New to scoping, a couple questions

[lonote]

I just finished rebuilding a '79 Ultra-linear Pro Reverb that I had previously sort of refurbished, but found it had chronic noise due to a leaky board & in the end, it got a total overhaul. So it became the first real guinea-pig for my oscilloscope learning curve.

Overall it is quite clean but does finally get to distortion & I was interested to see what section was breaking up first. So feeding in a 500Hz sine wave @ 200mA, I placed probes at; the input jack, the input of the reverb mixer/3rd gain stage, at the input to the PI & at the speaker output (into a 4Ω dummy load).

I decided to set the tone stack to roughly playing position with T,M & B all around 5. Reverb & Trem were off.

I was surprised to see that it was the output at the speaker that was the first to breakup, going into clipping on the bottom of the wave form, but well before the top of the wave form.

In fact, the preamp sections didn't really break up that much except with extreme tone settings.

I had the scope set to calculate RMS volts & I could reach about 15.9VRMS@speaker out, before the bottom started to clip. Using an online calculator (https://www.sameskydevices.com/product/ ... calculator) that puts the amp output at about 63W, which is right in the neighborhood of it's power rating as an Ultra-linear amp (70W).

I wondered if the power tubes were at the root of the asymmetrical clipping, so I swapped the power tube positions, but the onset of clipping remained on the bottom of the wave form.

So my two questions are;

Am I going about measuring this stuff in the right way? Insights, tips or tricks?

Second, a circuit question. It would seem that something in the PI is causing the asymmetrical clipping & where would I start to look? This question is more academic as the amp seems to work well at reasonable volumes & this would probably not be an issue in use, but would be nice to know.

[romberg]

Sounds like you are going about things right to me. A 200mV signal might be a bit on the hot side but it is in the ballpark.

I have found that many/most early amp designs breakup first in the power tubes and then work there way backwards towards the input. You can see this by attaching the scope to the speaker output and turning up the gain until the fist distortion is seen. Then you can verify where the breakup is happening by moving a probe one step/stage down the line. So, if you see a distorted signal at the speaker jacks but the signal on the power tube grids is clean then you know the distortion is in the power tubes and OT. And you also would then know that everything else before that is not distorting. You can then increase the gain and move the probes down the line to follow the breakup.

To check this on the PI, I'd attach probes to the PI input and the power tube grids. If you see distortion or clipping on the power tube grids but the input to the PI is clean then you know that is where the breakup is happening.

Mike

[lonote]

Mike, thanks for confirmation that I am not too far off base.

I was fooling with it more before I saw your reply, basically covered the same ground you suggested & it seems the clipping is occurring in the PI.

I also broke out my copy of Merlin's book & he has some very similar illustrations of the LTPI going asymmetrical. Maybe that is just how it works & is supposed to look. I just don't have the scope experience to draw from. But it seems like a pretty sharp cut-off to me.

Just for fun, there are 2 screen shots attached.

Both show a hard-plucked open E string. Vibrato Channel, No Reverb or Trem, TMB all at 5, Vol at 7


Yellow= input jack, Magenta= PI input

Pic 1 Blue= power tube grid, Pic 2 Blue= speaker out

It looks to me like the power tubes are just doing as they are told.

Scope Capture 1.JPEG

Scope Capture 2.JPEG