Audiokarma.org
Audiokarma Featured Sponsor

Go Back   AudioKarma.org Home Audio Stereo Discussion Forums > Tube Audio


We appreciate your help

in keeping this site going.
Reply
 
Thread Tools Display Modes
  #1  
Old 10-02-2013, 05:38 AM
Dandy's Avatar
Dandy Dandy is offline
AK Member
 
Join Date: Jul 2008
Location: Wiltshire, UK
Posts: 2,142
SE 12AX7 and 6BQ5 schematics please

I'm having another look at the little SE 6BQ5 amp I got from a scrapped record player. The original schematic is attached to this thread. http://www.audiokarma.org/forums/sho...d.php?t=514623

The feedback went via the tone and volume pots. Another feature is that the output transformer has a tapped primary and the B+ at the reservoir capacitor (C67) immediately after the rectifier goes to the tapping on the transformer at a point on the secondary so that the ripple current flowing in the output valve anode cancels with the ripple in the B+ current flowing to the rest of the amp including the smoothing capacitor (C50). This makes it nice and quiet in operation.

When I removed the tone controls and made it a power amp to use with a 50k pot, I used the values of the attached schematic. It works, but now I don't know where/how to apply feedback! Without feedback there is some high frequency distortion. If I apply the 22K feedback, either to cathodes as in the 'new' circuit, or the grids as in the original one, it oscillates with HF.

So, I'm thinking of starting again. Any favourite schematics?
Attached Images
File Type: jpeg 83-84 amp.jpeg (54.5 KB, 63 views)
Reply With Quote
  #2  
Old 10-02-2013, 08:10 AM
Pio1980's Avatar
Pio1980 Pio1980 is offline
AK Member
 
Join Date: Aug 2008
Location: Angel Station, Alabama
Posts: 13,654
For a stereo amp using those bubs, a twin of this example of German boilerplate is about as good as it gets.
__________________
"Different" is easy, "Better" not so much.

Goodness comes from getting the basics right, glory is to be found in tending to the details.

Last edited by Pio1980; 10-02-2013 at 05:37 PM.
Reply With Quote
  #3  
Old 10-02-2013, 08:10 AM
Tom Bavis's Avatar
Tom Bavis Tom Bavis is offline
Audiophool
 
Join Date: Jun 2002
Location: Macedon NY
Posts: 7,630
Try reversing the primary leads on the output transformer - if feedback phase is reversed, it's POSITIVE feedback, and will oscillate. If you're using the primary B+ tap, reverse the secondary - either one will do...
Reply With Quote
  #4  
Old 10-02-2013, 09:22 AM
Dandy's Avatar
Dandy Dandy is offline
AK Member
 
Join Date: Jul 2008
Location: Wiltshire, UK
Posts: 2,142
Thanks Tom. It's a while since I worked on this, and I thought I'd tried reversing those leads, but the point of asking for help is to look again! Will do.

By the way, please can someone explain to me the symbol (capacitor inside a circle with a dot) between the 330k resistor and ground at the end of the power supply in my attachment?
Reply With Quote
  #5  
Old 10-02-2013, 09:48 AM
Tom Bavis's Avatar
Tom Bavis Tom Bavis is offline
Audiophool
 
Join Date: Jun 2002
Location: Macedon NY
Posts: 7,630
Neon bulb - pilot light.
Reply With Quote
Audiokarma
  #6  
Old 10-02-2013, 04:47 PM
Dandy's Avatar
Dandy Dandy is offline
AK Member
 
Join Date: Jul 2008
Location: Wiltshire, UK
Posts: 2,142
Thanks again Tom. Wired like that to show the rising B+, I guess.
Reply With Quote
Reply


Thread Tools
Display Modes

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

Forum Jump


All times are GMT -5. The time now is 05:28 AM.



Friends of Audiokarma
We appreciate your help in keeping this site going.
Stereo Repair World
Herbies Audio Lab
Vintage Home Audio
Amps and Sound
Pro DJ Headphones - click here
Organ Donor Parts
Wargos Electronics
Parts Connexion
Sonneteer
Take Five Audio
Sound Stage Direct
Audio Salon
AudioClassics
Audience AV
AV Solutions
Simply Speakers
Tone Arm Audio
BD Enterprise
Musical Paradise
Howard Products
Classic Sound Repair
FM DX Antenna
McIntosh Cabinets
Siliconray
BOI Audio Works
Vinyl Magic Record Cleaner
Amplifiedparts
Grant Fidelity
RSL Speakers
Yesterday's Audio
Urban Antigue Radio
DeWick Repairs
Many Moons Audio
Vintage Electronics Repair
The Tube Store
Politicalchat.org
Audio Doctor
Antique Radio Classifieds
MCM Electronics
Videokarma.org
   
   
Powered by vBulletin® Version 3.8.6
Copyright ©2000 - 2014, Jelsoft Enterprises Ltd.
ęCopyright 2002-2014 AudioKarma.org, All rights reserved.