Go Back   Home > Forums > >
Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum

Plate Current VS Plate Voltage Graphs
Plate Current VS Plate Voltage Graphs
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
Thread Tools Search this Thread
Old 12th February 2009, 04:09 PM   #1
Captn Dave is offline Captn Dave  United States
diyAudio Member
Join Date: Jun 2008
Default Plate Current VS Plate Voltage Graphs

I have a 6L6 setup where the cathode current (therefore bias) is set by a bad and unmarked resistor. There is some evidence to suggest that 100ma is the correct cathode current. I'm trying to find the operating point for verification. Which Chart is relavent to the task of determining the correct cathode current?

I suppose what I'm asking is that given the relatively low current at the supressor grid and control grid, do designers assume the plate current and cathode current to be equal?

  Reply With Quote
Old 13th February 2009, 12:28 AM   #2
ray_moth is offline ray_moth  Indonesia
diyAudio Moderator Emeritus
ray_moth's Avatar
Join Date: Jan 2004
Location: Jakarta
I guess you mean "the relatively relatively low current at the screen grid and control grid". I would say it is quite a rough approximation to consider cathode cureent and plate current to be the same. The screen current, even in a beam power tube, is a significant proportion of the cathode current and varies with screen voltage.
  Reply With Quote
Old 13th February 2009, 08:57 PM   #3
Captn Dave is offline Captn Dave  United States
diyAudio Member
Join Date: Jun 2008
Yes I did mean to say screen grid, and I should have approached the question differenty. Allow me to try again.

I'm sure we've all seen plenty of designs that use a pair of 12 volt filaments in series to set the cathode bias (Is there a term for this design?). It seems that most of these designs in my collection of schematics specify a 20 or 22 volt bias.

The vintage guitar amp I have on the bench now uses a 39R resistor in series with the parallel combination of a single 12 v filament and a (failed) power resistor of unmarked value. As you would expect, with the open power resistor) the 12AX7 is seeing way too much voltage and the operating point of the 6L6 power tube is way down there. The power resistor must be replaced but with what value?

I set it up on the bench and noodled around with different resistors. 100R yields 12.7 for the tube filament and 22.9 for the bias. Not too bad for the tube filament but still pretty high for the bias. A 44R resistor drops the bias to about 20.6 volts but the filament only sees 8.8 volt. I'll probably set it up for a 22 volt bias which should yeild an 11 volt filament and call it a day. OR- a 21 volt bias and 10 volt filament is another possibility, humm.

But, I'm a geek and I so would like to know, without a schematic, how would one go about reverse engineering the design operating point and determining the correct bias?

And so that's why I was asking about cathode current.

  Reply With Quote


Plate Current VS Plate Voltage GraphsHide 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

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
ECC83 - Long Plate vs Short Plate poynton Tubes / Valves 16 17th February 2008 07:42 AM
6SN7 plate voltage, current and lifespan cbutterworth Tubes / Valves 1 25th October 2007 08:25 PM
does plate current/voltage effect tube sound???? benny Tubes / Valves 10 7th July 2004 01:18 PM
Subwoofer Plate Amplifier Base Plate supplier? jwells777 Parts 1 11th May 2003 07:40 AM

New To Site? Need Help?

All times are GMT. The time now is 11:02 AM.

Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 16.67%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Copyright ©1999-2018 diyAudio