Go Back   Home > Forums > >
Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum

Does anyone have any FFT plots from real Class-AB tube amps?
Does anyone have any FFT plots from real Class-AB tube amps?
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
Thread Tools Search this Thread
Old 13th May 2010, 12:27 AM   #51
Michael Koster is offline Michael Koster  United States
diyAudio Member
Michael Koster's Avatar
Join Date: Jan 2008
Location: Eureka, CA
Originally Posted by smoking-amp View Post
Hmm, I'm not seeing any combining of feedback signals in Schades schematic thru the driver xfmr. Although the two sides of the OT should nominally be the same anyway. What seems to be the main problem to me is the disparity in gm when handing over control from one tube to the other. Schade's biasing graphs seem to address that for smoothest net gm.
Oh, so right. The feedback signals are subtracted separately from a common drive voltage. There is no feedback current in the transformer.

So if the drive signals are mirrors of each other, the main issue must be the gm bump, which can be addressed by selecting an op point for best composite gm.

I wonder if that's all that's happening here. What happens if the idle current is reduced? Is there a minimum distortion point?
  Reply With Quote
Old 13th May 2010, 02:41 AM   #52
SGregory is offline SGregory  United States
diyAudio Member
Join Date: Apr 2003
Location: Southern Tier NY
Is it safe to assume that even without local feedback (only GNFB), there would be an optimal bias setpoint for minimum distortion.

I was playing around with THD vs. Bias Current when the amp is in AB. I have only really measured THD with bias >60mA on a KT88. At 80mA I found that the THD increased slightly from 70mA. In addition, as expected, max power decreased as a result of reduced b+ (due to PT load).
Living Life Doing the Waltz in 4/4 meter.
  Reply With Quote
Old 13th May 2010, 06:09 AM   #53
teemuk is offline teemuk  Finland
diyAudio Member
Join Date: Nov 2004
Location: Suomi, Finland
In addition to turning off compression of the simulation data and all the other tips mentioned previously, you need to set the simulation's time frame such that it starts and stops saving data at the wave's zero crossing point. Otherwise, LTspice will interprete the "partial" wave as distortion.

A good reference is also plotting the FFT of the actual input, which in SPICE software should be ideal. Very often it's not.
Attached Images
File Type: gif plot.gif (2.1 KB, 49 views)
  Reply With Quote


Does anyone have any FFT plots from real Class-AB tube amps?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

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
EU AMP Ecodesign Working Plan 2009-2011 - No way for new Class A - AB - H & Tube Amps qsa Solid State 0 4th May 2010 07:47 AM
Soundstage with Class A/AB amps Samuel Jayaraj Solid State 23 9th January 2008 12:35 PM
Safe Working voltage of output Transistors in class AB amps with +/- supply rajeev luthra Solid State 3 31st July 2004 06:58 AM
Any Class AB Chip Amps ? LS Chip Amps 1 22nd July 2004 08:07 PM
Thermistors in class AB amps ? LBHajdu Pass Labs 10 24th June 2004 09:38 PM

New To Site? Need Help?

All times are GMT. The time now is 08:57 AM.

Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 15.00%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.
Copyright ©1999-2017 diyAudio