running a preamp tube in class B! - diyAudio
Go Back   Home > Forums > Live Sound > Instruments and Amps
Home Forums Rules Articles diyAudio Store Gallery Wiki Blogs Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Instruments and Amps Everything that makes music, Especially including instrument 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 20th April 2013, 08:53 PM   #1
diyAudio Member
Join Date: Jun 2012
Default running a preamp tube in class B!

I have a tremolo circuit here using a 6au6 with no bias at all. There's no cathode resistor and no other source of bias in sight. After puzzling over it, I've come to the the conclusion that it was intentionally biased into class B in order to seriously deform the waveform.

It sounds really, really good. I'm looking at basically wholesale stealing this design to go in an very low noise, low gain amp I'm designing right now, but not only is the pentode noisier than need be, but it would be that much simpler as a triode. So I'm looking at revamping the circuit with a 12ax7 instead.

BTW all I know about using tubes for tremolo oscillators I learned from the Valve Wizard site.

Is this just like a power tube load line, where the class B load line is 1/2 the resistance of the class A line?

This 6au6 has a 68k anode resistor right now, which would provide a gain of around 100 on the usual class A loadline, but I'm thinking it's really operating in class B and the gain is that of a 34k class A loadline.

Also, what kind of voltage swing does an oscillator like this have?
Pretty much the whole way? Does it go into grid current limiting or cutoff at all? And is there any way to control all that?

Sorry I'm unable to post any schematics anytime soon.
Thanks all,
  Reply With Quote
Old 20th April 2013, 09:41 PM   #2
DF96 is offline DF96  England
diyAudio Member
Join Date: May 2007
What is the value of the 6AU6 grid leak resistor? If particularly large then it might be using grid current bias. On the other hand, oscillators often run in Class C.

Note that a valve with zero bias is not Class B, but Class A with grid current. You would probably get distortion but for a different reason.

Without the circuit we are just guessing.
  Reply With Quote
Old 20th April 2013, 09:51 PM   #3
diyAudio Member
Join Date: Jun 2012
The grid leak resistor is fairly large- it's 2.2M.

Again, sorry I can't post a schematic right now, but I may be able to at some point.

The screen is at 80V, the B+ is about 250v. Suppressor tied to ground.

"Note that a valve with zero bias is not Class B, but Class A with grid current."

So does this mean that the usual loadline applies?

Thanks again!
  Reply With Quote


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
Running a tube preamp into a solid state preamp? Pckid212 Solid State 5 6th January 2013 10:12 PM
Class A BJT buffer for tube preamp kacernator Solid State 6 22nd April 2012 12:47 PM
Class D amplifier with tube preamp on input any ideas jazzman06 Class D 9 4th July 2009 11:44 PM
Balanced tube preamp for class D power amps. jane Tubes / Valves 5 13th December 2006 07:41 AM
A Class Preamp schematic and tube amps question MtUmut Solid State 0 10th March 2004 09:30 PM

New To Site? Need Help?

All times are GMT. The time now is 11:50 PM.

vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2017 DragonByte Technologies Ltd.
Copyright 1999-2017 diyAudio

Content Relevant URLs by vBSEO 3.3.2