Difference bewteen ultralinear & triode mode? - Page 7 - 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 14th December 2012, 03:39 PM   #61
cotdt is offline cotdt  United States
diyAudio Member
 
Join Date: Oct 2005
Send a message via AIM to cotdt
I see, negative feedback does seem to add odd order harmonics, though it cancels out even more of it. It is still beneficial from all the evidence. Lower output impedance being the greatest benefit.

There is not enough gain in most tube amps to apply lots of negative feedback. Having installed a NFB switch on most of my amps, I still don't like negative feedback. Having some harmonic distortion in the amp is rather pleasing to listen to. I would only add negative feedback in a SET amp to keep the damping factor greater than 4.

One of my SS amps have less negative feedback than the vast majority of SS amps, and no global feedback, and it sounds great! Low and moderate amounts of negative feedback has worked out great for me so far.

Last edited by cotdt; 14th December 2012 at 03:42 PM.
  Reply With Quote
Old 14th December 2012, 04:17 PM   #62
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
Quote:
Originally Posted by cotdt
I see, negative feedback does seem to add odd order harmonics, though it cancels out even more of it.
It can add even orders too, although not if the basic amp has no even order distortion at all.

Quote:
Having installed a NFB switch on most of my amps, I still don't like negative feedback.
Generally not a fair test. When you add negative feedback you not only reduce distortion, you also reduce gain and widen frequency response. You may also cause peaks to occur at LF and HF extremes. NFB must be designed in, not slapped on as an afterthought or simply removed. I suspect that many people who don't like NFB have done the same as you.

It's like painting a wall, then complaining that the paint peels so deciding that you prefer unpainted walls - many paints require some surface preparation and an undercoat. You either need bare walls, or prepared and painted walls: unprepared painted or prepared unpainted are both inferior.

Quote:
Having some harmonic distortion in the amp is rather pleasing to listen to.
Yes, some seem to like it. Few admit it.
  Reply With Quote
Old 14th December 2012, 04:54 PM   #63
diyAudio Member
 
artosalo's Avatar
 
Join Date: Feb 2010
Quote:
Originally Posted by DF96 View Post
....NFB will create higher-order terms...
Can you give an practical example with vacuum tubes where these higher-order terms can be observed and measured ?
  Reply With Quote
Old 14th December 2012, 05:01 PM   #64
Pano is offline Pano  United States
diyAudio Moderator
 
Pano's Avatar
 
Join Date: Oct 2004
Location: Milliways
Blog Entries: 4
Quote:
Originally Posted by DF96 View Post
Yes, some seem to like it. Few admit it.
It's more complicated than that, and most of us know it.
  Reply With Quote
Old 14th December 2012, 06:21 PM   #65
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
Quote:
Originally Posted by artosalo
Can you give an practical example with vacuum tubes where these higher-order terms can be observed and measured ?
A variable-mu valve (remote cutoff) with a small cathode degeneration resistor. The natural 3rd order from the valve, and the re-entrant 3rd order from the feedback happen to have opposite phase so can cancel. The resistor should be about 1/3gm (or 1/2gm - I can't remember the factor).

You can see a similar effect, but perhaps for a different reason, in the intermodulation curves for a variable-mu valve (see Philips datasheets e.g. EF85?) without the resistor. There is a sharp peak in the allowable signal voltage for 1% 3rd order distortion, at about the usual bias point - which is why this is the usual bias point. I suspect that cathode coating resistance plays the role of the degeneration resistor.
  Reply With Quote
Old 14th December 2012, 07:14 PM   #66
TheGimp is offline TheGimp  United States
diyAudio Member
 
TheGimp's Avatar
 
Join Date: Aug 2009
Location: Johnson City, TN
With a good sound card and a program like ARTA or AutioTester, one can watch the harmonics while varying NFB.
  Reply With Quote
Old 19th December 2012, 05:05 PM   #67
diyAudio Member
 
artosalo's Avatar
 
Join Date: Feb 2010
Quote:
Originally Posted by DF96 View Post
Whether these extra terms are noticeable or not is a separate issue, but they are definitely there - unless somehow tubes manage to avoid using conventional mathematics.
I wanted this time to experiment with mathematics i.e. with LT Spice if I can find recognizable higher order harmonics to rise when small amount of NFB was used.

I used a very linear CCS loaded triode stage with no NFB and 6 dB voltage NFB. In both cases the output level was equal as well as the total load impedance.

Without NFB the harmonics from 7th to 9th were more than 100 dB below reference signal (1 kHz). This level represents THD below 0.001 %.

The results I got were surprising.
6 dB NFB reduced harmonics from 2nd to 6th between 5.4 dB to 7.4, which is expectable, but the reduction of 7th to 9th was much higher ( 12 db to 32 dB ), which was quite unexpected and fully against that has been stated here.

Click the image to open in full size.


Click the image to open in full size.

My conclusion now after two analysis, one with actual amplifier and one with "mathematics", is that it is incorrect to say that low NFB should be avoided in tube amplifiers because this creates high order harmonics.
If it does this, the level of such harmonics are less than negligible.
  Reply With Quote
Old 19th December 2012, 05:19 PM   #68
diyAudio Member
 
artosalo's Avatar
 
Join Date: Feb 2010
The circuits I used are as follows:

Click the image to open in full size.



Click the image to open in full size.
  Reply With Quote
Old 19th December 2012, 05:28 PM   #69
diyAudio Member
 
Shoog's Avatar
 
Join Date: Aug 2002
Location: Eire
Quote:
Originally Posted by Pano View Post
It's more complicated than that, and most of us know it.
Some people never will though.

Shoog
  Reply With Quote
Old 19th December 2012, 08:40 PM   #70
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
Quote:
Originally Posted by artosalo
My conclusion now after two analysis, one with actual amplifier and one with "mathematics", is that it is incorrect to say that low NFB should be avoided in tube amplifiers because this creates high order harmonics.
In your simulation you may have been seeing cancellation. Unexpected with that circuit, but possible.

Whatever your measurements, simulations or whatever say, it remains a fact that re-entrant distortion exists. The maths is clear. The fact that high orders changed by any amount not equal to the feedback ratio confirms this. Whether re-entrant distortion is significant is a separate issue.

I'm not sure whether you disbelieve the maths of feedback, or just misunderstand what I am saying. Maybe I have not been expressing myself clearly enough. I said people who worry about re-entrant distortion should avoid low amounts of NFB. I personally don't worry about it (although I admit I used to). Provided the forward path is sufficiently linear then re-entrant distortion, although always present with NFB, will do no harm.
  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
Resistors for ultralinear-to-triode conversion spendorspain Tubes / Valves 6 28th January 2007 12:47 PM
Using a spdt switch to select between triode mode and pentode mode on a guitar amp. G Tubes / Valves 19 15th March 2006 09:15 AM
Difference between Bridge & Parallel mode john_lenfr Chip Amps 13 4th July 2005 01:27 PM
Switching from ultralinear to triode Klimon Tubes / Valves 3 26th May 2005 11:50 AM
EF86 Triode Mode Family_Dog Tubes / Valves 4 21st March 2005 04:41 PM


New To Site? Need Help?

All times are GMT. The time now is 03:45 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