Measuring output power

[Randall]

I am measuring output power on some 5E3 builds. 400 Hz first, and then 1000 Hz sine wave in, scope and Fluke 77 on 8 ohm non-inductive dummy load. Bring up volume on amp until sine wave starts to clip, read 8.9 Vac on multimeter.

8.9 * 8.9 / 8 ohm = 9.9 watts

1. Am I doing this right?
2. Doesn't this seem low for a 5E3?

It is pretty close to the same on both builds. I did the same to my Deluxe Reverb build and got 19 and 21 watts respectively, which seems about right. Are these two 6V6 amps really that far apart in terms of output power?

[JazzGuitarGimp]

You are doing it right. I usually back the volume up at the onset of clipping so that the signwave looks undistorted, then measure. I'm getting 19.5 watts from my 5E3 build.

[Tillydog]

Does the reading on your meter agree with what your scope tells you? I'm not sure what the frequency response of the DMM would be.

[JazzGuitarGimp]

8.9V on your meter should look like 25.2V peak to peak on the scope.

[Randall]

You are doing it right. I usually back the volume up at the onset of clipping so that the signwave looks undistorted, then measure. I'm getting 19.5 watts from my 5E3 build.

Now I'm really confused. 19.5 watts from a 5E3 seems quite high to me. What would account for such a wide margin?

[Structo]

Does your meter measure RMS voltage?

[Randall]

Not a "true RMS" meter, it's a Fluke 77 IV. I get approx 24 v p-p on scope and 8.5 on the meter.

[vibratoking]

You're math is correct. I assume you are using a scope since you are measuring the clipping. The error is most likely your DMM. Most are not designed to measure RMS voltage accurately at EVERY frequency. I think the Fluke is calibrated to measure RMS at the mains frequency. There is an error at any other frequency. So, I am guessing you are seeing DMM error. Why not just use the scope as suggested? It has all the information you need. Keep in mind that the 1.414 conversion factor only applies exactly to true sine waves. Any distortion of the waveform will also create an error that increases as the distortion increases. Keep the signal from distorting heavily and you will get a pretty accurate measument.


Edited:

Not a "true RMS" meter, it's a Fluke 77 IV. I get approx 24 v p-p on scope and 8.5 on the meter.

24Vpp undistorted sine wave = 8.5Vrms You meter and scope are in sync, so the problem must be somewhere else. Some stupid questions: are you certain of the load, that you are at max undistorted output and that you are measuring across the load?

[Randall]

I have measured the 100 watt 8 ohm non-inductive load at 8 ohms. I ahve turned up the volume control until clipping and then backed off until a clean sine wave, at about 10 on the volume. I am measuring across the legs of the resistor.

[Tillydog]

All sounds OK, which leads to the question of the low output. What OT are you using? (I guess you would have noticed if your B+ voltage was off?)

[Randall]

I'm using the Triode designed Super TF110 paper bobbin made by Mag Comp. B+ was 373. I lowered it about 9v with a zener. Plate voltages are sitting at about 348v. Perhaps I should have left it alone, but it was dissipating about 15w with a 270 ohm cathode resistor, so I put in a reverse biased zener on the OT center tap to reign it in a bit. Could this be a problem?

[sepulchre]

I'm about to do this with a couple of my amps so I'm interested. My meter is not a Fluke but it is supposed to be "True RMS". It seems your Fluke is on the money without being so, so I'm curious what kinds of reading I'd get and how exactly to interpret them.

[Phil_S]

I'm using the Triode designed Super TF110 paper bobbin made by Mag Comp. B+ was 373. I lowered it about 9v with a zener. Plate voltages are sitting at about 348v. Perhaps I should have left it alone, but it was dissipating about 15w with a 270 ohm cathode resistor, so I put in a reverse biased zener on the OT center tap to reign it in a bit. Could this be a problem?

Some 5E3 amps have plate voltage in the 390's. I'd remove the zener. Screens should be ~365.

Remember that line voltage of the day will affect your readings. In the relatively narrow range of line voltage, it is very linear. If you have line voltage of 125v, then 125/120 = 104% of expected B+.

[vibratoking]

I'm about to do this with a couple of my amps so I'm interested. My meter is not a Fluke but it is supposed to be "True RMS". It seems your Fluke is on the money without being so, so I'm curious what kinds of reading I'd get and how exactly to interpret them.

I can't imagine how you can make this measurement without a scope, power meter, or some other piece of test equipment. Measuring output power with a DMM is impossible IMO. You'll measure something, that's for sure, but it won't have much meaning.

[martin manning]

Perhaps one of the preamp stages is clipping prematurely, or your PI is very unbalanced? Use your scope to find out. Also, what is the p-p signal level going into the power tube grids?