How do I design a pentode constant current source plate load? - 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 19th January 2009, 09:12 AM   #1
Bitrex is offline Bitrex  United States
diyAudio Member
 
Join Date: Jan 2009
Question How do I design a pentode constant current source plate load?

Hello everyone. Long time lurker, first time poster.

I am trying to design an amplifier with a 6Au6 pentode operating as a constant current source plate load for a medium mu triode. The equation I'm attempting to apply is in the back of Rozenblit's "Beginners Guide to Tube Audio Design":

ip = Epp - ep +mu*E)/rp+(mu+1)*R

where ip is the current through the CCS and triode, Epp is the supply voltage, E is the voltage differential between the triode's plate and the CCS's grid, mu is the gain of the CCS tube, rp is the dynamic plate resistance of the CCS tube, and R is the value of the CCS's cathode resistor.

This equation works fine when I use it for a triode loaded by a constant current source triode, but when I apply it to a pentode as the CCS (estimating at my selected operating point the 6AU6 has an rp of about 1Meg and a mu of 4,000) the equation breaks down and I start getting impossible values for R. I'm wondering if anyone could help point out where I'm going wrong? Or if there's a better way to analyze things when using a pentode as the CCS?
  Reply With Quote
Old 19th January 2009, 11:00 AM   #2
Yvesm is offline Yvesm  France
diyAudio Member
 
Yvesm's Avatar
 
Join Date: Jul 2004
Location: Ardeche
Hi Bitrex !

A 6AU6 is, in itself, a very good current source for a wide range of plate voltage.

In the attached graph you can see that to obtain, says 6mA you just need to bias the grid at -2 Volts and fix the screen at +150V, both relative to the cathode.
It's no so easy at it seems at first glance !

This done, the plate current will remain stable for plate voltages ranging from less than 100 to more than 400V relative to the cathode.

Beware, however, that the cathode current is the sum of the plate and the screen current wich is NOT constant.
This is usually a problem when the CCS is used to "source" current in the anode of another tube but cause no problem when used to "sink" current from a cathode.

Yves.
Attached Images
File Type: gif 6au6.gif (52.5 KB, 886 views)
  Reply With Quote
Old 19th January 2009, 01:35 PM   #3
diyAudio Member
 
Join Date: Mar 2008
Hi Bitrex and Yvesm,

r ≈ s x Rg2 x Rk and i = u / r

r = dynamic resistance of the current source
s = transconductance of the pentode
Rg2 = screen grid feed resistor
Rk = cathode resistor between g1 and cathode of the pentode.

Kind regards,
Darius
Attached Images
File Type: png pentode_ccs.png (9.1 KB, 897 views)
  Reply With Quote
Old 19th January 2009, 02:46 PM   #4
diyAudio Member
 
Join Date: Dec 2000
Location: Germany
Pentode current sources have a high dynamic resistance, but tend to be noisy.
  Reply With Quote
Old 19th January 2009, 04:51 PM   #5
diyAudio Member
 
Join Date: Dec 2000
Location: Germany
For even higher dynamic restistance use a higher value Rk and a voltage divider for G1,

Reinhard
  Reply With Quote
Old 19th January 2009, 04:59 PM   #6
diyAudio Member
 
Join Date: Nov 2007
Location: Dallas
Consider a Cascode of Triodes instead of a Pentode? Need not
have any screen forward biased. Same current, source or sink...

Of course, this is also an application where a depletion MOSFET
solves a lot of problems, needs no elevated heater, etc...

I have no idea which solution of these three might be least noisy.
  Reply With Quote
Old 19th January 2009, 05:49 PM   #7
diyAudio Member
 
Miles Prower's Avatar
 
Join Date: May 2005
Location: USA
Blog Entries: 6
Default Re: How do I design a pentode constant current source plate load?

Quote:
Originally posted by Bitrex
I am trying to design an amplifier with a 6Au6 pentode operating as a constant current source plate load for a medium mu triode. The equation I'm attempting to apply is in the back of Rozenblit's "Beginners Guide to Tube Audio Design":
My first preference would be to solid state that. If you still want to go with a pentode instead, I'd say fuggedabouddatequation. You'll get more accurate results from the plate characteristics.

I have a "white paper" here that describes how to make pentode based CCS's, but I can't find the thing on-line. If interested, e-mail me and I'll send it off as an attachment.
__________________
There are no foxes in atheistholes
www.dolphin-hsl.com
  Reply With Quote
Old 19th January 2009, 07:46 PM   #8
diyAudio Member
 
Join Date: Jul 2003
Location: Ann Arbor, MI
hey-Hey!!!,
I'd suggest a triode-pentode cascode. Triode on the bottom, pentode on top. I've done this with 12BY7/12HL7 with excellent results. Triode strap the bottom one as triode( or consider something like a triode like the 6C45 or strapped EF184). You'll need to create floating voltage references to allow for largest-possible R-set and to 'space' the pentode above the triode. See the Gary Pimm enhancement-mode MOSFET ccs biasing methods for ideas.
cheers,
Douglas
__________________
the Tnuctipun will return
  Reply With Quote
Old 19th January 2009, 11:21 PM   #9
Bitrex is offline Bitrex  United States
diyAudio Member
 
Join Date: Jan 2009
Thanks for all the helpful responses!
I have one further question - if I wanted to take the output of this circuit and couple it to a small power triode, for example, would it be preferable to take the output at the cathode of the pentode, or the plate of the triode? I know that the cathode of the pentode would have a lower output impedance, but it's sitting at such a high voltage that it would prevent direct coupling of the two stages. But would the plate of the triode have a low enough output impedance to drive the miller effect capacitance of a small power triode?
  Reply With Quote
Old 20th January 2009, 12:41 AM   #10
diyAudio Member
 
Join Date: Jul 2003
Location: Ann Arbor, MI
Take the mu-follower output at the cathode! use a bigger coupling cap.
__________________
the Tnuctipun will return
  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
PCF80 -Designing a Pentode Constant Current Sink - help needed. Shoog Tubes / Valves 25 31st July 2014 04:00 PM
Pentode Plate Load dsavitsk Tubes / Valves 20 13th March 2014 07:38 PM
constant current source hacknet Solid State 19 13th March 2006 02:46 PM
active plate load v.s. constant current... mwmitchell Tubes / Valves 3 8th August 2002 09:34 PM


New To Site? Need Help?

All times are GMT. The time now is 03:06 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