DC coupled cathode follower driving me nuts - 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 8th November 2011, 12:14 PM   #1
diyAudio Member
 
funk1980's Avatar
 
Join Date: Sep 2011
Location: Groningen
Default DC coupled cathode follower driving me nuts

I'm building a pre-amp for bass guitar, utilizing two 12ax7's. I'm fairly new to (tube) amp design, but did a lot of reading learned a lot on this forum. I am however struggeling with the second stage of the preamp. It's a direct (DC) coupled recovery stage/ low impedance driver. The recovery stage was easy, but I fail at the cathode follower.

The gain stage, biased at -1.5V, gives about 144V quiescent voltage at the anode and therefor the grid of the CF. I calculated the CF's cathode resistor should be about 120kOhm to get a voltage drop of about 145V for biasing. Looks good on paper, but after building it, I'm measuring around 180V at the cathode and there's no way I can figure to lower it. I've tried different values of Rk, but it's just lowering it by a couple of volts. It's getting nowhere near 145. I've double checked the wiring, stared infinitly at the schematics, but I'm obviously missing something.

Can anyone help me to understand why my design is failing? Pictures included ofcourse:

Click the image to open in full size.

Click the image to open in full size.

Last edited by funk1980; 8th November 2011 at 12:25 PM.
  Reply With Quote
Old 8th November 2011, 12:22 PM   #2
SY is offline SY  United States
diyAudio Moderator
 
SY's Avatar
 
Join Date: Oct 2002
Location: Chicagoland
Blog Entries: 1
The CF is doing what it's supposed to do. The issue is the plate voltage of the gain stage. Welcome to the world of real tubes that have tolerances. Try swapping a couple of tubes in to see how that varies.

One note: your circuit is set for early tube death. Be kind and wire a reverse-biased diode between grid and cathode of the CF. I hope you've respected maximum cathode-to-heater voltages...
__________________
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 8th November 2011, 12:32 PM   #3
diyAudio Member
 
funk1980's Avatar
 
Join Date: Sep 2011
Location: Groningen
Thanks for the quick reply!
I left the diode out for clarity. Also forgot to mention I raised the heater supply

But I fail understand what mechanism causes the voltages I'm measuring. Even lowering Rk to 82kOhm doesn't do much. So in my mind, the CF is biased at 30V, which offcourse is rediculous for a 12ax7
  Reply With Quote
Old 8th November 2011, 01:23 PM   #4
SY is offline SY  United States
diyAudio Moderator
 
SY's Avatar
 
Join Date: Oct 2002
Location: Chicagoland
Blog Entries: 1
Try measuring voltage directly from cathode to grid of the CF. I'm going to guess that your meter has about a 1M impedance.
__________________
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 8th November 2011, 02:36 PM   #5
diyAudio Member
 
EddieRich's Avatar
 
Join Date: Jun 2011
Location: North of Boston
If the CF is cathode biased at 145V, then the voltage measured from the cathode to the plate would be 195V. Remember, the tube references everything to its own cathode. The grid would be biased at -1V relative to the cathode, so looking at the plate curves, there would be about 2mA flowing through the tube and Ra3. So Ra3 = 145 / 0.002 = 72.5K.
  Reply With Quote
Old 8th November 2011, 03:17 PM   #6
Yvesm is offline Yvesm  France
diyAudio Member
 
Yvesm's Avatar
 
Join Date: Jul 2004
Location: Ardeche
In short, the job of the cathode follower is to force the cathode to follow its grid, you can't change the cathode voltage by changing the cathode resistor, this just changes the current thru the tube . . . until you abuse it !
To set another cathode voltage all that you can do is to set another grid voltage by altering either the current or the plate resistor of the first tube.

Yves.
  Reply With Quote
Old 8th November 2011, 03:28 PM   #7
diyAudio Member
 
artosalo's Avatar
 
Join Date: Feb 2010
If I understood right you said that the voltage at the anode (1st) is 144 V and at the cathode (2nd) is 180 V. This is impossible situation and I think that if the voltage readings are really these, then the cathode follower is possibly oscillating.
You could check this with oscilloscope. However, you can avoid the oscillation by inserting a series resistor of some 1k at the grid of the CTF.

Can you confirm the actual voltage readings from the anode of the 1st. 12AX7 and at the cathode of 2nd half ?

If you want to adjust the voltage level of the first anode and the bias of the cathode follower, this is done by fine tuning the cathode resistor of the 1st 12AX7.
  Reply With Quote
Old 8th November 2011, 05:08 PM   #8
diyAudio Member
 
artosalo's Avatar
 
Join Date: Feb 2010
This is what you should get:

Click the image to open in full size.


In addition:

Gain = 37.7 dB, THD > 1 % @ 13 Vrms out.
Frequency response ( -1 dB ) 12 Hz...>100 kHz.
  Reply With Quote
Old 8th November 2011, 05:20 PM   #9
diyAudio Member
 
Join Date: Feb 2011
Location: South Wales, UK
What are you using to measure the voltages? Is your meter loading the 470k Ra ? Try measuring the grid/cathode voltage. The grid will be negative with respect to the cathode. This when added to the cathode voltage across the 120k resistor will give you the anode voltage.
  Reply With Quote
Old 8th November 2011, 07:35 PM   #10
diyAudio Member
 
funk1980's Avatar
 
Join Date: Sep 2011
Location: Groningen
Thanks a million guys. I really appreciate it. Here's how I've been progressing

First, I followed artosalo's schematic (thanks for that) and I added the 1k between anode V1 and grid V2 as suggested. Next, I measured between V2's grid and cathode. 1 volt! That's more like it. But I couldn't dial in (pot) a lot more potential between the two. So I swapped the tube with a new Tung-Sol and I was able to set the potential to 1.5 volts with ease. Seems tube nr.1 has issues .

And then I checked what payitforwardeddie and SY wrote about my meter's impedance. It's a simple cheap multimeter, so input impedance won't be a lot more then 1M, and thus effectively lowering Ra1 from 470k to about 320k!
It's my first stage with such a high Ra, so that's a big rookie mistake I guess .

So, unexpected tube behaviour combined with a cheap multimeter was what's going on. Problem solved. Sounds great! 37.8 dB of measured gain. I couldn't have done it without your help.

Last edited by funk1980; 8th November 2011 at 08:00 PM.
  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
A Direct-Coupled Amplifier with Cathode Follower 6V6 mltube Tubes / Valves 6 31st March 2011 06:41 PM
Cathode follower amplifier, push-pull, xformer-coupled 00940 Tubes / Valves 3 19th November 2010 01:53 PM
Dave, it's driving me nuts bill_a Forum Problems 7 9th December 2009 11:54 PM
Direct-coupled cathode/source follower driver in PP? ray_moth Tubes / Valves 83 17th July 2008 02:53 AM
Power follower class A Mosfet DC coupled. Mr. Steve Solid State 30 15th February 2006 06:04 PM


New To Site? Need Help?

All times are GMT. The time now is 07:22 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