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sparton 28 transformer questions and help - Patrick - 10-10-2020

Both the input and output transformers were missing. Can't seem to locate an input transformer replacement anywhere. Can anyone help?

Also, since 2 47's need around 14K impedence, would 4 also need just 14K? And with 3 speakers in parallel, would a 4 ohm output be correct? Can't find the appropriate one for this, either.

Thanks,

Pat

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

RE: sparton 28 transformer questions and help - morzh - 10-10-2020

When tubes are paralleled, the impedance is expected to be 1/2 of what a single circuit can drive.

RE: sparton 28 transformer questions and help - Patrick - 10-10-2020

So you're saying the input impedence of the output transformer should be around 7K? And what about the secondary to the 3 speakers?

RE: sparton 28 transformer questions and help - morzh - 10-10-2020

Well, I do not know of this particular radio.
Let's just say you have a battery. The battery can output 1.5A at 1.5V. That is, it can see 1 Ohm load.
Then you parallel two of these. They will do exactly the same, output 0.75A each so together they output 1.5A.
But, each one being capable of 1.5A, you could reduce the load to 0.5 Ohm so the current now is 3A.
But each one outputs 1.5A which is equivalent of each seeing 1 Ohm load.

Same here. If the single pushpull wants to see 14K it means that two in parallel, in order to see 14K each, need to see 7K full load.

Now, you have to see what the transformer is.
This coupled with your load determines what the tubes will see at the primary.
When they say "14K" it means it could be less. But then the power transfer won't be optimal.

RE: sparton 28 transformer questions and help - Patrick - 10-11-2020

Well...that certainly clears thing up.

RE: sparton 28 transformer questions and help - Patrick - 10-12-2020

Anyone else?

RE: sparton 28 transformer questions and help - Patrick - 10-13-2020

Got the output xformer figured out. Now, what can I use for an input xformer with a turns ratio of .7:1?