Distortion problems

[george]

 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 ?

[loudthud]

Are you actually able to measure distortion? Anytime you talk about distortion, the question of signal level comes up. What signal amplitude are you using? If you are just banging some bass with active pickups and an 18V preamp into the thing, what do you expect? Vintage bass amps cannot handle modern active pickups.

[george]

No I'm looking at the FFT of the scope signal - the fundamental is say (signal level = 150mVrms) 1kHz and I get a nice second order "side band" at 2kHz. You can "hear" it with a guitar connected as well. Signal input is nice and clean so it's in the pre-amp. Was Sunn trying to add distortion and boost with the footswitch arrangement on these amps? The service manual I have shows pin #52 as distortion out and pin # 53 as distortion boost - for pre amp board 32-0040.

[loudthud]

Was Sunn trying to add distortion and boost with the footswitch arrangement on these amps?

It was the Nixon administration...

[george]

It was the Nixon administration...

and these amps sound terrible - I thought they were supposed to be "clean" with lots of "headroom"??

[loudthud]

and these amps sound terrible - I thought they were supposed to be "clean" with lots of "headroom"??

What do you expect? It was the 70s. Elvis was alive and hadn't left the building. Vintage amps are what they are. If you don't like them, get something else. They sound like what you make them sound like. If you don't have the instrument or technique or the right speaker cabs, they're not going to sound like you want them to. Plenty of people on here like the way they sound.

[george]

Yup I agree that it was the 70's and these poor things are almost 40 years old - Funny I spent 6 years "tunning" cars for THX in CA/MI/Germany and I thought that was a world based on just a little knowledge and many levels of beliefs.   Hey I'm just fixing these for folks - no vested interest here but to give them something that is consistent and relaible - as far as the notion that it's instrument and techique - its all in the fingers bro - no tube/DSP/speaker cabinet/etc will ever make up for that. Thanks for the help.