Measuring Distortion with a HP 339A and a Keithley 2015 THD 

Aisupporter thank you :) Gerry

With a 8-bit oscilloscope as most are one can only measure down to 0,4%.

​@Florian Hofmann agreed scopes are awful for measuring aptitude. Seeing the shape is more useful for troubleshooting distortion, you may know why it's distorted by looking at the wave or FFT. But the resolution is too poor for measurement.

Yeah its a known problem that can kill the meter, its the two big caps, you can simply replace them if you are concerned. The model I have is a newer model that apparently does not suffer form the problem.

almost certainly due to the output stage but in my experience tube amplifiers are less prone to dissonant tones when overdriven, which is why guitar players love them so much. The distortion characteristics of tube and transistor amplifiers are different in the frequency domain, that's visible when you compare both. In general, tube amps have a notably high THD figure compared to a well designed transistor equivalent, despite this, tube amplifiers tend to be sonically more pleasing to the ear, The only difference I can see between tube and transistors is the frequency spectrum is under moderate overdrive conditions even harmonic levels are more dominant than odd harmonic levels in a tube amp, so it makes perfect sense to me (as a guitar player too) that my Tube amps always sound better than their solid state counterparts. That being said, nowadays, modelling amps get so close to re-producing these tones there is really no justification for the expense and lugging 60's tube amps around any more, a good quality PA or a FRFR active speaker setup is much better today.

@@gerrysweeneyGuitar guys distort the single-ended stages. Typically a high gain triode like the 12ax7. If they overdrive the output stage it'll start adding odds and becomes very grungy sounding. Too much of that and it's rather unplesent. Transistors have a lot of high-order harmonics and IMD. That can sound pretty nasty. Whip up a class A gain stage with a single transistor. That could be a fun exercise in harmonics.

The softer clipping of class A tube output vs a class a transistor with a much more abrupt cutoff is probably what creates the warmer tone. But yeah the asymmetrical clipping is 90% of what makes class A overdrive sound good.

Eric D. No because you cannot generate a lower sine wave from the "original" sine wave. Well, obviously you could but.. The sine wave is already the perfect shape and perfectly is at the frequency you set it too. If there was another, lower frequency since wave than that would be the new original sine wave and the former original sine wave becomes a harmonic. Or it would just be another tone which would be ignored in this case

Raj, very difficult question to answer, you need to get a scope on there and have a look, it could be almost anything. Its generally not a good idea to replace output transistors without first making sure the rest of the amp circuit is in good shape. Every scenario is different I am afraid. Gerry

Yes in theory, but the difference would be so small you would not really be able to tell the difference between the noise you are trying to measure and the noise floor of your scope. There would also be other inaccuracies like the difference in gain calibration and signal delay on the different channels of your scope which would introduce unexpected results, and phase noise etc. So, in theory, yes your thinking is correct, but would require an ideal scope with two perfectly matched channels with a noise floor at least 10x lower than the noise you are trying to measure, so for all practical purposes the answer would be no, that would not work.

@@gerrysweeney Thanks for a really great answer! So the metode you show, have some sort of amplification to the signal, to get the difference after the filter?

@@friedmule5403 metode?

@@gerrysweeney metode = that what you are doing / describing on your drawing. :-)

@@friedmule5403 method I guess

If you are just starting, analog would be cheaper than digital and if you want something that would retain its value I would look at something like a second hand TEKTRONIX 2430A, great scopes, affordable (£250 ish) will probably always be worth about £250 in working order. Gerry

Gerry Sweeney Many thanks Gerry will google that model now and have a read up.

figured it out...

+SteelBlueVision thanks, yes you are right :) thank you for pointing that out. Its funny when you know something instinctively I have never really questioned my understanding of the naming. I know the frequency domain patterns pretty well by sight but not by name obviously :)

Raj, thats an Agilent InfiniVision MSO-X-3024A 200Mhz/4GSa/s. Gerry