Another Counterpoint SA-7.1 question - diyAudio
Go Back   Home > Forums > Amplifiers > Tubes / Valves

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
Reply
 
Thread Tools Search this Thread
Old 21st August 2009, 06:57 AM   #1
mishak is offline mishak  United States
diyAudio Member
 
Join Date: Mar 2007
Location: Chicago
Default Another Counterpoint SA-7.1 question

Recently I got myself into tube preamp upgrading “business”. To start with promising base, I choose the Counterpoint SA-7.1 preamp, found it, bought it, and, finally, received it a few days ago. I’ve read all threads here regarding this preamp, so I have some ideas what to do.

But here is a question: After initial listening I found that my preamp has a big rollover in frequencies greater than 12 KHz in the line stage.

After studying schematics (for comparison I used schematics posted here:
Counterpoint SA-7 Schematic), I found that my SA-7.1 rev. B has two Zener diodes 1N5368 B in series, cathode to cathode, in parallel with some capacitor connected between grid and cathode in the follower stage. See attached portion of schematics in red.

So, could it be original (looks like that)? Do I need it? My guess that capacitor was added to fix “troublesome cathode-follower (it likes to whistle)” how Michael Elliott put it. Could somebody give me an advice?

Second, I want to try a little bit of negative feedback to kill this gain and improve THD.
Initial idea was something like resistor in green on schematics. Do I need it to be more sophisticated?
Any input would be highly appreciated.
Attached Images
File Type: jpg schematics_line1.jpg (83.2 KB, 283 views)
  Reply With Quote
Old 21st August 2009, 07:28 AM   #2
diyAudio Member
 
Join Date: Mar 2004
Location: Cologne
Send a message via MSN to planet IX
You can remove the two zeners and the parallel capacitor and put a simple 1N4148 in this place instead (anode to the grid and cathode to the cathode of the tube).
This protects the grid of V4 from positive voltage.
  Reply With Quote
Old 21st August 2009, 03:23 PM   #3
mishak is offline mishak  United States
diyAudio Member
 
Join Date: Mar 2007
Location: Chicago
Thanks, PlanetIX.
Isn't it the same as keep zeners and remove capacitor? My question was about capacitor itself: should I replace it with the smaller value, or I can remove it totally.
  Reply With Quote
Old 24th August 2009, 01:56 PM   #4
diyAudio Member
 
Join Date: Mar 2004
Location: Cologne
Send a message via MSN to planet IX
With the two zeners you have the forward-drop of one diode and the zener-voltage of the other diode in series. One single diode will reduce the positive grid voltage to 0,6V.

I see no reason not to get rid of the capacitor. If V4 oscillates increase the value of the gridstopper...
  Reply With Quote
Old 25th August 2009, 01:14 AM   #5
mishak is offline mishak  United States
diyAudio Member
 
Join Date: Mar 2007
Location: Chicago
I see, it make sense, thank you.

What about global negative feedback schematics? (I don't want to discuss is it good or bad, just want to try and see the difference).
I couldn't find a good working example of such shematics on the web. There are plenty for the three tubes line stage, like in ARC SP-66, but no for the two tubes case as in SA-7.
  Reply With Quote
Old 25th August 2009, 01:15 AM   #6
mishak is offline mishak  United States
diyAudio Member
 
Join Date: Mar 2007
Location: Chicago
I meant ARC-6B
  Reply With Quote
Old 8th September 2010, 07:05 PM   #7
ditubo is offline ditubo  United States
diyAudio Member
 
Join Date: Sep 2010
Greetings!

I'm a noob here and have been lurking around the forum looking for infos on modifying the SA-7.1. Any updates on this thread?
  Reply With Quote
Old 22nd October 2010, 10:42 PM   #8
ditubo is offline ditubo  United States
diyAudio Member
 
Join Date: Sep 2010
Quote:
Originally Posted by ditubo View Post
Greetings!

I'm a noob here and have been lurking around the forum looking for infos on modifying the SA-7.1. Any updates on this thread?
Bump.
  Reply With Quote

Reply


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
counterpoint 3.1 intense-tavda Tubes / Valves 18 20th September 2010 03:16 AM
Counterpoint SA-12 vsr123 Tubes / Valves 0 3rd July 2003 03:54 PM
Counterpoint nickgreek Solid State 0 22nd March 2003 08:04 AM


New To Site? Need Help?

All times are GMT. The time now is 06:54 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2