Thermionic Cooling? - 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 9th June 2007, 03:34 AM   #1
diyAudio Member
 
Join Date: Sep 2006
Default Thermoionic Cooling?

The filament on my 304TL draws 10 amps at 10 volts warming up to glowing. After that the current draw of the filament drops to about 5 amps. The data sheet says it draws 10 amps.
Is this normal? Under maximum plate dissipation would the filament(cathode) be cooled by thermionic emission thus causing it to use more current?
  Reply With Quote
Old 9th June 2007, 07:31 AM   #2
ulibub is offline ulibub  France
diyAudio Member
 
Join Date: Nov 2006
Location: Lorraine
Hello!

This effect exists indeed, but I doubt that it is strong enough to cause a visible cooling of the cathodes in tubes. To leave the atomic lattice of a metal, each electron has to do a certain amount of "work function" to get free into the vacuum. That's the reason why we must heat the cathode - just to provide this work function for the electrons. And since this is a material constant (measured in electron volts), metals are used with a low work function - such as Barium (provided by thermolysis of Barium Oxyde on oxyde cathodes) or Thorium. The energy needed by the electrons to provide the work function is taken from the thermic power of the heating - and the more emission, the more work function power is needed, thus resulting in a certain drop of cathode temperature on high emission conditions indeed.

But I don't know of any case with tubes, that cathode heating power (voltage, current) had to be reduced during low-emission conditions (to prevent overheating). But I know it from the cathodes of mass spectrometers (tiny hard metal filaments, strongly heated) that these sometimes overheated and melted when no emission was needed (acceleration voltage switched off).

If your filament heating current drops to the half when plate voltage is switched on, you really should see the cathode getting much darker - it's a reduction of the heating power by 50%! And indeed, if a strong thermionic cooling would occur, you should see an increase (!!) of the heating current - a cooler filament draws more current than a hot one! And I think if there was a tube where heating power would have to be reduced during low emission, this would be marked in the data sheets...

I'm quite sure that your results have another cause... Maybe some current limiting effects in your power supply? Or simply a malfunction of your meter?

Uli
  Reply With Quote
Old 9th June 2007, 02:16 PM   #3
diyAudio Member
 
Gold_xyz's Avatar
 
Join Date: Dec 2005
Location: Italy
Hi

Datasheet Eimac says :

Mounting : vertical.
Cooling : convection and radiation.

http://frank.pocnet.net/sheets/090/3/304TL.pdf

With 300W anode + 100W heater = 400W dissipation
I think that this is near to limit of natural convection cooling.

If the filament current start from 10A and down to 5A
with a costant 10V supply perhaps it can be the exhausted filament.

Cooling with electrons emission is a very small amount.
(compared to heat of filament)
http://ieeexplore.ieee.org/xpl/freea...isnumber=15953
__________________
My English originates from here
  Reply With Quote
Old 9th June 2007, 02:49 PM   #4
diyAudio Member
 
astouffer's Avatar
 
Join Date: Jun 2004
Location: Pittsburgh, crumbling wasteland
Maybe the test leads in your meter are adding some resistance?
  Reply With Quote
Old 9th June 2007, 05:09 PM   #5
nhuwar is offline nhuwar  United States
diyAudio Member
 
nhuwar's Avatar
 
Join Date: Jan 2007
Location: Cypress Texas
Possible lose of cross sectional area of the filament causing a high heated resistance. How old is this tube?
  Reply With Quote
Old 10th June 2007, 07:20 PM   #6
diyAudio Member
 
Join Date: Sep 2006
Well i think its post ww2. It is NOS. I was just using a 10 amp dmm meter in series with the filament. It gets bright, just like a light bulb.

I am concerned since running a thoriated filament a little high or a little low can cause damage.
  Reply With Quote
Old 11th June 2007, 12:06 AM   #7
nhuwar is offline nhuwar  United States
diyAudio Member
 
nhuwar's Avatar
 
Join Date: Jan 2007
Location: Cypress Texas
The damage can be fixed usally by a procedure made by eimac you can find in the care and feeding of power grid tube manuals found on there website.
The damage can mainly be done by overload of emission.

Power on thorated W can also be ran at a slight lower voltage if you follow eimacs procedure for finding this operating point also found in the care and feeding manuals.

Heres a link to the manuals on eimacs website http://www.cpii.com/library.cfm/9


Nick
  Reply With Quote
Old 12th June 2007, 03:41 AM   #8
kevinkr is offline kevinkr  United States
diyAudio Moderator
 
kevinkr's Avatar
 
Join Date: Sep 2004
Location: Boston, Massachusetts
Blog Entries: 6
What are you using to power this tube's filament? Perhaps you should also measure what is happening to the filament voltage across the tube.. You might find it drops over time, that's the only explanation I can come up with for a reduction in filament current - from there you will have to determine the cause.
__________________
"Extraordinary claims require extraordinary evidence." - Carl Sagan
  Reply With Quote
Old 12th June 2007, 10:49 PM   #9
diyAudio Member
 
Join Date: Sep 2006
A lead acid battery. I checked for voltage drop and their was none.
  Reply With Quote
Old 12th June 2007, 11:08 PM   #10
d2134 is offline d2134  Romania
diyAudio Member
 
Join Date: Mar 2007
Send a message via Yahoo to d2134
I think your DMM is fault. Please check it.
It is normal for a cold filament to draw more curent then a hot one and the ratio is normal, I suspect your DMM.
  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
replacing op-amps with thermionic valves jenks Tubes / Valves 2 25th March 2008 01:45 PM
Thermionic Valve Analogue Stages PTSOUNDLAB Digital Source 7 5th May 2003 04:23 PM
air cooling karma Pass Labs 8 11th March 2003 04:54 AM
Cooling an amp with poor cooling BAM Solid State 8 31st December 2001 04:40 AM


New To Site? Need Help?

All times are GMT. The time now is 10:15 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