12AX7 heater filament resistance - 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 30th June 2009, 12:40 PM   #1
Ipanema is offline Ipanema  Malaysia
diyAudio Member
 
Ipanema's Avatar
 
Join Date: Jun 2004
Location: KL
Default 12AX7 heater filament resistance

I bought a new reissue tungsol 12AX7. The heater filament pin 4 and 5 measured 40ohms but a RSD 12AT7 only measured 12ohms. When I connect it to 12V DC, it doesn't produce the magic glow. I would like to know whether this tube is malfunction because I got it from ebay. Thanks.
  Reply With Quote
Old 30th June 2009, 04:58 PM   #2
diyAudio Member
 
Join Date: Oct 2007
This matter of "filamernt resistor" is not as simple as it sounds! Well, not so complex neither: probably the 12 ohms is the resistance measured now because it's used, I don't know, but, when the current is passing through it, the temperature makes the resistance grow up, so, could be 40 ohms in cold, and 12V/150mA functioning. The same happens to incandescent bulbs (I've done this at University), about 4 ohms cold, 90 ohms functioning.

How does the new one sound? I think it doesn't matters how it glows...

  Reply With Quote
Old 30th June 2009, 06:26 PM   #3
diyAudio Member
 
oldmanStrat's Avatar
 
Join Date: Apr 2009
The first 12AT7 (I know it's a different tube) I have in my amp worked fine for quite a while - but never really glowed at all. Even with the lights out you would only just barely see the heaters.

I recently had to replace the tube with a 6201- and that one glows like a flashlight !
__________________
For security reasons my name is changed daily...
  Reply With Quote
Old 30th June 2009, 06:40 PM   #4
Ipanema is offline Ipanema  Malaysia
diyAudio Member
 
Ipanema's Avatar
 
Join Date: Jun 2004
Location: KL
I'm asking because this 12AX7 does not work in my RH preamp build for 12AU7. Just want to test how it will sound with 12AT7 and 12AX7. I suppose all three can work in direct swapping.

http://www.tubeaudio.8m.com/Rpa/rpa.html

12AT7 - more gain
12AX7 - not working. No glow. No sound. So I just unplug it from the preamp and just supply 12V heater to pin 4 and 5. No glow. I suspect something wrong with this 12AX7. Since all 12A*7 require 150mA of heater current. I asume that their filament resistance should measured the same at cold. 12ohms vs 40ohms are very much different. Pls teach me how to confirm whether this tube had malfunction.

Thanks.
  Reply With Quote
Old 30th June 2009, 07:02 PM   #5
diyAudio Member
 
oldmanStrat's Avatar
 
Join Date: Apr 2009
measure the current with a meter - if you don't have 150ma, then that tube probably has a problem.
__________________
For security reasons my name is changed daily...
  Reply With Quote
Old 30th June 2009, 07:08 PM   #6
tomchr is offline tomchr  United States
diyAudio Member
 
tomchr's Avatar
 
Join Date: Feb 2009
Location: Greater Seattle Area
As others have said already: Cold resistance isn't much of an indicator. Measure the current draw when applying the specified heater voltage to the filament.

If you don't have an ammeter, you can use a 1 ohm resistor in series with the heater and measure the voltage drop across the resistor. Current, I = E/R, where E is the voltage drop and R is the resistance.

~ Tom
  Reply With Quote
Old 30th June 2009, 07:24 PM   #7
diyAudio Member
 
Join Date: Apr 2009
Send a message via MSN to bacon665
Whats the B+ voltage in this amp. I read on wikipedia.. i think it was the 12AU7 was originally designed for car applications where the maximum B+ would be 12v dc (or 6 depending on who made it)
  Reply With Quote
Old 30th June 2009, 07:57 PM   #8
diyAudio Member
 
oldmanStrat's Avatar
 
Join Date: Apr 2009
Quote:
Originally posted by bacon665
Whats the B+ voltage in this amp. I read on wikipedia.. i think it was the 12AU7 was originally designed for car applications where the maximum B+ would be 12v dc (or 6 depending on who made it)
Can't say I agree with that. I just checked the data sheets available on tdsl: http://tdsl.duncanamps.com/show.php?des=12AU7

and they show a Va max of 300V.
__________________
For security reasons my name is changed daily...
  Reply With Quote
Old 30th June 2009, 08:46 PM   #9
diyAudio Member
 
Join Date: Apr 2009
Send a message via MSN to bacon665
Sorry for the mistake
Quote:
The 12AX7 was the most common member of what eventually became a large family of twin-triode vacuum tubes, manufactured all over the world, all sharing the same pinout (EIA 9A). Most used heaters which could be optionally wired in series (12.6V) or parallel (6.3V), with respective current requirements of 150 mA or 300 mA. The variations include the 12AT7, 12AU7, 12AV7, and the low-voltage 12U7, plus many 4-digit EIA series dual triodes. The variations span a wide range of voltage gain, ruggedness, and transconductance.
Source: http://en.wikipedia.org/wiki/12AX7#T...ode_variations

Sorry about that. according to sylvania's datasheets its max rating is 30 volts. so it would be operating under voltage still and might be replaceable.

As for a 12AX7 not working in it either some rating is different or maybe the tube is bad? what was wrong with the 12AU7 that was in the socket?
  Reply With Quote
Old 1st July 2009, 12:48 AM   #10
anatech is offline anatech  Canada
diyAudio Moderator
 
anatech's Avatar
 
Join Date: Jun 2004
Location: Georgetown, On
Hi Ipanema,
The glow comes from the heater extending past the cathode sleeve. It's as simple as that. Some are contained within the sleeve (no to low glow) and some extend out some (large glow). Look carefully at the tube under a good light, off and cool! You will see what I mean if you pay attention to the centers of the two sections.

Measuring resistance is futile! All you get is a continuity indication and filaments rarely go open unless abused or cheap. Think "original equipment" or "El Menco" brands, they were poor types.

Hi bacon665,
Look somewhere else. The max plate voltage for a 12AX7/U7/T7 is about 330 VDC. oldmanStrat was correct.

These tubes are not interchangeable at all. They have completely different characteristic curves. You can plug them in and they will make sound, it just will not be low distortion sound. Also, be careful about installing a 12AX7 into another type as the plate current may be high enough to shorten the normal life of a 12AX7. Guitar players get away with this simply because they are creating a sound, and they don't really care about tube life.

-Chris
__________________
"Just because you can, doesn't mean you should" © my Wife
  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
Very Basic 12AX7 Heater Question cfitzger Tubes / Valves 6 1st May 2014 04:05 PM
Is 6V DC too low for heater filament? Sunsun22 Tubes / Valves 8 29th December 2008 08:45 AM
5687 filament heater question dr._sleep Tubes / Valves 2 25th August 2005 04:45 AM
Troubl-shooting heater voltage for filament andy2 Tubes / Valves 12 10th January 2005 05:41 PM
grounding heater/filament center metebalci Tubes / Valves 6 27th March 2004 09:24 AM


New To Site? Need Help?

All times are GMT. The time now is 09:56 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