Configuring an AB class amp to work in pure A class - 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 23rd February 2010, 01:05 PM   #1
jrme is offline jrme  France
diyAudio Member
 
Join Date: Jul 2007
Default Configuring an AB class amp to work in pure A class

Hi,

I have always been using solid state amps. In my experiences, I have reached the conclusion that pure class A sounds better than the others. Then, I am currently using pure class A amps since a long time.

Now I have bought a vintage tube amp, Pioneer SM-83, which is working in AB class. I am wondering if it is technically possible to make modifications on it, in order to bring it to work in pure class A only.

Do you have any experiences in transforming an amp in such a way?

In advance thanks.

Regards.

jrme
  Reply With Quote
Old 23rd February 2010, 02:46 PM   #2
Arnulf is offline Arnulf  Europe
diyAudio Member
 
Join Date: Feb 2009
Of course it is possible - adjust the load (change the output transformer) and adjust the bias point so that both tubes always operate in conduction (neither ever goes into cut-off).

Existing transformers might work, depending on how far into class (A)B the output tubes are biased.
__________________
mod verb, transitive /mod/ to state that one is utterly clueless about the operation of device to be "modded" and into "fixing" things that are not broken; "My new amplifier sounds great so I want to mod it."
  Reply With Quote
Old 23rd February 2010, 03:31 PM   #3
diyAudio Member
 
richwalters's Avatar
 
Join Date: Sep 2003
Location: Alps:Tube amp designs over 150W, SMPS guru.
In my 1950's service book, the cue to class A tube amps was to set the quiescent current so the anode +screen is at the max quoted dissipation figure (often red hot anodes). In reality this is somewhat true as the anode heating power is reduced when driven hard. However, it is also the quickest way to shorten tube life. Others may have other ideas.
Some of the cheap mass produced early radiograms did operate at very high quiescents = high dissipation and often close to being unstable.
richy
  Reply With Quote
Old 23rd February 2010, 04:04 PM   #4
DougL is offline DougL  United States
diyAudio Member
 
Join Date: Feb 2004
Location: Wheaton IL.
Blog Entries: 30
In general, you need to lower the Voltage and increase the current in order to change from class AB to A, and not destroy the output tubes. Also changing the load can help.

It may be enough to run the 8 ohm speakers on the 4 ohm tap, and modify the bias and power supply.

I would recommend the TubeCad PP calculator to give you a simulation of your output before you make changes.

Doug
__________________
Scienta sine ars nihil est - Science without Art is nothing. (Implies the converse as well)
Mater tua criceta fuit, et pater tuo redoluit bacarum sambucus
  Reply With Quote
Old 23rd February 2010, 05:19 PM   #5
jrme is offline jrme  France
diyAudio Member
 
Join Date: Jul 2007
Great! Thank you very much for your answers, I will try the ways you explain.

Regards.

Josť
  Reply With Quote
Old 23rd February 2010, 06:31 PM   #6
m6tt is offline m6tt  United States
diyAudio Member
 
Join Date: Aug 2007
Another way of thinking of it is that you need to put the idle current at or slightly above (toward current) the center of the loadline, and the waveform cannot cause cutoff on negative cycles. Equal headroom up and down. Unfortunately, this is point is usually very close to the max dissipation line. This makes the loadline critical, since the idle point needs to be inside the max dissipation line, requiring a lower load and higher current/less B+.
  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
floating supply class AB will it work? is it any good? ttako Solid State 9 14th March 2010 11:48 PM
Can a Class AB PP amp be said to be operating in Class A at low signal levels? ray_moth Tubes / Valves 19 23rd January 2009 07:52 PM
Pure class-A or class AB? neazoi Tubes / Valves 5 12th May 2008 07:51 AM
Help! 50w Class A or 150w Class AB debate Toobstheway Solid State 12 17th September 2007 07:05 PM
Class A and Class AB - How Does This Work? batee Pass Labs 18 28th January 2007 11:32 AM


New To Site? Need Help?

All times are GMT. The time now is 05:33 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