[Sourbone]

I've acquired a model 39-17. Nice looking little radio. I'm looking for an output transformer. The output tube is a 41. RCA literature for the tube says the load resistance is between 10,000 and 12, 000 ohms. The original primary resistance is 700 ohms. I've only replaced an output transformer once before with Ron's advice. Looking for the right transformer amoung the Hammond single ended universal series. As well, I'm not sure about the secondary lead choices. Let me know what might work or if there are less expensive choices out there. 

Thanks, 
Dave

[morzh]

700 Ohm has nothing to do with what you need. Or almost nothing.
You size up your transformer by the allowed wattage and then see if at your load impedance (say 4 Ohm) the input of the transformer gives you your 10 - 12kOhm.

So,
http://www.hammondmfg.com/pdf/5C_125SE.pdf


Step 1: get your speaker voice coil impedance (what is it?). Say it is 4 Ohm.
Step 2. 4 Ohm will yield 10,000 Ohms primary impedance if connected to BLK/ORG leads.

If your Voice coil impedance is different, see the chart for what you need. Basically the PRI impedance = VC impedance times square of the turn ratio.

[Sourbone]

Okay, thanks. I'll measure the voice coil tonight. Thanks for your help.

[Radioroslyn]

Okay, thanks. I'll measure the voice coil tonight. Thanks for your help.

Methinks your going to measure the resistance of the vc which is not what you need to know. Impedance is what you are looking for. The simple answer is try first few low impedance taps on the transformer secondary for loudest/ least distortion. More that likely it's going to be around 2 to 8 ohms.

Terry

[morzh]

Well, to make it easier a bit:

The impedance of the VC is never lower than the guaranteed "minimum impedance" which is the DC resistance of the coil.
Then for calculations is so called "nominal impedance" is used which loosely describes the behavior over most of the audio span.
This is given as 1.15*Zmin so it is about 15% higher than measured DC resistance.
It is imprecise but quite OK for transformer pickup.

In any case it is NOT going to be lower than the DC resistance.

So, DC ohming out is still useful.

PS> Again, this works for a single speaker. Not for acoustic systems that have crossovers and that complicates the heck out of calculations. But in this case it is the single speaker.

[Sourbone]

The Philco literature for the radio says its audio wattage is 2 watts. It looks like I should probably use the Hammond 125ASE and the secondary leads ORG and BK. Thanks for your help.

[morzh]

So your speaker has 4 ohm VC?

[Sourbone]

The voice coil measures 1.3 ohms.

[morzh]

Well.....125 SE though will work, is not the best one for the job. Your impefance is really low for it to match it well.
I was always surprised by the fact that 125CT does provide good match for low impedance speakers and SE does not.
I bought my xfmr for a similar speaker from Your Oldradioparts.

If you still are going with 125SE then yes, Blk/Org is the best one.

[Sourbone]

You know it seems Philco speaker voice coils are commonly low in impedance. I just repaired a model 650 where the voice coil was 1.1 as it was written on the Nostalgia air scan of the schematics. A model 39-40 I worked on has the voice coil impedance at .25 ohms. Might that low impedance contribute to the primary opening up?

[morzh]

No why would it?
They specifically use transformers with high turn ratio so the primary presents as a proper impedance....
In Philco 20 the turn ratio is 100, one of the highest so it converts 0.7 ohm into 7kOhm.

[codefox1]

Most tabletop tube radios of that vintage are not very fussy about exact impedance matches, no harm in trying something similar you may have on hand. Exception is for "portables" which have such low output that a severe mismatch reduces volume significantly. At least that's been my experience.