Cathode follower question - 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 30th June 2010, 08:35 PM   #1
rongon is offline rongon  United States
diyAudio Member
 
rongon's Avatar
 
Join Date: Jul 2009
Location: Across the river from Rip's big old tree...
Default Cathode follower question

Hi again...

Here are some squiggles I drew:

Click the image to open in full size.

I was thinking that if a CCS was a good thing as the load for a cathode follower, then a choke should be too.

I know the choke pictured above doesn't have much inductance (22H). Is that still a problem considering the very low output Z of a CF?

--
I know a choke isn't as good a load as a CCS, and will have bandwidth limitations a CCS might not, but my question is meant more for my learning/understanding than for building the best possible circuit.
  Reply With Quote
Old 30th June 2010, 10:05 PM   #2
diyAudio Member
 
Miles Prower's Avatar
 
Join Date: May 2005
Location: USA
Blog Entries: 6
Quote:
Originally Posted by rongon View Post
I was thinking that if a CCS was a good thing as the load for a cathode follower, then a choke should be too.

I know the choke pictured above doesn't have much inductance (22H). Is that still a problem considering the very low output Z of a CF?
For any cathode follower, the larger the tail load, the better. 22H corresponds to: Xl= 2K77 at f= 20Hz. You'd ideally be best off with an r(k)= 276R, which would mean a g(m)= 3.62mA/V, or better. This isn't so difficult to pull off, and so the choke load will work just fine. (Assuming it's a decent choke designed for the purpose and not something like a PS ripple choke.)

Quote:
I know a choke isn't as good a load as a CCS, and will have bandwidth limitations a CCS might not, but my question is meant more for my learning/understanding than for building the best possible circuit.
Asking is the first step to learning.
__________________
There are no foxes in atheistholes
www.dolphin-hsl.com
  Reply With Quote
Old 30th June 2010, 10:36 PM   #3
R.I.P.
 
Join Date: Dec 2001
Location: Schaffhausen Switzerland
Your theory is correct, a choke can replace a CCS - but only under certain conditions. But neither circuit is much good as drawn, as both will very easily overload with any sort of signal level. The cathode of a CF should sit well above gnd, ideally around half the B+ rail to allow maximum voltage swing before any clipping, up or down. This means the grid of the CF needs to be elevated to the same voltage, and the cathode impedance adjusted to suit. Note that a choke will not work with near half the B+ across it...something will smoke.

Regards, Allen
  Reply With Quote
Old 30th June 2010, 11:03 PM   #4
SY is offline SY  United States
diyAudio Moderator
 
SY's Avatar
 
Join Date: Oct 2002
Location: Chicagoland
Blog Entries: 1
Thanks for sitting in for me, Allen!
__________________
You might be screaming "No, no, no" and all they hear is "Who wants cake?" Let me tell you something: They all do. They all want cake.- Wilford Brimley
  Reply With Quote
Old 30th June 2010, 11:18 PM   #5
diyAudio Member
 
Wavebourn's Avatar
 
Join Date: Sep 2006
Location: Pleasant Hill, CA
Send a message via Skype™ to Wavebourn
Quote:
Originally Posted by Allen Wright View Post
Your theory is correct, a choke can replace a CCS - but only under certain conditions. But neither circuit is much good as drawn, as both will very easily overload with any sort of signal level. The cathode of a CF should sit well above gnd, ideally around half the B+ rail to allow maximum voltage swing before any clipping, up or down. This means the grid of the CF needs to be elevated to the same voltage, and the cathode impedance adjusted to suit. Note that a choke will not work with near half the B+ across it...something will smoke.
A choke does not need to. It may sit at zero (ideally) DC, and swing way below zero.

In case of CCS load it should sit at (B+ - Vs)/2
where Vs is a saturation voltage on the max current.
In case of resistor load it should sit at (B+ - Vs)*3/4
__________________
The Devil is not so terrible as his math model is!
  Reply With Quote
Old 30th June 2010, 11:34 PM   #6
Jeb-D. is offline Jeb-D.  United States
diyAudio Member
 
Jeb-D.'s Avatar
 
Join Date: Nov 2005
Location: SoCal
Quote:
A choke does not need to. It may sit at zero (ideally) DC, and swing way below zero.


I'm glad somebody is thinking outside of the sand box.

Last edited by Jeb-D.; 30th June 2010 at 11:39 PM.
  Reply With Quote
Old 30th June 2010, 11:40 PM   #7
R.I.P.
 
Join Date: Dec 2001
Location: Schaffhausen Switzerland
Quote:
Originally Posted by Wavebourn View Post
A choke does not need to. It may sit at zero (ideally) DC, and swing way below zero.

In case of CCS load it should sit at (B+ - Vs)/2
where Vs is a saturation voltage on the max current.
In case of resistor load it should sit at (B+ - Vs)*3/4
1/ This may be all correct, but it's really splitting hairs. The OP had some basic questions, he doesn't need or want a learned maths lession.

2/ The schema the OP drew shows a choke in a zero biased triode CF. Assuming the choke has enough DCR to act as a suitable cathode bias R, it will still have VERY limited output swing, and make massive 2nd harmonic distortion before clipping.

Regards, Allen
  Reply With Quote
Old 30th June 2010, 11:56 PM   #8
diyAudio Member
 
Wavebourn's Avatar
 
Join Date: Sep 2006
Location: Pleasant Hill, CA
Send a message via Skype™ to Wavebourn
Quote:
Originally Posted by Allen Wright View Post
1/ This may be all correct, but it's really splitting hairs. The OP had some basic questions, he doesn't need or want a learned maths lession.

2/ The schema the OP drew shows a choke in a zero biased triode CF. Assuming the choke has enough DCR to act as a suitable cathode bias R, it will still have VERY limited output swing, and make massive 2nd harmonic distortion before clipping.
Unfortunately without some math your "very" and "massive" are undefined.
Triode CF is not ZERO biased. It is biased up to the current needed for a maximum of a negative voltage swing on a given output load. Second harmonic distortions will be way below what you may get from an identical common cathode stage with choke loaded anode.

Edit: you may search for Altec 1570 schemo.
__________________
The Devil is not so terrible as his math model is!

Last edited by Wavebourn; 30th June 2010 at 11:58 PM.
  Reply With Quote
Old 1st July 2010, 12:04 AM   #9
diyAudio Member
 
Wavebourn's Avatar
 
Join Date: Sep 2006
Location: Pleasant Hill, CA
Send a message via Skype™ to Wavebourn
Quote:
Originally Posted by Jeb-D. View Post


I'm glad somebody is thinking outside of the sand box.
Thank you. I'm not so old to forget physics' basics I learned 40 years ago.
__________________
The Devil is not so terrible as his math model is!
  Reply With Quote
Old 1st July 2010, 12:19 AM   #10
rongon is offline rongon  United States
diyAudio Member
 
rongon's Avatar
 
Join Date: Jul 2009
Location: Across the river from Rip's big old tree...
A little controversy!

Quote:
2/ The schema the OP drew shows a choke in a zero biased triode CF. Assuming the choke has enough DCR to act as a suitable cathode bias R, it will still have VERY limited output swing, and make massive 2nd harmonic distortion before clipping.
With a DCR of about 1k ohms, that simple resistance with 8mA through it should raise the cathode to +8V, right? That should give us the necessary grid bias, right? Not enough of a load, but then you add...

...The inductance of the choke, which will resist changes in current, creating an AC load that causes a signal voltage swing across the resistance... I think.

Being a buffer with a grid bias of +8V, wouldn't a stage like this CF be used to match a previous high-Z gain stage (amplifying a 500mV signal to let's say 2Vrms) to a subsequent stage requiring a lower Z? So an 8V grid bias would be plenty in that application, right?

Quote:
In case of CCS load it should sit at (B+ - Vs)/2
where Vs is a saturation voltage on the max current.
In case of resistor load it should sit at (B+ - Vs)*3/4
That little bit of math I can do. Only thing is, what defines Vs? In the CF with choke in the cathode, the bias is +8V, is Vs = 8V?

--

--
  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
Cathode follower question woody Tubes / Valves 6 10th October 2005 02:10 AM
cathode follower vlljpior Tubes / Valves 6 22nd March 2005 07:50 PM
Question about direct coupling a anode follower into a cathode follower. G Tubes / Valves 45 29th July 2004 06:47 PM
cathode follower ackcheng Tubes / Valves 2 6th July 2004 03:12 PM


New To Site? Need Help?

All times are GMT. The time now is 06:44 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