Stromberg-Carlson AU-32 distortion in preamp - diyAudio
Go Back   Home > Forums > Amplifiers > Tubes / Valves
Home Forums Rules Articles diyAudio Store Gallery Wiki Blogs Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Thread Tools Search this Thread
Old 15th November 2010, 01:09 AM   #1
diyAudio Member
Join Date: Oct 2009
Default Stromberg-Carlson AU-32 distortion in preamp

Hello all,

I've been reading for a while, but this is my first post.

I've got an SC AU-32, which I am updating and optimizing for guitar. So far I have added a grounded cord, replaced most of the caps, (including rebuilding one of the cans) and changed out the inputs and outputs for 1/4" jacks. I added a selector switch to select the output impedance.

It's working and sounds great, if you keep the volume under 3. Above that it starts to distort, much sooner than it should.

I pulled out the o-scope and hooked it up with a sine wave input. Through the input tube (6SN7) all is good, nice symmetrical amplified signal.

The distortion shows up at the input of the mixing tube (6J5). The two input channels are tied together with 1M resistors, tied together, with the junction fed to the mixer tube.

On the input side of the 1M resistor, I see a nice symmetrical sine wave. On the output side of the resistor, I see a clipped signal.

And there I am, out of my depth. I don't see how the signal is becoming asymmetrical across the resistor.

The only caps I didn't replace are in a can with 3 10 uF caps, one of which is circled on the schem photo. This hookup is confusing, since the cathode resistor/cap are connected not to ground but to the B line. Any ideas?
Attached Images
File Type: jpg 6SC7 output.jpg (39.7 KB, 36 views)
File Type: jpg 6J5 input.jpg (36.1 KB, 35 views)
File Type: jpg Schem.jpg (40.8 KB, 36 views)
  Reply With Quote


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off

Similar Threads
Thread Thread Starter Forum Replies Last Post
Stromberg-Carlson project majorarmspeed Tubes / Valves 58 3rd September 2013 05:15 PM
FS: Stromberg Carlson AU-57 tube amp whitelabrat Swap Meet 8 18th December 2010 05:26 AM
strange Stromberg Carlson m6tt Tubes / Valves 1 11th January 2010 06:56 PM
Converting a Stromberg Carlson AU-35 into a Bass guitar amp? Pckid212 Instruments and Amps 14 30th November 2009 10:10 PM
Stromberg Carlson speaker...please help. the_manta Multi-Way 5 7th March 2007 02:20 AM

New To Site? Need Help?

All times are GMT. The time now is 10:22 AM.

vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.
Copyright 1999-2017 diyAudio

Content Relevant URLs by vBSEO 3.3.2