[Patrick]

Yeah, I saw that. On my missing speaker, the mounting holes are 7 1/2" center to center, same as the speaker I actually have. That speaker measures 6 3/4", so I guess Sparton would call that a 6" speaker. But wouldn't I need to find one with a field coil that measures either 2400Ω or 3700Ω, if not 6100Ω (and center-tapped)?

[morzh]

Look at the before discussion.
You could use any speaker that uses a field coil that takes the same current (or close) as the original speaker. Of course provided that the speaker is the type you are looking for (full range or woofer?) and fits the size.
Then you simply add resistors to make up for the divider that you need.

[Patrick]

Like you said, I can measure the current needs using a 6100Ω resistor, providing everything else works ( I just recapped and rewired it), but how am I to determine "any speaker that uses a field coil that takes the same current (or close) as the original speaker" on a speaker that I find on ebay?

[morzh]

Well....in the older times you would find datasheets.

Today...well, I would find out what radio the speaker from and then do that same calculation, using the dropout across the coil and its resistance which often enough is either shown or known if asked on the forums.
If the speaker works at lower currents but is big and powerful enough, a shunting divider would do taking part of the current while providing the needed biases for the tubes. Higher currents....nothing much could be done; you could use them but you will loose the efficiency of that speaker.

[Patrick]

Found this speaker on ebay. Looks like field coil is 720 Ω fed with 320V. Would this work?

[Image: https://i.imgur.com/dcjP9Br.jpg]

[morzh]

Patrick,

It is not fed with 320V. It is fed with the difference between the voltages on its left end (let's take your number, 320V) and right end (I do not know but let's say 270V) which then would make for 50V. So you need to know the voltage at the centertap of the audio xfmr to see what voltage it is, and then by dividing it by 720 Ohm you will know the current.

[Patrick]

So the left end is connected to Pin 8 of the 5Y3, which is 320V. The right end, is connected to pin 4 of the 6F6, which is 235V. That gives us 85V. Divided by 720 gives us 118 milliamps. Since we need a speaker that only draws about 35 ma, this one would not work.

Are my calculations correct?

[morzh]

Yes it seems correct if your numbers are right.
Has to be a fairly warm field coil with 10W dissipation.
The current seems a bit high, but even if it is 90mA it is still too high.

Your coil is the load type, not the filter type.
So look amongst the speakers whose coils are not part of filtering, but are loads on the B+, those are higher resistance and are likelier to fit your application.

[Patrick]

Got it. Thanks for all your help. I believe I'm beginning to understand, now. I'll keep looking.