Has anyone here replaced a model 70 speaker with a modern p.m.? If so, what method of bypassing the field coil did you use (I have read that a choke is better than resistors) also, what would be a good output trans to look for? I have an original model 20 speaker but the transformer is different.
Any information would be appreciated. Todd
If you are replacing with a PM,
1. It not fully correct: 20, 70 and 90 all have chokes in the filters that are separate from the field coil.
2. Simply putting a resistor instead of field coil is just fine.
3. Yes you also could find an additional choke to put instead of the field coil. Try to match the resistance to what it is (in all three radios it is about 3.2kOhm). If using choke, add the resistor equal to the difference of 3.2kOhm and the choke's DC resistance.
4. Transformer should match your impedance to about 7kOhm. The voice coil is 0.7 Ohm and the ratio of the transformer in all three radios is 100, so the impedance is transfered as 10,000 times the voice coil impedance.
A 125C Hammond transformer is a universal taht works fine.
Between 6.5K and 8K is just fine.
5. You need a SE transformer as this is what 70 (and some 90s) use.
Todd, is the 20 speaker in good condition? There is some info in the tech section about those speakers and how to convert. If the speaker is ok, do the transformer swap and you would have a more or less original speaker.
Actually transformer form 20, 2766, could be used as an output transformer for 70, as it is a universal PP/SE transformer, in the latter case the centertap is not used, when it is used for 1x47 output. So the 20 speaker is fully good for 70 except you have to re-wire the way the transformer is connected
I looked up the 47 tube and the specs showed 7000 load ohms, so the 20 transformer was within range. So, like morzh mentioned, this should work. Clip and insulate the center tap wire, use the outside wires and see what you have.
I would not even clip teh wire - what if one day you get the real thing and decide to use the 20 for some 20 you at the time will be working on.
Just roll it up tight and wrap in insulating tape.
BTW the 2766 transformer is shown as both PP and SE for 47 tube in Chuck's transformer table on the Philco Repair Bench website.