Marshall Jubilee Unusual Clipped Waveform Shape

[bobloblaws]

Got an 87 Marshall Jubilee 100W head here, I hooked it up to my scope to measure output power. I was surprised to see a strange shape to the waveform when it started to clip. I expected the tops and bottoms to square off but instead the distortion appears to be on a different portion of the wave as in the attached photo. For fun I started over and left the master volume at lower level but with the preamp gain cranked and it was pretty much a perfect square wave. Any idea what might cause this strange shape?

B

[Enzo]

Yes. It is not a hifi. Seriously.

A hifi might clip the flat tops and bottoms off because it was passing a nice clean sine wave through the amp up to that clipping point.

[Greg_L]

Got an 87 Marshall Jubilee 100W head here, I hooked it up to my scope to measure output power. I was surprised to see a strange shape to the waveform when it started to clip. I expected the tops and bottoms to square off but instead the distortion appears to be on a different portion of the wave as in the attached photo. For fun I started over and left the master volume at lower level but with the preamp gain cranked and it was pretty much a perfect square wave. Any idea what might cause this strange shape?

B

[ATTACH=CONFIG]49655[/ATTACH]

Where are you probing to get this waveform?

[Dave H]

Got an 87 Marshall Jubilee 100W head here, I hooked it up to my scope to measure output power. I was surprised to see a strange shape to the waveform when it started to clip.

That looks like clipping in the preamp to me. Scope the waveform at the power amp input to make sure it's a sine wave.

[pdf64]

Presence control set to min?
Resistive load?

[Chuck H]

Right. Presence at zero, tones at five. Use 100mV input signal and adjust for amp output with the amplifiers volume and gain knobs.

EDIT: Oh... And very often it's necessary to futz the tone knobs to an unusual setting, like treble zero, bass ten, mids five, or something to get a guitar amp to demonstrate even close to a proper square wave. They ARE signal processors after all and NOT reference amps. I guess Enzo already covered that.

[bobloblaws]

Thanks for the tips guys. I didn't realize that the position of the tone controls had a dramatic effect on the shape of the waveform. Per Chuck H, futzing with the controls more or less squared it off.

Dave, the preamp output was a perfect sine.

pdf64, yes I was using a 4 ohm dummy load. What impact does that have?

B

[eschertron]

Despite the expectation that a 'clean' sine wave will look correct all the way through the signal path, I've noticed that even with a very low amount* of low-order harmonic distortion from 1st and 2nd gain stages the tone stack can make crazy shapes from the phase shifts between treble and bass.

*low enough that it looks OK to me, and I don't have a distortion analyzer, so...

[Chuck H]

Despite the expectation that a 'clean' sine wave will look correct all the way through the signal path, I've noticed that even with a very low amount* of low-order harmonic distortion from 1st and 2nd gain stages the tone stack can make crazy shapes from the phase shifts between treble and bass.

*low enough that it looks OK to me, and I don't have a distortion analyzer, so...

The first time I scoped the signal out of a VOX type TS I though something was wrong with the amp. You can actually flip the phase with the tone control settings!