Cathode follower driver - 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 29th September 2006, 03:10 PM   #1
diyAudio Member
 
croccodillo's Avatar
 
Join Date: Mar 2005
Location: Bergamo
Default Cathode follower driver

Hi guys,

I just wrote down this schematic.
It is a three stage integrated amplifier:
- 1st stage (12AX7) is a CCS loaded SE driver, should be very linear.
- 2nd stage (6N6P) is a cathode follower
- 3rd stage (KT88) is a parafeed output stage, loaded by a CCS (actually a KT88 pentode CCS).

I know first stage can work, already tried it in the past; I have also built the third stage (parafeed output stage, at the moment on my desk under test and development), and drove it with a 1:10 transformer, fed by my PC sound card.

The new stage is the cathode follower in the middle, included to add some juice to the KT88 driver.

The feedback is optional, at the moment there's no feedback at all in the prototype.

My questions:

1. Is the second stage correctly developed? Would it work well?

2. How can I determine the values for R4 and R5? I would like to fix the idle operating point of 6N6P tube to around 120V@20mA.

3. What happen if I substitute the R4 resistor with a CCS@20mA?

4. The alternative to this topology is to use the 6N6P tube as a plate follower, using a CCS to load it: would this topology work worse or better than cathode follower?

5. 12AX7 and 6N6P have two triode each, so I could use one of them for each channel (I'm building a stereo amplifier, of course): is there any problem using the same valve for both stereo channels?

Thanks in advance.

Ciao,
Giovanni Albergoni
__________________
In Nomine Libertatis Vincula Edificamus,
In Nomine Veritatis Mendacia Efferimus.
  Reply With Quote
Old 29th September 2006, 03:49 PM   #2
Gluca is offline Gluca  Italy
diyAudio Member
 
Gluca's Avatar
 
Join Date: Nov 2004
Location: Back to Italy
Best thing to do with that cathode follower is to direct couple it to the following grid (ie get rid of that coupling capacitor): this will require a complete redrawn of the amp as you will probably need a negative supply.

Ciao
Gianluca
  Reply With Quote
Old 29th September 2006, 04:31 PM   #3
diyAudio Member
 
Join Date: May 2005
Location: California
I second Gluca's opinion. Direct coupling to the power tube by driver as a cathode follower. The Italians have used this topology a lot; ie the Geloso amps. You will need to have a split supply for the driver, so the driver's cathode will feed the proper negative bias voltage to the output's grid. This makes for a powerful driver that is extremely fast; this will sound good.
Using a cathode follower direct coupled to the power tube has many advantages but is harder to implement.
Capacitors are generally bad but a necessary evil and if you can avoid using any, then that is the way to go.
Daniel
  Reply With Quote
Old 29th September 2006, 04:44 PM   #4
diyAudio Member
 
Wavebourn's Avatar
 
Join Date: Sep 2006
Location: Pleasant Hill, CA
Send a message via Skype™ to Wavebourn
You don't need R4.
Value of R5 depends on a maximum current you need to drive your output monster from one side, from the other side it is limited by max current 6N6 can sustain (power dissipation) without significant distortions and plate melting.
And I would use some high voltage power PNP instead of pentode for CCS...

Gentlemen who suggest you direct cupling forgot about the 1'st triode that needs some positive voltage on the plate to work properly.
  Reply With Quote
Old 29th September 2006, 11:27 PM   #5
diyAudio Moderator Emeritus
 
ray_moth's Avatar
 
Join Date: Jan 2004
Location: Jakarta
Quote:
Gentlemen who suggest you direct cupling forgot about the 1'st triode that needs some positive voltage on the plate to work properly.
Well spotted! Two possible solutions? (a) move the coupling cap so it lies between the first and second stages; or (b) use the negative line instead of ground at the first stage.
  Reply With Quote
Old 30th September 2006, 12:06 AM   #6
diyAudio Member
 
Miles Prower's Avatar
 
Join Date: May 2005
Location: USA
Blog Entries: 6
Default Re: Cathode follower driver

Quote:
Originally posted by croccodillo

1. Is the second stage correctly developed? Would it work well?

2. How can I determine the values for R4 and R5? I would like to fix the idle operating point of 6N6P tube to around 120V@20mA.

3. What happen if I substitute the R4 resistor with a CCS@20mA?

4. The alternative to this topology is to use the 6N6P tube as a plate follower, using a CCS to load it: would this topology work worse or better than cathode follower?

5. 12AX7 and 6N6P have two triode each, so I could use one of them for each channel (I'm building a stereo amplifier, of course): is there any problem using the same valve for both stereo channels?

Thanks in advance.

Ciao,
Giovanni Albergoni
1) Looks OK, however, I would add a negative rail, and DC couple the cathode follower to the final. That way, you get the advantages of fixed bias, with less power wasteage, better THD, enough current to charge up Ci + Cmiller + Cstray without overdrive blocking. Move the AC coupling to the first preamp.

2) Loadline. Also, R4 either isn't necessary, or pick one with 1 -- 5V across it and bypass with an electrolytic if stability and/or PSRR is gonna be a problem. With a cathode follower, you want the plate to be at AC ground.

3) You waste a CCS, and invite instability. If you're gonna do that, then the CCS belongs in the cathode lead, not the plate lead.

4) Much, much worse. Do that and your feedback becomes positive instead of negative. All you'll get is a fixed frequency square wave generator.

5) Yes. No.

Furthermore, I would ditch C4, and connect that gNFB at the load on the OPT secondary, That way, the xfmr is included in the error correction. This will help to linearize it, and reduce the Zo for better woofer damping and better bass performance.
__________________
There are no foxes in atheistholes
www.dolphin-hsl.com
  Reply With Quote
Old 30th September 2006, 12:10 AM   #7
diyAudio Member
 
Wavebourn's Avatar
 
Join Date: Sep 2006
Location: Pleasant Hill, CA
Send a message via Skype™ to Wavebourn
Quote:
Originally posted by ray_moth

Well spotted! Two possible solutions? (a) move the coupling cap so it lies between the first and second stages; or (b) use the negative line instead of ground at the first stage.
And what's the reason?


"There were rocks in the pool. We took racks and removed rocks. Now there are racks in the pool" (C)
  Reply With Quote
Old 30th September 2006, 01:29 AM   #8
diyAudio Member
 
Join Date: Mar 2003
Location: Zürich
Hi Wavebourn

I can only use other people arguments, but actually that is not that bad at all! I will use the argument's by Morgan Jones, in his Valve amplifiers book. Hmm...before spitting rubbish here (2:33 in the morning) I will read Morgan Jones again, and understand exactly why. If noone explained why ray moth's approach is better, I will try to do it, maybe tomorrow.

But it has to do with overloading of stages and class of operation. THe cathode follower can't be overloaded, but the output stage can...

I have to go to bed... ray moth, jump in (I know you have experience with the Crystal Palace).

Erik
  Reply With Quote
Old 30th September 2006, 01:41 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 ErikdeBest
Hi Wavebourn

I can only use other people arguments, but actually that is not that bad at all! I will use the argument's by Morgan Jones, in his Valve amplifiers book. Hmm...before spitting rubbish here (2:33 in the morning) I will read Morgan Jones again, and understand exactly why. If noone explained why ray moth's approach is better, I will try to do it, maybe tomorrow.

But it has to do with overloading of stages and class of operation. THe cathode follower can't be overloaded, but the output stage can...

I have to go to bed... ray moth, jump in (I know you have experience with the Crystal Palace).

Erik
I mean, what reason in moving the coupling capacitor one stage ahead?

Possible answer is that a grid current of the output toob will charge the cap causing dynamic bias.
__________________
The Devil is not so terrible as his math model is!
  Reply With Quote
Old 30th September 2006, 01:51 AM   #10
SY is offline SY  United States
diyAudio Moderator
 
SY's Avatar
 
Join Date: Oct 2002
Location: Chicagoland
Blog Entries: 1
That's very much it. "Blocking" is the word you're looking for. In most designs, the output stage will clip first, and direct-coupling the cathode follower to the output tube grid minimizes the pain caused by grid current piling up charge on the coupling cap, overbiasing the stage for a time constant or three.
__________________
And while they may not be as strong as apes, don't lock eyes with 'em, don't do it. Puts 'em on edge. They might go into berzerker mode; come at you like a whirling dervish, all fists and elbows.
  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
Direct-coupled cathode/source follower driver in PP? ray_moth Tubes / Valves 83 17th July 2008 03:53 AM
Question about direct coupling a anode follower into a cathode follower. G Tubes / Valves 45 29th July 2004 07:47 PM
cathode follower ackcheng Tubes / Valves 2 6th July 2004 04:12 PM
Cathode Follower? Yay or nay? SHiFTY Tubes / Valves 25 16th March 2004 07:38 AM
Cathode follower driver stage Joel Tubes / Valves 23 16th November 2002 07:55 PM


New To Site? Need Help?

All times are GMT. The time now is 04:11 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