Hello ,I have a Philco model 65 that I need to get an output transformer for.It is like the Philco 20 and uses push/pull 71A tubes.Would a 6L6 or 6V6/6BQ5 type output transformer be better for the impedance? Also even after all this time I still don't understand ,let's say.... if an output tansformer is 5000 ohms plate to plate.....,does that mean 2500 ohms each side of the centre tap ,or 5000 ohms each side of the centre tap......, or if a transformer has 8000 C.T. does that mean 4000 ohms each side of the centre tap or 8000 ohms each side of the centre tap.I wish books etc.. or catalogues for radio/amp parts would explain this better!!argghhh!Thanks for info.

Chris, we need think about this differently. Purge the idea that an audio output transformer has any impedance, CT or otherwise.

The transformer will REFLECT whatever load impedance connected to the secondary winding (i.e., the speaker coil) to the primary side (i.e., the output tube), by a factor of the turns ratio squared. So, if the turns ratio is 25 and the speaker impedance is 8 ohms, the impedance seen by the ouput tube(s) is 25 x 25 x 8 = 5,000 ohms.

A tube manual will tell you the recommended impedance across the output tube(s). For a pair of 6L6s in push-pull, it wants to see a load resistance, plate-to-plate, of 5K ohms (which is the reflected impedance across the whole secondary).

So forget everything about the xfmr except the turns ratio. And go by the tube manual.

The load at the secondary is not very critical, at all. For an antique radio using 6L6s in push-pull, probably plus-or-minus 25% or even 50% would be inaudible.

:Chris, we need think about this differently. Purge the idea that an audio output transformer has any impedance, CT or otherwise.
:
:The transformer will REFLECT whatever load impedance connected to the secondary winding (i.e., the speaker coil) to the primary side (i.e., the output tube), by a factor of the turns ratio squared. So, if the turns ratio is 25 and the speaker impedance is 8 ohms, the impedance seen by the ouput tube(s) is 25 x 25 x 8 = 5,000 ohms.
:
:A tube manual will tell you the recommended impedance across the output tube(s). For a pair of 6L6s in push-pull, it wants to see a load resistance, plate-to-plate, of 5K ohms (which is the reflected impedance across the whole secondary).
:
:So forget everything about the xfmr except the turns ratio. And go by the tube manual.
:
:The load at the secondary is not very critical, at all. For an antique radio using 6L6s in push-pull, probably plus-or-minus 25% or even 50% would be inaudible.
:
Where this is coming from is I have a guitar amp book that says if you use a transformer where the impedance is too low it will work but burn up the tubes quicker..decrease tube life.... something about plate dissipation...so that is where that fear is coming from especially old tubes that are no longer made

>Where this is coming from is I have a guitar amp book that says if you use a transformer where the impedance is too low it will work but burn up the tubes quicker..decrease tube life.... something about plate dissipation...so that is where that fear is coming from especially old tubes that are no longer made

There will be less distortion if the load resistance is higher than the recommended value - but less power output. The maximum power output occurs when the load resistance is equal to the plate resistance, but the tube manual will always show a recommended load resistance higher than the plate resistance - to reduce distortion, even though it is not optimum for power.

In push-pull, you can always run your output tubes cooler by not blasting the volume. (But single-ended amps run full-blast at idle.)

My understanding is that guitar amps run their output tubes very hard, unlike our radios. Not to worry - but if you want to shade the load resistance a bit higher, that will be fine.

Note: load impedance is NOT the same as plate resistance.

But back to your response: "...if you use a transformer where the impedance is too low it will work but burn up the tubes..." Please get "transformer impedance" out of your mind. The important thing is the load resistance seen by the output tubes - which is only a function xfmr turns ratio and the speaker impedance. The transformer, either the primary winding or the secondary, does not have a characteristic impedance at audio frequences.