Hi everyone.
On vacuum tube gear, power factor is mostly related to the derating that the power transformer suffers when rectifying and using a capacitor input filtering scheme.
But I never read about power factor on output transformers.
I would say that the inductive component of a loudspeaker could add enough reactance at higher audio frequency as to degrade the power factor. In other words, make apparent power greater than real audio power, and placing a heavier burden on the primary winding.
Am I wrong?
On vacuum tube gear, power factor is mostly related to the derating that the power transformer suffers when rectifying and using a capacitor input filtering scheme.
But I never read about power factor on output transformers.
I would say that the inductive component of a loudspeaker could add enough reactance at higher audio frequency as to degrade the power factor. In other words, make apparent power greater than real audio power, and placing a heavier burden on the primary winding.
Am I wrong?
Perhaps, if you only listen to high frequency sine waves at full output...
The primary and secondary might each dissipate 5% of the output power with a resistive load, but the amount of actual power when playing music is small, especially at high frequencies where the inductive reactance increases. The "burden" on the transformer is tiny.
The primary and secondary might each dissipate 5% of the output power with a resistive load, but the amount of actual power when playing music is small, especially at high frequencies where the inductive reactance increases. The "burden" on the transformer is tiny.
I think an inductive load will cause higher voltage swing on an OT. Could be an issue I guess at full power and high freq., but that would be unusual. More of a problem for the output tubes overheating.
with the 60hz power supplies, power factor is easy to figure out, but how do you figure it out at frequencies that go from 30hz to 16khz....? how do you even begin?
You calculate power factor exactly the same way. When the power factor is 70%, you are going to be down 3dB. This happens when R and jX are the same magnitude. The power into the actual resistive portion of the load is half (0.5 and 0.5 added at right angles = 0.707). For a 30 to 15kHz transformer, the power factor is actually quite good compared to many mains transformers.
At high frequency (but below any resonance) impedance goes up, giving less load on the output tubes/transistors. The voltage swing on an OPT can get stupid, And the limiting factor becomes overvoltage, rather than overcurrent. Things will run cool until something arcs.
Where the low power factor can excessively load the tubes is at the low end. Where you have insufficient primary inductance, and more current flows thru that path than thru the resistive portion of the load. Fortunately, tubes can deal with this as long as it isn’t sustained.
At high frequency (but below any resonance) impedance goes up, giving less load on the output tubes/transistors. The voltage swing on an OPT can get stupid, And the limiting factor becomes overvoltage, rather than overcurrent. Things will run cool until something arcs.
Where the low power factor can excessively load the tubes is at the low end. Where you have insufficient primary inductance, and more current flows thru that path than thru the resistive portion of the load. Fortunately, tubes can deal with this as long as it isn’t sustained.
Yes. But usually impedance has to raise quite a bit to be an issue. (it is a lower impedance which most of the times make the anode dissipation go above its max rating).
What about measuring on one frequency at a time?
If power factor is negligible below (let's say) 5 kHz, then it should be ok to test at 10 kHz, 15 kHz and 20 kHz, and then just guess an approximation.
Is that a supposition? Or did you actually made measurements that confirmed to you that assumption?
A quick simulation leads to power factors as low as 0.3 @ 10 kHz for a load of 8 ohm + 0.5 mH and turns ratio 25:1.
Averaging across the full spectrum could yield a better figure, but still 0.3 is alarmingly low power factor, which makes me wonder if I'm missing something here).
Yes, that really seems a concern.
Totally agree. I just ignored that fact because here I was wondering about the burden on the OPT, not on the tubes.
The transformer cannot be held responsible for the poor power factor of the load. If the transformer had 0.5 mH of secondary - referred leakage reactance it would have a 2.5kHz -3dB roll off when tested into an 8 ohms dummy load. Most OPTs are much better than that.
Most loudspeakers do have an inductive rise in impedance. This will swamp the leakage reactance, and would normally extend the frequency response that you would theoretically get if loaded purely resistively. Even causing mild peaking in an amp with no NFB (or instability in an amp with too much, because loop gain increases). But it also brings the resonance with the winding capacitance down (total. L is now higher), and that could get you in trouble sooner. That happens, and everything (including power factor ) take a dump.
If you think about it, power factor in effectively dealt with in textbooks anytime they talk about elliptical loadlines.
Why is this an issue at all? Are you just trying to understand transformer theory? Because in most cases of amp design this is a moot point when you specify the desired frequency response and power rating for the OPT and buy one with a flat FR at the max power you want it to handle. ...right? I've got a feeling this is about another design effort to use cheap toroidal power transformers in place of a regular OPT.
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All tube amps i have encountered will make rated power from lowest
( sometimes 20hz sometimes 30hz ) to 20khz. And this within about a dB )
Same applies for transistor amps. The difference could be how an
amp deals with reactive load. This is however not power sensitive,
any defects here is heard at low and "normal" listening levels.
Difficult speakers generally does better with tubeamps
True.
And not only at High frequencies, where typical speaker inductance increases it impedance, but much worse ar resonance, where a woofer can easily reach between 45 to 90 ohm impedance.
Right.
Wrong.
Not what you expected, and let me put it clear for you.
Apparent Power and Real Power, just to use your terms, have a ratio.
Do we agree so far?
But that ratio can come from 2 very different situations.
What you assume is that out power remains constant, so primary power injected into transformer must increase and so placing a heavier burden on the primary winding.
What I see (as will anybody who focuses on that aspect and knows how speakers work) is that higher speaker impedance (at resonance and higher frequencies) means the speaker will take LESS power than before, so burden on OT will actually DECREASE.
Yes, you still have a ratio, but not between constant secondary, increased primary power BUT based on decreased secondary power.
Which often will mean decreased primary power too.
Nevermind Tony. Thanks for hanging around anyway 🙂
I meant 0.5 mH of speaker inductante, not secondary leakage.
Good point. Thanks, Merlin. Your book on valve preams, for instance, has a short but very nice reference to elliptical loadlines. But I never read on any book about the implications on transformer degins.
Yes 🙂
That's my experience too. But I wasn't sure if there is a burden on the primary (higher rms current) due low power factor.
Definitely.
Thank you. That makes it clear.