LED cathode biasing in push-pull input stage - 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 31st July 2008, 09:36 PM   #1
exeric is offline exeric  United States
diyAudio Member
 
Join Date: Mar 2008
Default led cathode biasing in push-pull input stage

I'm a newbie around here but have experience in electronics. Like most people I have significant gaps in that knowledge though. I have 3 related questions.

1. I want to create a balanced push-pull input stage so I don't have to deal with unequal output impedances from a single tube. The other reason I'm going that route is that its an easy way to create a constant current draw to that stage. Since I'm using a transformer attenuator as volume control to drive them there's no worries for a balanced grid drive. I plan on using leds to bias each 5842 cathode and a single IXSY CCS supplying both plates.

My question is this: tubes are never exactly equal in current draw or mu and its also possible for each 5842 tube to age differently. What's the best way to handle this. I'm thinking a small value pot with one leg for each cathode but it seems like it defeats the purpose of using leds. Any ideas out there?

2. An informal line stage standard output seems to be 2 V rms which equals 5.8 V P-P. Yet I keep seeing threads here where individuals are using leds that don't equate to this standard. It seems to me that 2 leds approximating 3 volts is the way to go. Will that work for the 5842 tube or is it over biasing it? It seems like its a good idea to have some margin for transient peaks right at the input of amplification.

3. I'm midway in learning pspice but haven't yet learned how to model a generic CCS that computes. I'm not sure what I'm doing wrong. Any ideas? Thanks guys in advance.
  Reply With Quote
Old 31st July 2008, 10:41 PM   #2
exeric is offline exeric  United States
diyAudio Member
 
Join Date: Mar 2008
I should add that the P-P input, (is "P-P" preferred or "differential"?), is driving a P_P output. Duh!
  Reply With Quote
Old 31st July 2008, 11:05 PM   #3
Gluca is offline Gluca  Italy
diyAudio Member
 
Gluca's Avatar
 
Join Date: Nov 2004
Location: Back to Italy
I am not sure I got that single CCS thing properly, but I'd rather use a single CCS under the cathodes of the tubes (i.e. differential pair). You will need a negative rail. Sorry for not answering to your questions.

Re
Gianluca
  Reply With Quote
Old 31st July 2008, 11:49 PM   #4
diyAudio Member
 
Join Date: Jul 2003
Location: Ann Arbor, MI
Quote:
Originally posted by Gluca
I am not sure I got that single CCS thing properly, but I'd rather use a single CCS under the cathodes of the tubes (i.e. differential pair). You will need a negative rail. Sorry for not answering to your questions.

Re
Gianluca
Gianluca,
You hit on the same thing I was thinking. The CCS works best in the cathode load.

Given equal plate loads( resistive that is ), one can even use dis-similar triodes. Say 6DN7 in place of 6SN7. If you can get the plate V matched with a biasing arrangement, they have excellent balance( quite a suprise when I first saw it ). All that took away most of my worry about tube aging and asymetrical changes in the performance.
cheers,
Douglas
__________________
the Tnuctipun will return
  Reply With Quote
Old 1st August 2008, 07:00 AM   #5
exeric is offline exeric  United States
diyAudio Member
 
Join Date: Mar 2008
It sounds like I'm seeing the beginning of a consensus for not placing the CCS on B+ side. My original reason for doing that was that, at least in single sided amplification, it has a lot of advantages over the cathode side. But maybe you guys are right that in PP that doesn't apply.

I still would like to be able to cut down on the resistance in the cathode circuit used for biasing the grid, ala LEDs. Maybe I've been thinking about this all wrong and the CCS in the cathode side is just an alternative solution to LEDs that creates a similare advantage. Is there a way to think about it or implement it that could give me a better feel for this idea. Is a CCS in the cathode just another way of reducing dynamic resistance in this situation or am I thinking about this wrong?
  Reply With Quote
Old 1st August 2008, 07:36 AM   #6
Gluca is offline Gluca  Italy
diyAudio Member
 
Gluca's Avatar
 
Join Date: Nov 2004
Location: Back to Italy
Well, CCS's can help in PP anodes too. Old WE schematics used chokes, a passive CCS as a matter of facts, after the supply and before entering the B+ connection of interstage trannies (have a look onto the Lynn's paper on www.nutshellhifi.com)

The last PP amp I built had the cathodes grounded and the tubes biased with a fixed negative voltage at the grids. I could not make a direct comparison between cathodes CCS's and grounded cathodes unfortunately. OH! ... ground has a lower impedance than LEDs :-)

Now I am following that path for all my amps and preamps even if they are SE: grid bias and CCS on anode or choke if a final stage is involved.

Ciao
Gianluca
  Reply With Quote
Old 1st August 2008, 09:53 AM   #7
SY is offline SY  United States
diyAudio Moderator
 
SY's Avatar
 
Join Date: Oct 2002
Location: Chicagoland
Blog Entries: 1
You're thinking about it wrong.

In a p-p circuit, you want the signals at the anodes to be as equal-but-opposite as possible. To achieve this, what is commonly done is to use equal plate load resistors, then the highest possible impedance at the cathode, i.e., a CCS.

If that cathode impedance is essentially infinite, a minute or two of sketching will convince you that the signals at the plate are indeed equal-but-opposite: the sum of the two tube currents must equal the cathode CCS current, so any reduction in one side MUST have an equal-but-opposite reduction in the other side, sicne there's nowhere else for the current to go (grids should not draw current here).

Now, where a plate CCS really helps you in a single-ended circuit is reduction of second harmonic. For a p-p stage, the second harmonic components cancel at the output, so you pick up much of the advantage anyway, at least as long as the plate load resistors aren't excessively small.
__________________
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 1st August 2008, 10:47 AM   #8
diyAudio Member
 
Join Date: Jul 2003
Location: Ann Arbor, MI
hey-Hey!!!,
The plate Z is modified by the cathode impedance. If you put a CCS there, it is in parallel with the other cathode. Cathode Z is ~R_p/(mu+1), so for medium mu, high gm tubes the cathode Z modification to the plate Z is fairly small.
cheers,
Douglas
__________________
the Tnuctipun will return
  Reply With Quote
Old 1st August 2008, 07:08 PM   #9
exeric is offline exeric  United States
diyAudio Member
 
Join Date: Mar 2008
Maybe I should start from the beginning how my thought process developed. As Gianluca aptly mentioned many of my ideas came from Nutshell hi-fi. The early WE P-P circuits there are modified by a CCS at the center tap of the primary on the interstage tranny. The explanation for implementing it that way was that a CCS creates a much more linear load impedance for the tubes to work against. In fact with a perfect CCS the load would be a horizontal line instead of the deeply slanted line you usually get.

Now, I don't plan on using an interstage transformer but I figured capacitor coupling would work equally well for that topology. I think the only fly in the ointment of that topology is the high dynamic resistance of the cathode resistor. But it seems to be a can of worms to try to eliminate it through led biasing.

It seems like the guy at Nutshell knew what he was talking about so it can't be as simple as requiring a CCS on the cathode circuit. There must be a middle ground here. The only real question it seems to me is if additionally a led can also be used to bias the P-P circuit. And if it can be, something not at all certain, which topology is the best for that.
  Reply With Quote
Old 2nd August 2008, 10:22 PM   #10
exeric is offline exeric  United States
diyAudio Member
 
Join Date: Mar 2008


Just thought I'd update those interested (Sy) what the disconnect in the previous discussion may be about. I finally figured out, sort of, how to put in a CCS in LTspice. So when I tried a CCS in the plate circuit of a PP capacitor coupled circuit it didn't work. But the same circuit worked fine in the equivalent transformer coupled circuit. There's a strange anomally happening where an unuseful voltage develops across the coupling capacitors if the are linked with a common CCS on one side of both caps.

I don't much care for the idea of a CCS for each side of a PP so it looks like I'll go with a CCS on the cathode and forego LEDs on the cathode. I knew there must be something that was being left out of the equation. CCS plate loads in PP don't seem much good if the circuit is cap-coupled but work great with interstage trannies.
  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
Push-Pull Cathode Bias Resistors Kashmire Tubes / Valves 5 6th January 2008 07:38 PM
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
Biasing Push Pull Triode amp into Class A G Tubes / Valves 11 4th November 2002 02:27 PM


New To Site? Need Help?

All times are GMT. The time now is 03:44 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