Making a custom tube characteristic graph - 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 22nd February 2014, 11:24 PM   #1
diyAudio Member
 
LATUBEGEEK's Avatar
 
Join Date: Mar 2013
Location: Orange County, CA
Default Making a custom tube characteristic graph

I want to know how a 12AT6 will operate below 100VDC on the plate. The tube data sheet graph (attached) barely lets me plot 108VDC vs 0.20 mA (what I have running currently on a circuit I am working on). There must be some assumptions made to get the data plotted (lines for various grid voltages), such as plate load, input impedance, etc. Some data sheets list example operating conditions and spec a "plate resistance", or a "cathode resistance", but these are for one set of conditions.... What are the standards used to create these plots? Thanks!
Attached Images
File Type: jpg 12AT6 PLATE GRAPHs.jpg (197.3 KB, 112 views)
  Reply With Quote
Old 23rd February 2014, 03:08 AM   #2
diyAudio Member
 
LATUBEGEEK's Avatar
 
Join Date: Mar 2013
Location: Orange County, CA
Well I got started anyway...! 12AU7 with 0-290VDC B+ available on a big Ohmite slider, variable cathode 5Kohm bias pot (to dial in grid voltage lines on tube data graph), 68K plate load, and 1Meg grid input (to ground). Measured plate VDC, dialed in -2.0 volts on grid, calc'd plate current thru plate load..... Got 1.25mA, data sheet predicted 1.5mA. Close but expected better... changed grid resistor to 5Meg, values changed, but even with cathode grounded couldn't get lower than -3V (with respect to cathode, correct?). Back to drawing board...
Attached Images
File Type: jpg TUBE DATA TEST SETUP.jpg (234.0 KB, 99 views)
  Reply With Quote
Old 23rd February 2014, 05:49 AM   #3
Keit is online now Keit  Australia
diyAudio Member
 
Join Date: Feb 2014
Location: Australia
The graphs are plotted for the configuration stated - so many volts on plate, so many volts on grid, and with these the tube determines the plate current. Things outside the tube like resistance in series witrh the plate, grid resistance, etc have nothing to do with it. It's just a more complex situation than grabbing a certain resistor and applying a voltage across it. You get a certain current and that's it.

To design for conditions at very low plate currents (which mean a high bias voltage unless the tube is stuffed), you need a bit of basic mathematics. The current in a triode follows the "three halves power" rule. That is, the current is equal to:-

Ia = k x Vak^(3/2)

where k is a constant that depends on the dimensions and placement of the structure within the grid, and the grid voltage. So if yoiu find a value of k that fits a given curve on a graph of playte curves (Ia versus Vak), that value will be valid for all values of plate current well below the cathode's emission capability.

The formula breaks down near and below zero bias, but that is immaterial to your situation.

Very high values of grid resistance lead to lower than expected palte currents due to cathode sapce charge. Electronics impinge on the grid, driving it negative. So if the external leak resistance is high, this negative charge on the grid can't drain away. You can't measure this charge with an ordinary multimeter as the meter will drain away the charge.

Keit

Last edited by Keit; 23rd February 2014 at 05:54 AM.
  Reply With Quote
Old 23rd February 2014, 06:15 AM   #4
diyAudio Member
 
jazbo8's Avatar
 
Join Date: Jan 2011
Location: In Transient
Quote:
Originally Posted by LATUBEGEEK View Post
Some data sheets list example operating conditions and spec a "plate resistance", or a "cathode resistance", but these are for one set of conditions.... What are the standards used to create these plots? Thanks!
Those operating conditions in the datasheet are examples of how the tubes can be used, BUT they are not the conditions used to generate the plate characteristic curves, here is a very simple manual curve tracer using off the shelf parts, you can ignore the G2 supply when tracing triode curves:

Simple curve tracer courtesy of PRR:
Click the image to open in full size.
  Reply With Quote
Old 23rd February 2014, 06:00 PM   #5
diyAudio Member
 
LATUBEGEEK's Avatar
 
Join Date: Mar 2013
Location: Orange County, CA
Keit - I will definitely look into that "3 halves power" rule, and appreciate leaning how unseen tube nuances affect the tubes response.
Jazbo - I see the PRR circuit biases the grid with batteries. Will try that method (was just raising cathode potential with a pot). I was having trouble getting a solid zero grid voltage.
Thanks!
  Reply With Quote
Old 23rd February 2014, 09:54 PM   #6
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
The 3/2 rule can break down at low anode currents too. Don't assume it when measuring a valve. Instead, do the measurements and then afterward see if the rule applies.
  Reply With Quote
Old 24th February 2014, 12:27 AM   #7
Keit is online now Keit  Australia
diyAudio Member
 
Join Date: Feb 2014
Location: Australia
Quote:
Originally Posted by DF96 View Post
The 3/2 rule can break down at low anode currents too. Don't assume it when measuring a valve. Instead, do the measurements and then afterward see if the rule applies.
Errr... Yes it can. Actually, the rule still applies, but at very low currents, things link imperfections in the regularity of the grid wire turns, electron leakage at the top and bottom, and leakage of micas due to contamination make cut-off more remote than the math predicts.

Note that with biasing by means of a resistor in series with the cathode, cutoff at an anode voltage of ~100 volts occurs somewhat about 3 volts. Therefore this form of biasing cannot bias in excess of 3 volts. You cannot develop a voltage across a resistor unless there is current through it.

If you need to operate a tube at very low anode currents, don't use cathode resistor bias, as the current you get isn't set at all precisely.
  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
Tube characteristic - Originates from pre or power amp tubes? Defo Tubes / Valves 20 25th May 2013 02:25 AM
Making my own custom low tech 12" fullrange from an old Akai woofer:) impuls60 Full Range 3 6th December 2011 12:19 AM
Making custom 15pin control pod? sabbathcrazy Digital Line Level 0 30th July 2011 11:14 PM
Need help with making a port for a custom enclosure. the cobbler Construction Tips 2 16th May 2011 09:02 PM
Custom Tube DAC vman71 Digital Line Level 0 26th June 2009 08:37 PM


New To Site? Need Help?

All times are GMT. The time now is 01:47 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