12AX7 gain stage output impedance - diyAudio
Go Back   Home > Forums > Amplifiers > Tubes / Valves
Home Forums Rules Articles diyAudio Store Gallery Wiki Blogs 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

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 3rd February 2008, 02:25 PM   #1
Dave Z is offline Dave Z  United States
diyAudio Member
Join Date: Dec 2007
Default 12AX7 gain stage output impedance

I'm used to dealing with op-amps in a solid state world, so how do I calculate the output impedance of a single gain stage using a 12AX7 so that I have a reference point I am familiar with? And for that matter what about the output impedance of a cathode follower stage?

  Reply With Quote
Old 3rd February 2008, 02:32 PM   #2
EC8010 is offline EC8010  United Kingdom
diyAudio Moderator Emeritus
EC8010's Avatar
Join Date: Jan 2003
Location: Near London. UK
The quick answer to "Output resistance of a 12AX7?" is, "More than you'd ever believe!" Typically, 75k. Sometimes more. To find the output resistance properly, we'd need a diagram.

As for a cathode follower: (ra + RL)/(mu + 1). Assuming the supply impedance seen at the anode is zero, that approximates to 1/gm if mu is high.
The loudspeaker: The only commercial Hi-Fi item where a disproportionate part of the budget isn't spent on the box. And the one where it would make a difference...
  Reply With Quote
Old 3rd February 2008, 05:20 PM   #3
Dave Z is offline Dave Z  United States
diyAudio Member
Join Date: Dec 2007
OK being new to this, I'm assuming that for the cathode follower ra is the anode resistance, RL is load resistance, but mu?

Attached Images
File Type: jpg 12ax7 gain stage.jpg (10.7 KB, 283 views)
  Reply With Quote
Old 3rd February 2008, 05:29 PM   #4
SY is offline SY  United States
On Hiatus
SY's Avatar
Join Date: Oct 2002
Location: Chicagoland
Blog Entries: 2
Mu is amplification factor, i.e., the change in plate-cathode voltage divided by change in grid-cathode voltage at constant current.

The circuit you attached is a common cathode voltage amplifier. Its source impedance will be 100k in parallel with the plate resistance of the tube (since the cathode is at AC ground). The latter is something like 60-80k, as EC8010 says.

My relentless plug: get a copy of "Valve Amplifiers" by Morgan Jones. Pictures, equations, thorough explanation of terms with lots of examples.
"You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is."
  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
output stage with high impedance alex0 Solid State 54 1st November 2011 05:40 AM
Dead 12ax7 gain stage - Help... mouldiwarp Tubes / Valves 13 22nd March 2009 10:25 PM
3C24 in a SE output stage - output transformer impedance recommendation !!! aldovan Tubes / Valves 15 6th September 2008 10:18 AM
How to calculate gain and output impedance? RogerGustavsson Solid State 1 7th June 2008 05:09 PM
Output stage with gain rtarbell Solid State 18 27th October 2006 07:15 PM

New To Site? Need Help?

All times are GMT. The time now is 08:44 AM.

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