Replacing an Output Transformer with tapped secondary - Printable Version

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Replacing an Output Transformer with tapped secondary - davemc - 10-01-2007

My 40-216 has a defective output transformer. I doubt I'll find a GOOD replacement any time soon (both halves of the primary were open, and the connections are in the middle so I cannot get to them without removing the secondary!) Likely many potential donor chassis out there ALSO have bad transformers.

Anyway, this set, as well as other large Philcos, use a transformer with a tapped secondary. The tap feeds the tone and volume control circuit - I assume some sort of feedback arrangement.

The secondary was good, so I applied about 2 volts from my HP 200A oscillator at 500 hz and determined that the tap is about 10% of the total.

Can I get away with just shunting the secondary with a couple of resistors to provide the 10% tap? For example, 100 ohms in series with 10 ohms. Perhaps they can even be higher such as 1K and 100 ohms. I realize any parallel resistive load will be reflected back into the primary, so the higher the better.

I calculated that the ratio of the replacement transformer should be about 41. So a 110 ohm load will be reflected as 184K in parallel with the output tubes plate resistance. That should not hurt too much. The tube manuals calls for 10K plate to plate load impedance.

Opinions? Will this work? Is there any limit on the values of the shunt resistors?

Thanks!
Dave

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