[NostalgiaRadioTime]

Well I started working on the Stewart-Warner 'cube' and am seeing signs that the field coil is open. I'm only getting a reading / continuity on two of the 4 pins on the plug. Those two are going to the audio output transformer. So I'm hoping someone can give me an idea of the resistance I'm shooting for on both the field coil and audio output transformer? I'm trying to see if I can find a suitable 6.5 inch diameter speaker that I can use rather than try to rewind the field coil.

Here's a link to the schematic:
http://www.nostalgiaair.org/pagesbymodel...021218.pdf

Many thanks.

[Arran]

Before you give up on the speaker, you may want to check it over carefully to make sure that it didn't suffer from a broken lead going into the field coil. I will have a look at how the speaker on my console is wired, some sets had four pin plugs but one wasn't connected to anything since one side of the output transformer was often connected to the output side of the field coil so a fourth wire was unneccessary. Resistance of my field coil won't help you, it has a 12'' in it not a 6''.
Regards
Arran

[Mondial]

You can estimate the field coil resistance by looking at the voltage measurements on the schematic and the normal current draw of the tubes. The voltage difference between the 5V4G cathode and the 6K6G screen is 80 V (325V-245V). This is the voltage across the field coil.

The normal current draw of the tubes would be about 55 mA, so calculating by Ohms law,   80V/.055A= 1454 ohms.

So you are looking for a field resistance around 1500 Ohms.

The output transformer primary resistance should be around 400 Ohms. (15 V/.035A=428 Ohms) This is the DC resistance, not the rated load impedance which is about 7500 Ohms.

Actually, most any output transformer for a 6V6, 6F6, 42, or 41 tube should work fine.

[NostalgiaRadioTime]

Before you give up on the speaker, you may want to check it over carefully to make sure that it didn't suffer from a broken lead going into the field coil. I will have a look at how the speaker on my console is wired, some sets had four pin plugs but one wasn't connected to anything since one side of the output transformer was often connected to the output side of the field coil so a fourth wire was unneccessary. Resistance of my field coil  won't help you, it has a 12'' in it not a 6''.
Regards
Arran

Thanks, Arran. I spent a good deal of time this afternoon checking over the speaker and it is definitely the field coil that is open. I carefully peeled back some of the paper covering the winding to expose the connections where the wires protrude out, just to be certain it wasn't a bad connection or broken wire. It's not.... where ever the break is, it's internal : All four pins are in use on the plug; the two larger diameter pins are for the field coil, and the smaller two are for the audio output transformer.

[NostalgiaRadioTime]

You can estimate the field coil resistance by looking at the voltage measurements on the schematic and the normal current draw of the tubes. The voltage difference between the 5V4G cathode and the 6K6G screen is 80 V (325V-245V). This is the voltage across the field coil.

The normal current draw of the tubes would be about 55 mA, so calculating by Ohms law,   80V/.055A= 1454 ohms.

So you are looking for a field resistance around 1500 Ohms.

The output transformer primary resistance should be around 400 Ohms. (15 V/.035A=428 Ohms) This is the DC resistance, not the rated load impedance which is  about 7500 Ohms.

Actually, most any output transformer for a 6V6, 6F6, 42, or 41 tube should work fine.

Many many thanks for that info!   

[NostalgiaRadioTime]

I'm going to put this here, even though it's not related (well, maybe indirectly) to the speaker. I found that the schematic calls for a 250K ohm volume control with off/on switch. Well when I took the control out to try to clean it, I measured it and found there was a 1.5 meg ohm volume control in its place! I happened to have a brand new 250K ohm control with switch on hand and installed it instead. It seems to work just fine, although the "V" of the 6E5 magic eye tube (sorry guys I did decide to install one after all), is jittery as the radio plays. I'm using a small field coil speaker to try the set out while waiting for a more suitable sized replacement but am wondering why someone would sub a volume control of such greater value? Could it be it was to compensate for a smaller value field coil on the speaker? The one that was in the set with the open field coil was not the original. It was date stamped 9/23/54 and was only 6.5 inches in diameter. The schematic says it was an 8 inch speaker. But I have no idea what the field coil ohm value was on the replacement. I have a line on an 8 inch speaker at 1100 ohms, which is closer to the 1500 ohms Mondial suggested. Any thoughts on how the value of the volume control can help or decrease the performance of the set?

[Radioroslyn]

> Could it be it was to compensate for a smaller value field coil on the speaker? 
No. Probably what was on hand at the time of repair. Value isn't very critical. The
lower the resistance will lower the gain of the amplifier stage but probably not enough
to notice.