Plate dissipation vs. Power output at speaker

[Nekle]

How do these differ? Is the power consumed at the plates different from the power as it comes out of the speaker? How do any losses occur? Sorry if this has been asked before.

[Old Tele man]

...power at the PLATE must pass through the OUTPUT transformer (OT) before making it to the SPEAKER...

...OT's are typically about 85-90% efficient, so if the power tubes are cranking out 50 Watts, the speaker will only "get" about 42-45 Watts...(most Fender/Schumacher "606" OT's are about 90% efficient).

...hence, the power tubes are producing MORE power than the speaker(s) will be receiving.

[Nekle]

thank you old tele man.
Is 90% then a good "general rule of thumb" type thing to use? And is that with matched loads only?

What about when the OT primary impedance is coupled to a 16 ohm secondary and you tag an 8ohm speaker on it? this will reflect back to the primary half it's original impedance (correct?). So if the tubes originally needed a plate to plate resistance of say, 6600 ohms, they would now be seeing 3300 ohms meaning more current would be travelling through both sides of the OT. Since the plate voltages have stayed the same does this mean that we are increasing our power output at the speaker simply by mismatching the loads?
Thank you for your help.

[Blue Strat]

Mismatching speaker loads reduces power to the load. There's no way around this.

[Nekle]

thanks blue strat. I don't think I'll ever understand this stuff:(

[Wakarusa]

Originally posted by Nekle
thanks blue strat. I don't think I'll ever understand this stuff:(

Try this: Electron-Tube Circuits (http://www.wakarusaamp.com/library/manuals/Electron_Tube_Circuits_seely.pdf)

Go to Chapter 9 (Untuned Power Amplifiers). Excellent description of exactly how power to the load (vs. dissipated in the plates) is worked out. Not nearly as technically dense as RDH4.

Edit: Look at figure 9-2. It's a graph of the equation right above it. This is for a triode, but still gets the idea across. The salient point is that maximum power transfer is achieved only when the plate and load resistance is equal.

[Nekle]

Thank you for that Wakarusa!

[Old Tele man]

excepted from ELECTRON TUBE CIRCUITS, 2nd Ed., 1958, McGraw-Hill, by Dr. Samuel Seely...Chapter 10, page 314, Figure 10-2 and text:

...here's the rationale behind the tube amp "rule-of-thumb" which states: "...total speaker load of 'HALF-to-TWICE' (ie: Z/2 to 2*Z) the 'optimum' load is OK."

http://home.comcast.net/~elmccaul/GTRS/Half_Exact_Twice.JPG

...the key words are:

(A) "...the power is at least 88 per cent of its maximum value for values of Rl/rp ranging from 0.5 to 2.0" [emphasis mine]

(B) "...shows that the power loss is less than 2.25dB for all values of R between 0.5*rp and 2*rp." [emphasis mine]

...this means there's only about a 12% power loss when using speaker loads that are either: (A) One-half (Z/2) of optimum; or (B) Twice (2*Z) optimum...but, that assumes you started with a "perfect" impedance match to begin with, which is seldom true...so, the actual power loss is usually higher, probably closer to 15-20%.

...and that means, about 1/5th of the power which SHOULD be transferring to the speaker is staying within the tubes as HEAT...which can eventually cause "red plates" due to over heating (also depends upon how 'long' you're playing at full power, ie: the TIME-duration).

Note: Rl = Load resistance; rp = Plate resistance

[StompBoxBlues]

I know that the power output transformer works, that as the current collapses on the negative going side of the wave, it induces a postive going on the other side.

This is why there is no "cheating" the trafo, getting something for nothing...it is only optimal when the impedance matches for effective transfer.

BUT...I had a big question rolling around in my empty brain for a while now, trying to decide and theorize on what the answer could be...

I was wondering this, does it make any difference which of the settings you choose while they match?

But this I mean, if you have a selectable impedance cabinet AND amp, go extreme and say you can either set both cabinet and amp to 4 ohm impedance, or 16 ohm impedance.

Is one of those inherently a little better than the other, or is it exactly the same? Do the speakers work any better/worse/harder or the amp when it is one or the other?

[aleclee]

Originally posted by StompBoxBlues
I know that the power output transformer works, that as the current collapses on the negative going side of the wave, it induces a postive going on the other side.

This is why there is no "cheating" the trafo, getting something for nothing...it is only optimal when the impedance matches for effective transfer.

BUT...I had a big question rolling around in my empty brain for a while now, trying to decide and theorize on what the answer could be...

I was wondering this, does it make any difference which of the settings you choose while they match?

But this I mean, if you have a selectable impedance cabinet AND amp, go extreme and say you can either set both cabinet and amp to 4 ohm impedance, or 16 ohm impedance.

Is one of those inherently a little better than the other, or is it exactly the same? Do the speakers work any better/worse/harder or the amp when it is one or the other? They'll sound different due to differing damping factors.

Do a search for "speaker AND damping" for the 411.

[StompBoxBlues]

Originally posted by StompBoxBlues
I know that the power output transformer works, that as the current collapses on the negative going side of the wave, it induces a postive going on the other side.

This is why there is no "cheating" the trafo, getting something for nothing...it is only optimal when the impedance matches for effective transfer.

BUT...I had a big question rolling around in my empty brain for a while now, trying to decide and theorize on what the answer could be...

I was wondering this, does it make any difference which of the settings you choose while they match?

But this I mean, if you have a selectable impedance cabinet AND amp, go extreme and say you can either set both cabinet and amp to 4 ohm impedance, or 16 ohm impedance.

Is one of those inherently a little better than the other, or is it exactly the same? Do the speakers work any better/worse/harder or the amp when it is one or the other?

Thanks for the tip. Interesting stuff.

[Nekle]

Yes, thank you all very much for helping with this:)
I'd like to try and build one of my own some day but I'm not even close to being ready for it yet. Slowly though I'm hoping to get there!