Primary current feedback - any good? Help! - Page 3 - 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 28th February 2012, 08:27 PM   #21
diyAudio Member
 
Wavebourn's Avatar
 
Join Date: Sep 2006
Location: Pleasant Hill, CA
Send a message via Skype™ to Wavebourn
Quote:
Originally Posted by Globulator View Post
Wavebourn, is the current feedback from the 200R pot at the right/bottom pot (Rk2 perhaps but it's a bit fuzzy)?
I'm guessing the voltage nfb is from Rf1 (60k)..
Right.
Zobel in parallel with primary is very essential also.

Quote:
I.e. as the 6V6 conducts it pulls the centre of the 200R pot upward (current sense) and the anode pulls downward (voltage of anode vs centre point of 200R pot).
Right.

Quote:
It's interesting because at no time does the voltage feedback actually reference the voltage of the primary, just of (B+ - primary voltage - a bit of Rk2) which means PSU ripples etc will not get corrected?
No, ripples on speaker can't be corrected by such feedback, since it decreases effective Rp of the tube.

Quote:
I wonder if there would be a benefit to drive the OPT with a cathode follower and ground the centre tap so the cathode feeds back the actual primary voltage? What do you think - has anyone done this?
Do you mean split-load transformer, or negative feedback by voltage from transformer to cathode?
Yes, it had been done many times.
__________________
"Our youth [...] have bad manners, contempt for authority; they show disrespect for their elders, contradict their parents, [...] and tyrannize their teachers.” -- Plato, 447-367 BCE
  Reply With Quote
Old 28th February 2012, 08:47 PM   #22
diyAudio Member
 
the_manta's Avatar
 
Join Date: Nov 2006
Location: Munich, Bavaria
with a cathode follower output stage you simply shift your problems to the driver stage. Furthermore it's kinda waste of tubes. Simply use a Source follower and you don't need a tube, OPT and huge amount of signal voltage from the driver.

Quote:
I wonder if there would be a benefit to drive the OPT with a cathode follower and ground the centre tap so the cathode feeds back the actual primary voltage? What do you think - has anyone done this?
I don't exactly get what you mean. Can make a drawing or explain more ?

BTW, Wavebourn posted a very nice EL84 amplifier circuit some time ago which uses neg. voltage- and pos. current feedback:

Click the image to open in full size.

BTW-2, I attached a copy of a triode (contin. line) and with pos current feedback (dashed). It is quite interesting what the feedback does here. It's from the book 'Rothe, Kleen - Elektronenröhren als Anfangsstufen-Verstärker'
Attached Images
File Type: jpg pos_feed.jpg (68.6 KB, 61 views)
__________________
Терпенье и труд все перетрут
  Reply With Quote
Old 28th February 2012, 09:20 PM   #23
diyAudio Member
 
Globulator's Avatar
 
Join Date: Aug 2004
Location: Cambridge, England.
Thanks for the info guys, yes my cathode follower musing was this:

Click the image to open in full size.
http://www.tubecad.com/2005/June/blog0048.htm

Which gives the low impedance drive and proper feedback path (the primary is grounded), but the demands on the driver and heater supplies rules it out.

The positive current feedback is an excellent idea though - because at least that is pure - do I need a zobel with that too?

Would a zobel improve 'normal' amps too?

Then I'll want some kind of aikido feedback of the PSU, I know push-pull helps that but I can't help thinking that some of the time one tube may not be conducting - at which point the PSU noise is again in the frame.

So I guess I'm now looking at 1) positive current feedback (easy to do via cathode resistors), 2) negative voltage feedback via the anode + cap (feeds back a version the PSU B+ minus the primary voltage), and then a term of the PSU voltage (a la aikido) that will add in and make sure the actual feedback is equal to the primary voltage, and cancel the hum and audio variations.

The aim is to create a push-pull amp (with GU50 pentode mode with 500V plate) that presents a very low impedance to the primary, and to leave the secondary alone to do its own thing with the speaker (no GNFB).

Maybe Schade feedback combined with positive current feedback and PSU noise cancellation injection is the way forward here.

Last edited by Globulator; 28th February 2012 at 09:22 PM.
  Reply With Quote
Old 28th February 2012, 11:05 PM   #24
diyAudio Member
 
Wavebourn's Avatar
 
Join Date: Sep 2006
Location: Pleasant Hill, CA
Send a message via Skype™ to Wavebourn
I would suggest to deal with ripples at their lair; in PS. I prefer sand assisted tube amps, it is the way to go when nice sand is available, as opposed to 50 years ago when it was only in dreams of home audio designers. One MOSFET and string of Zeners, and it is hollow like in the tank.
__________________
"Our youth [...] have bad manners, contempt for authority; they show disrespect for their elders, contradict their parents, [...] and tyrannize their teachers.” -- Plato, 447-367 BCE
  Reply With Quote
Old 29th February 2012, 07:39 PM   #25
diyAudio Member
 
Globulator's Avatar
 
Join Date: Aug 2004
Location: Cambridge, England.
Quote:
Originally Posted by Wavebourn View Post
I would suggest to deal with ripples at their lair; in PS. I prefer sand assisted tube amps, it is the way to go when nice sand is available, as opposed to 50 years ago when it was only in dreams of home audio designers. One MOSFET and string of Zeners, and it is hollow like in the tank.
It's a good idea, especially for stability. Sometimes I like to think of an amplifier as a voltage controlled power supply, as we have the power source, the output tube and the means to modulate it. I.e. to move some of the regulation function of the PSU into the amp itself.

One day I'll get enough 6s (24V) Lipo batteries (and an easy way to charge them without burning my house down) to make a battery powered set - that would be an interesting listen
  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
Fuses, primary, rails, current limiters and channels? rgrayton Chip Amps 4 5th August 2005 04:15 PM


New To Site? Need Help?

All times are GMT. The time now is 04:33 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