Distortion problems - again

[gbono]

Can you not attach/post gif.s?

I have three Sunn Concert series amps that are showing various degrees of second order harmonic distortion in the preamp section. For example, I have a Concert Bass which doesn't shown any 2nd order harmonics at Q101 but it shows up at Q102, Q103, etc. I have the distortion feature turned off but I'm wondering if there is a ground path that I don't see

If R112 or R126 were shorts then the distortion would be "on"? I'm assuming the distortion feature works by adding the RC feedback networks into the output of Q102 and the input of Q104 via the ganged 25K pot ?

http://sunn.ampage.org/site/schematics/concertbass.gif

[J M Fahey]

How do you know that you have 2nd order distortion? 

[gbono]

Since I can't seem to attach files.... here are some links to waveforms:

http://i284.photobucket.com/albums/ll33/bonosurf/DSCF01031.jpg

This is the signal from the pre-amp output jack on the rear panel - 150mVRMS into the amp (at around 1kHz) maybe 1-2 on the volume control - the signal into and through Q101 is clean right up to the gate of Q102. Then it looks like what you see above. I have the distortion and boost disabled.

http://i284.photobucket.com/albums/ll33/bonosurf/DSCF01011.jpg

This is the output waveform from the output jack on the rear panel - 3.2 ohm dummy load same conditions for input as before but 1-2 on the volume control - as you can see this is low level output with notch in the crossover region (duh). I didn't see this before because the scope puts limit lines on the display and they hid the notch.

http://i284.photobucket.com/albums/ll33/bonosurf/DSCF01041.jpg

Moderate to high level signal measure as above maybe 4-5 on the volume control - this amp will almost "soft clip" like a tube circuit.

So I have multiple problems here. I'll check the bias on the output transistor. I tried to adjust the bias trimmer but it doesn't seem to improve the crossover distortion issue - though it will adjust the idle power (measured at the AC mains).

[J M Fahey]

Hi Gbono.
I don't think that you have "a problem", but rather an "old" amplifier, which still can give battle, just don't expect from it what it never did.
Looking at your pictures:
1) I don't understand it, the image is blurry and shows 2 waveforms: a perfect blue sinewave and a green sinewave which looks like output noise/hiss plus the occasional popcorn (the bursts) .
Don't like the image quality at all, is it a PC scope or something like that?
2) *This* is what I expect to see on a scope screen; is it a real one or a PC scope? Very good.
That signal is loud, clean and normal.
Yes, it has crossover distortion, that kind of transformer driven transistor amp is famous for it, you can live with it (please do) or thermally runaway and die in a nasty way.
Leave it alone. Anyway it can't be heard when playing loud, so ... *do* play LOUD !!! (now you have the perfect excuse)
3) These transformer driven amps are famous for "tubeish" clipping (same as old Vox), enjoy.
PS: that bias control adjusts the driver transistor only, so put it back where it was originally or as stated on the schematic.
:tu:

[gbono]

Ouch - you mean these amps are supposed to perform like this?

The first picture/waveform is from an Agilent portable scope. The scaling for the verticle and horizontal are a bit confusing since you are looking at the time domain signal (blue) and the frequency domain transfer function (green). Crude but effective way of looking at realtive distortion/modulation of a test signal.

The other pictures are from an older HP scope and yes that vector CRT display is nice to look at.

[J M Fahey]

Well, the blue signal looks fine to me, considering the lack of definition; the lower, derived one, may be a representation of distortion, but which is many dB below the sinewave, essentially inaudible.
You have a LOUD workhorse, use it as intended , as in not in bedroom situations.
Good luck.