Push-Pull Cathode Bias Resistors - 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 4th January 2008, 09:02 PM   #1
diyAudio Member
 
Join Date: Dec 2003
Location: Midwest
Default Push-Pull Cathode Bias Resistors

Question: Why do the KT88 datasheets emphasize that two cathode resistors are essential for push-pull operation with KT88 tubes?

Why not a single shared cathode bias resistor? (bypassed or not bypassed, depending on A/AB operation)

Also, if you feel like debating bypassing the cathode bias resistor, check out the old article just posed on Tube Cad Journal (TCJ):
“Effects of the Cathode Capacitor on P-P Output Stage.”
Excellent article.
  Reply With Quote
Old 5th January 2008, 02:14 AM   #2
diyAudio Moderator Emeritus
 
ray_moth's Avatar
 
Join Date: Jan 2004
Location: Jakarta
Quote:
Question: Why do the KT88 datasheets emphasize that two cathode resistors are essential for push-pull operation with KT88 tubes?
I can only presume that they must be talking about Class AB operation, which ideally requires fixed (grid) bias but can be achieved, to a limited extent, with separate cathode bias resistors, provided that they are bypassed. If you use a single, shared cathode bypass resistor, you are pretty well stuck with Class A, whether you bypass it or not.

I didn't have time to dig for the article in Tubcad but, as I understand it, bypassing a shared cathode resistor is advisable if there is any chance (which there always is) of imperfect matching between the OP tubes. Whether the cap should be between cathodes and ground or cathodes and B+ is also an interesting topic. I believe the latter has some nickname or other - 'Shining Path', perhaps?
  Reply With Quote
Old 5th January 2008, 02:51 AM   #3
diyAudio Member
 
tubewade's Avatar
 
Join Date: Jun 2007
That is an interesting question for which I have no answer. In my datasheets the same recommendation is made for KT66, 6550, and KT90. For the most part these have commonly been run with fixed bias but there are some amplifiers that use them and use a single cathode resistor for both valves; Altec Lansing A340A, Leslie 147, and Leslie/Vibratone 22H, 122 and 145 to name a few.
  Reply With Quote
Old 5th January 2008, 03:03 AM   #4
Jeb-D. is offline Jeb-D.  United States
diyAudio Member
 
Jeb-D.'s Avatar
 
Join Date: Nov 2005
Location: SoCal
Quote:
I can only presume that they must be talking about Class AB operation, which ideally requires fixed (grid) bias but can be achieved, to a limited extent, with separate cathode bias resistors, provided that they are bypassed. If you use a single, shared cathode bypass resistor, you are pretty well stuck with Class A, whether you bypass it or not.
I'd have to second that assumption.

Related note:
If you have them Pentode wired and sharing a G2 resistor you will still have to add a G2 capacitor in class A. But you can get away without cathode bypass cap. I prefer no bypass cap on shared resistor push pull designs.
  Reply With Quote
Old 5th January 2008, 03:46 PM   #5
diyAudio Member
 
tubewade's Avatar
 
Join Date: Jun 2007
Default Re: Push-Pull Cathode Bias Resistors

Quote:
Originally posted by Kashmire
...
Also, if you feel like debating bypassing the cathode bias resistor, check out the old article just posed on Tube Cad Journal (TCJ):
“Effects of the Cathode Capacitor on P-P Output Stage.”
Excellent article.
That is a really excellent article. I found it, along with some other great reading, here...

http://www.aikenamps.com/TI_Other.htm
  Reply With Quote
Old 6th January 2008, 07:38 PM   #6
diyAudio Member
 
Join Date: Dec 2003
Location: Midwest
tubewade, good articles. Thanks.


From http://www.aikenamps.com/TI_Other.htm:
Quote:
For those of you who *still* think class AB amplifiers are "class A at lower volumes", read the difference in this next paper:
This is a nice summary paper. For the A/AB argument, the paper could have stated things a lot more clearly, but I don't think the intent of the paper was to do this.

I've pretty much talked myself into build a fixed negative bias supply, but I'm annoyed with myself for building an amp that will require potentiometer adjustments. DC servo loops would solve this problem, but that's just more circuity, making an otherwise simple amplifier into, well, a less simple amplifier.

Being from the Zen school of thought, it's hard to me to keep throwing components into an amp until I can't draw the schematic onto the back of a napkin easily. Is there a good way to have reliable auto-bias for class-AB amplifiers? It that the big hubub over the CCS threads on this forum? One CCS per valve?

Hmmmm ... maybe back to cathode resistors ... http://www.meta-gizmo.com/tri/sunschematic.html

Quote:
The Japanese ego is very different, which makes possible such Zen circuits.
It's hard for westerners to understand the Japanese school of thought, but a worthwhile exercise to go through.

K.
  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
6T9 push pull with Cathode Coupled input Brit01 Tubes / Valves 5 18th January 2009 11:58 AM
LED cathode biasing in push-pull input stage exeric Tubes / Valves 14 18th August 2008 01:35 AM
UL to triode mode conversion on cathode bias amp-different cathode resistors needed? bigwill Tubes / Valves 6 30th December 2007 12:37 AM
Separate cathode resistors in push pull output oldheathkitphil Tubes / Valves 1 23rd May 2007 04:29 PM
The cathode FeedBack in Push-Pull amp jmartins Tubes / Valves 3 13th May 2005 03:17 PM


New To Site? Need Help?

All times are GMT. The time now is 08:58 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