Output Waveform

[Carlosraj]

Hi

This is the first time I am using a scope. Attached is an image I am getting on my scope attached to the output of an amp. The waveform in yellow is the input and the one in blue is the output. I notice a shift in the two waveforms and a dent at the centre of the output waveform. What does the deformation in the waveform means, and what likely can cause it.

Thanks

 

[Netlist]

The dent at the center is crossover distortion, most likely caused a underbiased output transistors.
The shift is a fase shift from in to output.
How about the other channel?

 

[Carlosraj]

This is a Marshall tube guitar amp, only one channel output.

Thanks

 

[Netlist]

I think the tubes need a bit more bias. 🙂

 

[turk 182]

which model number Marshall is it?

more bias and confirming the phase inverter coupling caps aren't bad/leaky

 

[rayma]

At 1kHz, the phase shift is probably in the tone stack.

 

[Carlosraj]

OK, Its a JCM2000.

 

[turk 182]

too many versions under the JCM2000 banner can you be more specific? DSL50 DSL 100 and which revision?

https://www.drtube.com/schematics/marshall/JCM2000 Bias Instructions.gif take a look here

 

[Carlosraj]

Its a DSL100.

Thanks

 

[turk 182]

is the bias stable?
any red plating?

it might be that the bias was intentionally starved to counter a "know" issue.