measuring output power

[amptweeker]

I must be missing something, I tried two different amps with a scope signal Gen and Dummy load,
In both cases the power seemed to be about 1/2 of what it should be.

One amp was cathode bias 5881 push pull about 387 on the plates 4K opt
16 ohm 200W dummy load about 13.8 VAC measured clean sine wave,

gives me about 11.9 watts?

I would guess about twice that 23- 24 Watts would be expected as it is cathode bias

I also measured a small 6v6 push pull amp and it came out to like 6 watts, maybe 12 is more like it.

I am using V-squared over R for power, but it seems like it needs to be about times 2

Sorry if this question had been asked before I did some searches.

[Enzo]

Did you apply your test signal directly to the phase inverter grid? or did you run through the preamp? If you ran through the preamp, you have to make sure the signal is not distorting there. If the power stage receives a distorted signal, it has no way to clean it up.

[J M Fahey]

If you want to live happy donīt read your girlīs stored messages and donīt measure Tube power amps

Seriously now, power ratings are "optimistic" , specially with modern tubes (which are weaker) and in modern amp versions (which lower voltages precisely because modern tubes are weaker to begin with)
In the old days Fender reached 500/530V and some Marshall reached hair rising 560V , today most are tamed to 400/420V or so under penalty of replacing lots of tubes under warranty, a losing game.

If I measure 80/85W *clean* out of a Fender or Marshall I am more than happy, in any case thatīs LOUD.

Only amps which regularly meet or surpass official ratings are old Ampeg (think V4 and such), 6550 equipped Marshalls . both of which are based on "stronger" tubes such as 7027 or 6550, not your run of the mill 6L6 or EL34, and MusicMan which feed their tubes 600 to 700V and cathode drive them balls to the wall and beyond.

Yours should put out somewhat more, but nothing groundbreaking.
Test fresher tubes, vary cathode biasing resistor, modify load up and down, you might improve things.

EDIT: there might also be some measurement procedure error:
such as:
* if you are using 1kHz signal and measuring with a standard multimeter, many donīt reach that high so display less than what you actually have.
Try 250 or 440 Hz from: http://www.mediacollege.com/audio/tone/download/

get the 30 second MP3 and set player to Loop or Repeat 1 to have a continuous tone.

* if you are measuring by injecting 60Hz (many do, as you have it "free" straight from a filament tap) Guitar amps usually lose power/efficiency that low.
A Bass amp "should" play 60Hz well.

[pdf64]

What are you using to measure the 13.2Vac?
Cheap meters have a limited freq range, may not cope >200Hz, and only good for low distortion sine waves, so will mis-read tube amps that don't have negative feedback. True RMS meters are preferred.
The plate voltage at idle (eg 387Vdc) is kinda irrelevant. What matters is the Vdc between the cathode and the HT (eg at OT CT) at max signal output.

[amptweeker]

Did you apply your test signal directly to the phase inverter grid? or did you run through the preamp? If you ran through the preamp, you have to make sure the signal is not distorting there. If the power stage receives a distorted signal, it has no way to clean it up.

I ran the signal through the preamp but was on the load with the scope and watching for clipping

[amptweeker]

What are you using to measure the 13.2Vac?
Cheap meters have a limited freq range, may not cope >200Hz, and only good for low distortion sine waves, so will mis-read tube amps that don't have negative feedback. True RMS meters are preferred.
The plate voltage at idle (eg 387Vdc) is kinda irrelevant. What matters is the Vdc between the cathode and the HT (eg at OT CT) at max signal output.

I was using a fluke 87 on Ac volts

[Enzo]

OK, you were watching the output for clipping, but where did that clipping occur? It is easy to overdrive the preamp into distortion, which will make the output appear weak.

[amptweeker]

OK, you were watching the output for clipping, but where did that clipping occur? It is easy to overdrive the preamp into distortion, which will make the output appear weak.

I guess I was trying to make sure the sine wave was clean or not clipped, and measure before any distortion or clipping happened.
I think there is a flaw in my method somewhere

[Jazz P Bass]

Try again with an 8 ohm load.

[Enzo]

What I am clumsily trying to say is that your tubes may well be capable of greater clean power, but your preamp stages may be limiting how large a signal can get to them before distortion occurs. You are looking for onset of distortion in the output, but I THINK you are assuming the clipping occurs there. It may or may not.

Think of this, what if we ran a fuzz pedal before the amp, so no level of signal came out the speaker undistorted. it is real distorted no matter what because of the pedal. It would not be fair to the power tubes to claim they had zero power.

[Tom Phillips]

To support what Enzo is getting at, please tell us the amplitude and frequency of the test signal you are applying to the amp's input.

[J M Fahey]

I guess I was trying to make sure the sine wave was clean or not clipped, and measure before any distortion or clipping happened.
I think there is a flaw in my method somewhere

No flaw, what you measure is truth, anything else is smoke and mirrors.
That said, amp might be improperly set, biased, loaded.
For example, you didnīt state cathode voltage, idle current.
Maybe 4k Zaa is too low under your conditions.
Maybe tubes are worn.
>>>Are you connecting 16 ohm load to 16 ohm tap?<<<<
Less than a Month ago, a colleague in an all Tech closed group had a similar problem, he was, say, using a 16 ohm load into a 4 or 8 ohm tap and stubbornly insisted that "amp should put out full power anyway" ...... finally he properly matched load and, no surprisingly, expected power was there .
Ok, a little less , but reasonably close.
Please post schematic.

[amptweeker]

I was using a 1khz sine wave I did not measure the input signal, I guess I will perhaps try again and gather more details
I was assuming there was a "standard way to measure the output power of a tube amp, at this point all I know
is it is a very loud cathode bias amp perhaps the loudest I have heard, I was just curious if I should call it 20W or 25W
or even 30W, I guess it does not matter, I was just curious

[Dave H]

I ran the signal through the preamp but was on the load with the scope and watching for clipping

I guess I was trying to make sure the sine wave was clean or not clipped, and measure before any distortion or clipping happened.
I think there is a flaw in my method somewhere

Amplifier power output is quoted at a certain distortion level (5% or more) so you don't want a perfectly clean not clipped output. Turn it up until there's a little flat on the peaks (like the plot below which measures 5% THD) then make sure the pre-amp isn't clipping by scoping the PI input.

The plot below is an amp similar to the one in the first post i.e. 2 x 5881 cathode biased 4K OT, B+ 375V. It measures 25W @ 5% THD.

[mozz]

Try again with an 8 ohm load.

Exactly. I have 4, 8, 16 ohm dummy loads, when i build from scratch or a clone, I will find 1 impedance where you get more power out.