I have an 6500 primary to 8 ohm OPT. If I have a theoretically flat 4 ohm load is the OPT functionally equivalent to a 3250:8 OPT? Is it as simple as that?
Yes but the power is also limited to the half if you consider the secondary gauge then you may want to limit the power trough the transformer to roughly the half of the nominal power capability and also the tube will have more trouble with the low end the windings both primary and secondary will have more DCR than a normal 3.2k:4 designed unit the damping and overall loses will be greater
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You probably meant 3250:4, and this is true enough through the large middle part of its frequency range. It will have higher inductances, both good and bad, and capacitances than an equivalent transformer optimized for 3250:4, but it's fine for audio amp use. Speakers are too varied and too complex a load to get upset over a 2:1 difference in parasitic reactances in the transformer.
YOS,
Chris
YOS,
Chris
Power output will be roughly the same.
Some math:
Let's say the maximum plate voltage swing is 150 VRMS.
Winding ratio of the transformer is 28.5 : 1.
So, with a pure 8 ohm load (forgetting some losses) the secondary swing is 5,26 VRMS.
That is 3,46 watt @ 8 ohm.
Now, when connecting a 4 ohm load, the tube will see a 3250 ohm load.
It will not reach the same 150 VRMS at it's plate, let's say some 120 VRMS.
Secondary swing at 4 ohm now is (120/28.5)² divided by 4 is 4,43 watt @ 4 ohm.
Power wise, you will not notice the difference.
Some math:
Let's say the maximum plate voltage swing is 150 VRMS.
Winding ratio of the transformer is 28.5 : 1.
So, with a pure 8 ohm load (forgetting some losses) the secondary swing is 5,26 VRMS.
That is 3,46 watt @ 8 ohm.
Now, when connecting a 4 ohm load, the tube will see a 3250 ohm load.
It will not reach the same 150 VRMS at it's plate, let's say some 120 VRMS.
Secondary swing at 4 ohm now is (120/28.5)² divided by 4 is 4,43 watt @ 4 ohm.
Power wise, you will not notice the difference.
We rarely melt audio transformers. We size the wire for low loss, and speech/music is all peaks with low average.
Power on w-i-d-e-band audio is often limited by bass distortion. Half the impedance allows twice the power; but 10% off for increased resistance losses.
Of course bass distortion may not come into play if your OT is aimed for heavy bass and you only play girl+guitar tracks. I have seen a core I would call "50 Watts" working over 600 Watts in a speech-only old-school (no bass) football stadium announce rig.