removing adjacent-channel interference. They probably went out of use when the FCC began assigning channels far enough apart to avoid the heterodynes.

High voltages appear at the output-tube plates when static crashes cut off the tube momentarily. That's why there are usually capacitors from plate to ground, or across the transformer primary.

Gernsback's 1931 Radio Service Manual shows such a filter on an unamed dynamic speaker. It's a pi filter in series with the output transformer primary with 0.015µf capacitors and a 1 Henry choke "for improved quality". It seems to be more of a tone compensating filter than a whistle filter, though it would act as a top cut filter and significantly reduce any 10 kHz whistle.
I've just got one of these Magnavox 80 speakers at a club swap meet. The speaker unit needs some work but seems to be intact. However the field power supply and speaker transformer aren't original. When you get yours working, can you please measure the field AC voltage and DC after the rectifier across the field coil. Also do you know what year(s) they were made please, I'm guessing about 1926 / 1927.
Thanks, Don Black.

: I suspect that's a 10 kc whistle filter, for removing adjacent-channel interference. They probably went out of use when the FCC began assigning channels far enough apart to avoid the heterodynes.

: High voltages appear at the output-tube plates when static crashes cut off the tube momentarily. That's why there are usually capacitors from plate to ground, or across the transformer primary.