Distortion problems

[gbono]

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 ????

[Enzo]

First, I don;t assume that because I hear distortion that it is caused by the distortion circuit. Turn the distortion on, does it sound just like the unwanted distortion from when it is off?

Just an opinion, but I see shorting R112 as simply enabling pot R111 which can dial in the amount of gain increase caused by C105. Exactly as we might switch in a cathode bypass cap to up the gain in a tube stage. And shorting R126 just enables the other half of R111 as a tone rolloff. C115, C116 and that half of R111 form a simple tone control. SO as you dial up the gain in the earlier stage, this tone control rolls off some parts of the freq response to make it more pleasing. So the R112 part is involved with making overdrive distortion, the R126 part is simply a tone control, no distortion.

SO apply a test signal, are you overdriving any of the stages? Can't cram 20v of signal through this. Are any clipping? What does your distortion look like?

Have you replaced all those small e-caps like the source bypass caps? Q101, 102, 103 each have one, plus coupling caps C122, C124.

[dumbassbob]

Distortion

I had that problem found out it was dirt on jacks and a brand new P.Tube gone south

[gbono]

Enzo,

Your always the sanity check - though the preamp stage is distorted - it looks like there is some nasty crossover distortion (a "notch" at the zero crossing). It's very noticeable distortion at low levels and seems to decrease with drive level. This model has a bias trimmer that doesn't seem to change the distortion though you can change the idle AC power consumption.

I did try swapping the output devices but no change. Probably should make sure the bias is correct on the output devices. Any other suggestions?

[Enzo]

This Xover distortion is coming from the preamp? You have the jacks for it, don;t you? Plug a test signal right into the power amp, and also send the preamp signal out and see what it looks like.

[gbono]

Enzo,

sorry for the confusion - the low/moderate level output from the power amp into a 3.2 ohm dummy load is here

http://i284.photobucket.com/albums/l.../DSCF01011.jpg

This is the output from the preamp at a moderate level

http://i284.photobucket.com/albums/l.../DSCF01031.jpg

Output from the power amp at moderate level

http://i284.photobucket.com/albums/l.../DSCF01041.jpg


The input signal is 150 Vrms @ about 1kHz - I didn't notice the crossover distortion before because the scope placed limit lines on the waveform right at the "notch".

Isn't Q102 a source follower with split bias? I would assume unity gain - no? The input signal to and through Q101 is clean all the way to the gate of Q102.

[Enzo]

Wow, that is not what I see at all. Q102 has the same circuit as Q101, same 15k up top, same 18k - or is that 1.8k? - underneath. Both strung between +25 and -15. The coupling caps are different for tone shaping between stages, but that doesn;t affect the basic circuit type. The Q101 bypass cap is wired in, while the Q102 bypass is switched in and made variable with the control. In fact Q103 has the same circuit as well.

I don't even see anywhere a signal could leave Q102 source to follow and go anywhere.

SO, no, I do not expect unity gain. I expect each stage to have positive gain. Then after each stage it is divided back down. R108,109 is a divider, and R114,116 also form a voltage divider.

[gbono]

So I'm assuming you feel these are typical waveforms for an amp of this vintage? Ouch.

So the "boost" feature is always on and a footswitch is needed to turn it off.
Also if the boost feature is turned off and distortion is on - you get distortion feature to work. See truth table below. Assume this is also normal for these amps?



RESULT --- DIST -- BOOST
boost -------- on ---- on
nothing ------ off ---- on
distortion ---- on ---- off

[Enzo]

Oh, beats me. I don't know what is normal or how well the circuit is operating. I was just responding to your source follower assessment.

In my estimation the footswitch enables the feature, not disable. Look at R112, when the footswitch shorts across it, then the control can vary the series resistance with the cap from about 15k down to nothing. Turn off the FS and now you can't go below 68k, which seriously limits the cap effect. So to me, you need the FS to turn the effect there on.

And same story at R126. With an open FS, that 470k will pretty much prevent that tone circuit from doing much. Turn the switch on, shorting R126, and now you have caps and a 25k variable shunting the signal path