[Tomie]

Hi Ron, morzh,

Thanks for the input.  I have 2 identical Philco speakers.  The one came out of the 41-300, the other came out of a 41-295 donor radio I got.  The original speaker measures 5.0 ohms across the voice/bucking coil combination.  The donor measured 5.1 ohms.  I used the formula of multiplying the DC resistance reading by 1.25 and came up with 6.25 ohms and 6.375 ohms.  I've wired the speaker for 6 ohms and then just about every other option on the chart.  I always got sound, I never got quality sound.   
 
I can see no easy way to mechanical isolating the voice coil from the bucking coil to determine what the voice coil all by itself would measure if that is important to the overall calculations.  

Thanks morzh,

I am using the Zenith speaker with the output transformer that is attached to it.  Yesterday I removed the field coil covering from that speaker and to my delight discovered a single solder point where I could isolate one side of the secondary of that transformer, splitting the voice coil away from the bucking coil for that speaker.  The voice coil measures 4.7 ohms DC resistance all by itself, the bucking coil adds .5 to that total.  5.2 ohms is almost identical to what the two Philco speakers measure.  

I was able to obtain one more bit of information that I didn't have before.  The Zenith output transformer is wound 48.6 turns to one.  I find that interesting in as much as the original output transformer for this Philco radio measured 23.65 VAC applied to the half of the primary winding that still had continuity in order to obtain 1 VAC on the secondary.  Doubling the voltage for that half winding to provide the necessary voltage across the total winding to generate 1VAC on the output,   23.65*2=47.3 VAC or 47.3:1 ratio.  Again almost exactly what the Zenith is measuring.

I would love to hear what the Philco speakers sound like if being driven by the Zenith output transformer.  But I'm unwilling to dig that deep into that Zenith speaker just to find out.  It belongs on a Zenith set I haven't started to restore yet.  I went the distance yesterday, temporarily removing the one wire.

I have also been corresponding with transformer folks and I got the following reply from the Hammond technical representative regarding the 125E transformer I bought as a replacement:

"The DCR is not that critical, actually a perfect transformer would have zero DCR!
What’s important it the turns ratio. In this case it’s 10K to his speaker load. Assuming he did his homework and the 10K is correct and he is using the proper hook up to the speaker load then it’s correct.

Try reversing the Blue and Brown connections. Sometimes that works." (can't do that it squeals hooked up the other direction.)

"A key thing to remember is this is for a 15 watt output and it’s not Hi-Fi. It’s designed for audio only. The frequency range is 150 – 15 KHz so you won’t get any low end bass out of it."

Regarding any correlation between the primary winding DC resistance and the low end frequency response of a given output transformer, Randall Aiken has a nice article on the subject, Copyright © 1999, http://www.aikenamps.com/index.php/outpu...-explained.  I quote: “Primary inductance.  The transformer does, however, have a primary inductance, which has a direct effect on the low frequency response of the transformer.  ...This means that if you want better low frequency response from your transformer, you have to increase the primary inductance, which means a larger core and/or more turns on the primary.”

More wire means more resistance.  I've been told that the plate to plate impedance is important to insure that the tubes function properly.  But that circuit properly established may or may not insure quality sound.  For that the inductance of the transformer is important.  To a point, and in a very unscientific way, more inductance means more wire and that means more resistance.  

An old tech I know who has been doing this for more than 40 years told me, “I try to match, as close as I can, the primary winding resistance.  I can count on just part of one hand how many times that hasn't worked.”  That's why I went looking for primary winding DC resistance in the first place and it's why I tested Zenith speaker as an option.

Thanks much for all the input.  I'm totally dumb and I admit it.  I depend on your experience and understanding to help keep me on the page.

Tomie