Oscilloscope coming soon - please help!

[xtian]

power equals voltage (RMS) squared divided by resistance

P = V^2 / R

To get RMS voltage, you need HALF the peak-to-peak amplitude of your sine wave. That's the 0.3535 multiplier, because 0.3535 is half of 0.707.

Finally, your resistance is your speaker load.

[rogb]

How's this looking now?

85.3v p-p. I can't get any more level out of the source to get a clear clip.
My dummy load is good and hot though!
So the math(s):

85.3 x85.3 that's the square, OK.

Divided by 16 = 45.47w

Maybe.....

[martin manning]

0.707 is not the area under a sine curve. That would be 2 (and meaningless for this discussion).

0.707 is the RMS amplitude of a sine wave who's peak amplitude is 1. That sine wave has p-p amplitude of 2, so If you start with Vp-p the V RMS for any sine wave is given by Vp-p/2 * 0.707 or Vp-p * 0.354.

For 85.3V p-p, that's (85.3*0.354)^2/16 = 57W

[xtian]

Isn't this true? A sine inscribed inside a square of width 1 has an area of 0.707?

[martin manning]

Isn't this true? A sine inscribed inside a square of width 1 has an area of 0.707?

No. In that case the area under the sine is 0.637 times the area of the square, which you can think of as representing the average amplitude with respect to the peak.

[rogb]

0.707 is not the area under a sine curve. That would be 2 (and meaningless for this discussion).

0.707 is the RMS amplitude of a sine wave who's peak amplitude is 1. That sine wave has p-p amplitude of 2, so If you start with Vp-p the V RMS for any sine wave is given by Vp-p/2 * 0.707 or Vp-p * 0.354.

For 85.3V p-p, that's (85.3*0.354)^2/16 = 57W

I took this to the pub and the guys there explained how it works. I can understand the formula now, which is a bit late in life, but never too late to learn something. One of the guys there used to do something with tube-powered missiles in the RAF so he has forgotten more about tube technology than I will ever know!

As far as measurement of the power output goes, I set the 1kHz signal up to read 1v p-p, but to see the wave clip I had to change sensitivity so I could see the clip start, so I am not 100% sure I am doing that part right, guys.
I need to get a function generator so I have more level to play with.
Many thanks so far, TAG is the place to extend your knowledge, totally unique

[martin manning]

Never mind that, what beers did they have on? I can see you guys writing equations and sketching on the backs of beer mats... The other patrons probably thought you were nuts.

What procedure are you using to set power just below clipping?

[xtian]

As far as measurement of the power output goes, I set the 1kHz signal up to read 1v p-p, but to see the wave clip I had to change sensitivity so I could see the clip start, so I am not 100% sure I am doing that part right, guys.

I don't think the input amplitude is critical. Elsewhere on the web, soneone suggested setting the input wave at 250mV instead of 1v. So I tried that, too. The end result was the same, at least for the 10 amps I tested.

[rogb]

Well I played the guitar through it and it seemed to hit 80-90v before I could see clipping so I guess my sine wave might be OK.

I just kept turning the phone volume up until saw clipping start. I also ran a sound file through the laptop to compare.

The probe was on x10 and I compensated it as per Hantek's instructions.

MM: The beers are Fuller's - ESB and London Pride. I like Foster's though and it is the best pint around in that pub, so I guess the bitters are similarly good.

[rogb]

I just ran the 100w 2nd gen through and came up with:

96.0V p-p, that's (96.0*0.354)^2/16 = 72W.

Seems a bit low? Maybe it's the tubes apparently they are 30w plate dissipation TAD 6L6GC-STR?

I have a function generator on order so I will retest when I get it.

[martin manning]

I would set the input signal level and start with the volume controls turned down to 3 or so and the tone controls maxed. Then bring the preamp volume up until you see clipping at the output and then back off until it cleans up. Then bring up the master until you see clipping and back it down until the waveform is clean again. You don't want the preamp to clip before you get up to maximum power.

These clean power measurements are only going to be approximate since you are just eyeballing what appears to be a clean sine wave. Really you need to use the spectrum analyzer feature on your scope (if it has one) to set power at some reference THD. It doesn't really matter though, since guitar amps are supposed to generate distortion.

If you have a true RMS meter you can measure the voltage across the load and calculate power using that to see how it compares to your calculation assuming a perfect sine wave.

[rogb]

I would set the input signal level and start with the volume controls turned down to 3 or so and the tone controls maxed. Then bring the preamp volume up until you see clipping at the output and then back off until it cleans up. Then bring up the master until you see clipping and back it down until the waveform is clean again. You don't want the preamp to clip before you get up to maximum power.

These clean power measurements are only going to be approximate since you are just eyeballing what appears to be a clean sine wave. Really you need to use the spectrum analyzer feature on your scope (if it has one) to set power at some reference THD. It doesn't really matter though, since guitar amps are supposed to generate distortion.

If you have a true RMS meter you can measure the voltage across the load and calculate power using that to see how it compares to your calculation assuming a perfect sine wave.

Right! I am going straight in with the 1kHz to the P/A input and just adjusting the media player and laptop volumes for the clip reading.
I hope it will be more accurate with the function generator.
I don't think I have a spectrum analyzer on this model.
I will try your method and report back. Many thanks MM.

EDIT: I was getting 89-91v p-p running through the preamp as you described.

[xtian]

I just ran the 100w 2nd gen through and came up with:

96.0V p-p, that's (96.0*0.354)^2/16 = 72W.

Seems a bit low?

No, sounds great. As we learned in my thread <https://tubeamparchive.com/viewtopic.php?t=24113>, this is a very conservative method of power estimation. Once you get used to the numbers, they start to feel familiar, and I have found this method consistent across many amps.

[rogb]

I just ran the 100w 2nd gen through and came up with:

96.0V p-p, that's (96.0*0.354)^2/16 = 72W.

Seems a bit low?

No, sounds great. As we learned in my thread <https://tubeamparchive.com/viewtopic.php?t=24113>, this is a very conservative method of power estimation. Once you get used to the numbers, they start to feel familiar, and I have found this method consistent across many amps.

Thanks bud, I read that thread during my research but it didn't make sense until now. I have learned a lot this week.
Seems the 102 50w is pretty lively in the watts dept, it's a nice amp though I think I prefer my 2nd gen hybrid for the overall clean/OD "vibe"

[JazzGuitarGimp]

You mean, there's an Amp Garage AND an Amp Pub?!