Strange feedback method - 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 12th April 2008, 09:19 AM   #1
diyAudio Member
 
Join Date: Mar 2008
Default Strange feedback method

I have been restoring some old radios (as an aside, what are the best forums on the net for radio restoration discussion?) and found a mantel radio that used a feedback method for the output, that I've not seen before.

The output is a 6M5 pentode, single ended, into a typical output transformer and speaker. They have provided feedback from the speaker to the cathode of the 6M5 by putting the speaker (in parallel with the transformer output winding of course) in series with the a.f. bypass capacitor. So, the cathode circuit of the 6M5 is a resistor to ground, and in parallel, the series combination of the bypass cap then the speaker//transformer_output (with one side of the speaker grounded).

I wonder what the merits of doing this might be? Has it any place in a hifi amp?
  Reply With Quote
Old 12th April 2008, 12:20 PM   #2
diyAudio Member
 
Join Date: Mar 2003
Location: Zürich
This sounds like 'cathode feedback'. A search will turn up some threads: member Tubelab (George) has done some tests with it. Also Morgan Jones covers it in the design of the 'scrapbox challenge, described in Valve amplifiers, third edition.
__________________
my surname is indeed 'de Best': neither misspelling nor snobbism! Ask SY!
  Reply With Quote
Old 12th April 2008, 01:13 PM   #3
diyAudio Moderator Emeritus
 
ray_moth's Avatar
 
Join Date: Jan 2004
Location: Jakarta
It allows the AF from the cathode to go through the OPT secondary, while the DC goes through the bias resistor straight to ground.
  Reply With Quote
Old 12th April 2008, 04:41 PM   #4
diyAudio Member
 
tubelab.com's Avatar
 
Join Date: Jul 2005
Location: West Virginia panhandle
Quote:
I wonder what the merits of doing this might be? Has it any place in a hifi amp?
This is one of the methods of accomplishing cathode feedback. I use this exact circuit in the SimpleSE HiFi amp. It is a type of negative feedback applied to the output stage only. In some cases especially with undersized OPT's it will really wake up the bass response without many of the sound killing attributes of global negative feedback.
__________________
Too much power is almost enough! Turn it up till it explodes - then back up just a little.
  Reply With Quote
Old 12th April 2008, 10:38 PM   #5
diyAudio Member
 
Join Date: Mar 2008
Quote:
This is one of the methods of accomplishing cathode feedback. I use this exact circuit in the SimpleSE HiFi amp. It is a type of negative feedback applied to the output stage only. In some cases especially with undersized OPT's it will really wake up the bass response without many of the sound killing attributes of global negative feedback.
I just looked up the circuit on your web page. Yes, that is that same arrangement. I like it - easy to implement. In fact, I have some SE amps that I have built over the years, one very similar to your SimpleSE amp, that might benefit from this. Thanks
  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
Another feedback compensation method roender Solid State 4 24th February 2013 05:40 PM
New capacitive motional feedback method Nuutti Multi-Way 31 14th March 2006 03:04 AM
stupid question and strange testing method ,da pinch test fortuosity Solid State 11 7th February 2006 04:35 AM
Unnamed feedback method explored. Mr Evil Solid State 30 23rd November 2004 04:03 PM


New To Site? Need Help?

All times are GMT. The time now is 09:50 PM.


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