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Philco 41-300 output transformer question. - Printable Version

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Philco 41-300 output transformer question. - Tomie - 10-03-2017

I know this question has been discussed here as well as in other antique radio forums. However I am not finding the same results that others have had and I post here in the hopes someone can point me to the error in my ways.  The output transformer on my 41-300 had an open primary.  I had a donor chassis, and that transformer was good from center tap to one side.  The DC resistance measured 320 ohms on that winding.

I bought the Hammond 125E transformer which I was told is what others were using but it sounds terrible on my radio. All of the base tones are washed out and the output sounds cheap and tinny. However I own a  Zenith speaker and output transformer which I tried and it sounds much better.

The only difference I have been able to find between the two is the Hammond transformer has about 125 ohms DC resistance on the primary winding but the Zenith output transformer has 580 ohms DC resistance on the primary winding.  Here I am looking only at the DC resistance and not the overall impedance of that speaker output circuit.  I have not checked the turns ratio on the Zenith output transformer.

FWIW I have grounded the tone and volume controls that previously attached to the center tap of the output winding on the original Philco output transformer.  I had used various combinations on the Hammond transformer without different results.

I determined that the solution is to find a transformer that has between 550 ohms and 700 ohms DC resistance on it's primary winding as that seems to solve the problem.  But I'm posting here in the hopes that someone may see something in that amplifier/output circuit that I have missed which could be the cause.  I don't get it how others use the Hammond transformer and have it work well while it doesn't work well on this one.  I have checked and rechecked the hook up, switching between plates to the primary and I have used all of the various output combinations, but nothing seems to improve the performance until I use a transformer with a higher DC resistance on the primary.

I don't think I am asking too much from this radio, I have others that are older and they sound much better.  I would accept the quality of sound I get from the Zenith speaker but to be honest I had greater sound expectations for this radio.

The Zenith speaker and output transformer that works best is a model 7S685, Rider's 13-30.  The speaker part number is 49-475.  On the outside of the transformer is stamped 9448 7.  

Thanks for any help.
Tomie

RE: Philco 41-300 output transformer question. - morzh - 10-03-2017

Tomie

The DC resistance is not that important.

What is important, first of, is the input/output impedance ratio.

If you use the 125E CT transformer, make sure you connect the outputs that will match your impedances accurately.

Then, to a point, the primary inductance (or rather parasitic primary inductance) also matters.
Also the core material.

125 series has 150Hz-15kHz band that means that it should work fine for your purposes, should the match be good.

So, find out the speaker impedance, find out the primary impedance, and use the chart in the datasheet to see what connection provides for the closest match.

You want 10K P-P impedance. If your speaker is 3.2 to 4 ohm, you need 2-4 outputs to get you from 9K to 11K impedance.
For 3.2 Ohm, if it is the case, also try 1-3 outputs.

Last, if you could find a transformer with better low end characteristics, try that. Maybe this is what you are having, and the Zenith xfmr simply has better low end number.

RE: Philco 41-300 output transformer question. - Tomie - 10-03-2017

Morzh

Thanks. Do you know how to determine the low end characteristics of the Zenith and or Hammond transformers? I won't trash the Zenith speaker to remove the transformer, it belongs in another set.
So my speaker is basically 8 ohms, (I think the DC resistance on the voice coil was something like 5.? ohms.) I originally use 1 and 4 terminals then 2 and 4, etc. No difference. Lots of sound, just very low quality sound.
Is this Philco radio known for low quality sound issues?

RE: Philco 41-300 output transformer question. - Radioroslyn - 10-03-2017

I have the opposite issue my 41-300 sound pretty bassy if anything it's missing some highs. I grounded the feedback loop, I couldn't get it to work well w/the Thordarson multimach transformer. I've come to the conclusion that the tone control pot is intermittent it's kinda finicky. I did clean it when I serviced the chassis. It's a bit of a pita to replace because it's gear driven and has a short shaft. Would probably have to find a used one or develop some welding skills.

In terms of opt and low frequency response my best guess is that has to do with inductance of the primary and the core size. The more inductance and the larger the iron core is the better or more efficient it should be at lower frequencies. Would happy to entertain other thoughts on the matter as I'm no rocket scientist[Image: http://philcoradio.com/phorum/images/smilies/icon_lol.gif] just a hobbyist.

RE: Philco 41-300 output transformer question. - morzh - 10-03-2017

If it is 8 ohms, 1-4 outputs are the ones.
Also try 4-6.
Maybe yes, you just need another xfmr.

Also 125E is huge, you could use 125C. I also has twice the prim inductance and might sound different.

RE: Philco 41-300 output transformer question. - Tomie - 10-04-2017

This from the Hammond Factory representative:
"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 hook 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.

I'm going to go back and try to do some more testing.  I'll see how difficult it would be to measure the turns ratio for the Zenith transformer.  I have to isolate the voice coil and I'm reluctant to do that.  Short of other information about the low end frequency response for that transformer the turns ratio is all that i can calculate with what I have to work with.

Is there some way to determine the frequency response based on that information?

Thanks for any input anyone has.
Tomie

RE: Philco 41-300 output transformer question. - morzh - 10-04-2017

The rep, as you can see, has said exactly what we had said before. You need 10K impedance.

As for the turns ratio, it is the simplest of all:

1. connect the primary of your transformer to a Variac and inject, say, 20V in it (measured with the primary connected). This is your Vin.
2. While continuing injecting the voltage in the primary, connect the meter to the secondary and measure the voltage. This is your Vout.
3. Your Vin/Vout ratio is your turns ratio W.

Now,

4. Your Turns ratio squared times your output impedance (say you have 8 Ohm) is your primary impedance. Say you have W=35 (you measure Vout = 0.57V at Vin =20V). Then your input impedance is 8*35*35 = 9800 Ohm which is what you need.

RE: Philco 41-300 output transformer question. - Tomie - 10-04-2017

Thanks for the input morzh,

So let me take this one step farther.  Can I assume that if the turns ratio is comparable between one transformer and some other that the frequency response of those two transformer is similar?
Further, knowing the turns ratio, is it possible to calculate the frequency response on any output transformer?

Tomie

RE: Philco 41-300 output transformer question. - morzh - 10-04-2017

(10-04-2017, 05:52 PM)Tomie Wrote:  Thanks for the input morzh,

So let me take this one step farther.  Can I assume that if the turns ratio is comparable between one transformer and some other that the frequency response of those two transformer is similar?

No.

(10-04-2017, 05:52 PM)Tomie Wrote:  Thanks for the input morzh,

Further, knowing the turns ratio, is it possible to calculate the frequency response on any output transformer?

Tomie

Also, no.

Frequency response is a complex function, dependent on iron type, geometry of the winding, winding itself, parasitics etc.
Turn ratio only gives you the transfer function for voltage (also current or impedance).

RE: Philco 41-300 output transformer question. - Ron Ramirez - 10-04-2017

FYI - the voice coil should have an impedance of 4 ohms - assuming it is the original speaker.

RE: Philco 41-300 output transformer question. - morzh - 10-04-2017

>>>>>However I own a Zenith speaker and output transformer which I tried and it sounds much better.


Just want to make sure we understand each other: you tried the Zenith transformer with the Philco speaker, not with Zenith speaker?

RE: Philco 41-300 output transformer question. - Tomie - 10-06-2017

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/output-transformers-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

RE: Philco 41-300 output transformer question. - Steve D - 10-06-2017

More turns on a larger core will give higher inductance and better low frequency response but the resistive wire losses will be higher due to a longer mean length per turn and more turns.  So a manufacturer may increase the wire gauge to compensate and the DC resistance may be not much higher.  Again DC resistance is meaningless in determining transformer response.

Steve D

RE: Philco 41-300 output transformer question. - morzh - 10-06-2017

>>>>I am using the Zenith speaker with the output transformer that is attached to it.


See, from your first post and the line of posts thereafter (comparing transformers) I thought you only changed transformers but used them with the same Philco speaker. And your quality of sound change thereby had been attributed to the transformers. But now I think your sound quality problem lies with the speaker. Simply put your Zenith speaker is better. Which could well be the case with some 1940+ speakers from Philco: I am not overly impressed with my 41-285 sound and I think it is the speaker.

RE: Philco 41-300 output transformer question. - Electrothaumaturgist - 10-09-2017

That's so disappointing to hear. That 41-300 looks so similar to the 42-400 I restored a few years back... Looks to be about the same speaker in it (though I may be wrong), but mine was a total basket case, as was the speaker (had major holes in it... I fixed the speaker (and everything else), and the sound was absolutely impressive! Did you replace ALL crumbling wires? That baby should be belting it out! There were some caps and rotten wires under the push button sector that are tough to get to. I hope you isolate the problem, and good luck!!!