Working on an old shortwave radio which mice had made a meal out of the 4" speaker and its mounted audio XFMR. Parts list gives no specs. The operating spec say 1.7W and the 50C5 is rated for 5W max. What would be the correct parts to put on the output? I don't see anyone selling the combo speaker with audio XFMR except for some old pulls from popular radios. I did some measurements on the XFMR. Primary 186.4Ω, Secondary 0.748Ω and put a 1kHZ @1Vrms signal on the secondary and Primary was .204VAC. So looks like a 5.4:1 XFMR? I might be able to patch and salvage the XFMR but have no idea what the ouput impedance is although I want to say I saw 3.2Ω impedance somewhere. I don't see anything even close for an audio XFMR on Mouser or DigiKey... So open to new ideas.

 

One like this would likely work.
Look for 5W tube audio transformers.

 

Hammond makes all sorts of tube output transformers.
http://www.hammondmfg.com/125SE.htm
http://www.hammondmfg.com/clsover.htm

 

One like this would likely work.

Those are a bit cheaper than the Hammonds I was looking at. So, any audio XFMR that matches the tube max output and then its output impedance determines the speaker input impedance? That's simple enough.

EDIT: "Primary 5 K OHM;Secondary 0-4Ω-8Ω;Winding method:3+2 fever grade winding" No idea what the heck "3+2 fever grade winding" is but assume it depends on which of the 3 secondary wires used determines output impedance?

 

There are cheaper ones on AliExpress, if you are willing to chance it.

 

Those are a bit cheaper than the Hammonds I was looking at. So, any audio XFMR that matches the tube max output and then its output impedance determines the speaker input impedance? That's simple enough.

You need to also check the transformer MAX DC bias limits for single ended tube outputs.

The normal spec for a 50C5 is about 50mA of plate current for a class A amp. Your radio might be lower but It's best to have something with actual specs instead of hope for the best.
https://frank.pocnet.net/sheets/049/5/50C5.pdf

 

There are cheaper ones on AliExpress

Not with their "updated" shipping charges these days which for this is ~$20 plus the XFMR. This is when I don't buy without a spec sheet. Unless I've missed something, I'll go back to Mouser and DigiKey and price out a 5W XFMR and match its output impedance to a speaker. Probably shoot for 4Ω impedance. FWIW china has screwed their part sales with increased shipping to the point they are not the cheapest and selling junk for what real stuff costs here in the US after their shipping charges get added in. Costs about 4X more to ship from china than what the parts cost these days.