[Guest]

All,
I picked up a Philco 70 with a bad field coil. I'm new to tube radios, and after some research, it looks like the field coil cannot be easily removed to be rewound (let me know if this is wrong). In order to move forward, I'll be hooking up a PM speaker (to use while finishing the chassis work), while looking for a good original (let me know if you've got one).

I've got a few good 8 ohm speakers, and I probably need to build a test speaker for use in the future anyway. I know that I need to get a universal OPT, and it looks like there are some choices from Hammond. Here are my questions:

1) For the purposes of troubleshooting this radio (and potentially other desktop radios), what wattage should I aim for? I was thinking 5W would probably be adequate. This still looks like a $40 - $50 purchase.
2) Has anyone tried this much lower cost ($5 ... too good to be true) alternative: http://schmarder.com/radios/misc-stuff/t-725.htm

It looks to me like this OPT could be used for matching; I have my doubts about performance, but for troubleshooting ... thoughts?

M100C

[exray]

Since you are talking in terms of a temporary xfmr for testing you can use most any xfmr like a 6.3v filament xfmr, a number of wall wart varieties and so forth. If the question were about a long-term solution I wouldn't say that. Output transformer of any ilk will not prevent you from testing your radio and sallying forth with the restoration. Get 'er working first.

About the field coil. No problem in having a pro rewind it, in fact thats probably the best option but you still need to have some equivalent in the circuit for the aforementioned testing and that means an offboard choke or something.

The two go together in that respect. You need the field coil AND the AF output xfmr to make it work. There's plenty of workarounds...some, ahem, more 'temporary' than others.

[Texasrocker]

Hello M100C. A good way to sub for your open Spkr field-coil is by using a modern-type wirewound power resistor. According to Rons orig Philco Spkr data here, your orig spkr ( mod K-3) had a FC resistance of approx 3100 ohms.
Until you can find a orig replacement, or have your vintage spkr repaired, you could sub a 5 or 10 watt power resistor of approx 3K- to 3100 ohms as close as possible to 3100 ohms. You can trace back to your chassis orig spkr connection ,the correct 2-pins that originally used the spkr field, disconnect those 2 wires, then solder in the new ceramic wire-wound power-resistor across those 2 pins under chassis on the orig female-side of spkr connector. Since the orig spkr voice-coil was 0.7 ohms, using a modern pm type 8 ohm should work fine. Monitor the heat of the new power resistor if you decide to use one, as it will get hot which is normal in this case. The 5-watters usually are all I use, however the 10-watters will operate with less heat dissapated, but take up more space under the chassis.Hopefully, you can find a good orig spkr in the future if total "originality" is your preference. I have found the modern pm type full range spkrs usually sound much better overall anyways compared to the originals.Just my opinion though. Good luck with your vintage Philco restoration!

[etech]

I picked up a Philco 70 with a bad field coil. I'm new to tube radios, and after some research, it looks like the field coil cannot be easily removed to be rewound (let me know if this is wrong). In order to move forward, I'll be hooking up a PM speaker (to use while finishing the chassis work), while looking for a good original (let me know if you've got one).

You are correct. The Philco 70 uses a field coil assembly which must be removed using an arbor press. The cone has to be removed before you can do this. While it is not impossible to do, you will not find many speaker rebuild shops willing to do this. When faced with this situation, I waited until one came up for auction on Ebay. The good news is that they are available pretty regularly on Ebay. The bad news is that they are somewhat pricey, typically going for $75 to $100 per copy. You will need the K3 type speaker which was used on the Philco 70 and 90 cathedrals. If you get one from a Philco 90, just be sure that it is for the middle version Philco 90 that uses the single 47 output tube. The other Philco 90 sets use a push/pull output arrangement and you would need to change the output transformer to use it. I hope this helps.

Best regards,

Ed

[etech]

One other thought came to mind also, which I will mention for your consideration. Many times a field coil winding will fail at the connection point where the external wires attach to it. Since your field coil is already open, you cannot do much harm. You may want to carefully slice back the insulation around the field coil where the external wires enter, to see if there is a bad connection where the windings attach to the external wires. I'm not sure about these, but some field coils actually have small terminal lugs inside that the fine field coil windings are soldered to. In general, I have been lucky about 25% of the time in finding the open at these connections, on other speakers. Note that you may have to remove the output transformer to do this. Good luck with it.

Ed

[radiomikee]

Here is the other option no one mentioned, I did this once. Use a hacksaw, cut open the frame holding the field coil in place, slide it off the metal shaft it rides on. Remove all the wire from the coil form, and rewind the form with new wire. Attach larger stranded insulated wire to the magnet wire, tape it securely and presto, new field coil. Wind enough #36 wire to get at least 200-2500 ohms in the field coil itself, and series mount another 500-750 ohms under the chassis to make up any shortfall. Put the field coil back in place, rewire it into the speaker unit as previously. Have a metal shop weld the back end of the frame back in place. It's alot of work, but you will have saved another bad speaker which keeps another set running!