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Ipanema 30th June 2009 11:40 AM

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.

slideman82 30th June 2009 03:58 PM

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...


oldmanStrat 30th June 2009 05:26 PM

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 !

Ipanema 30th June 2009 05:40 PM

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.

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.


oldmanStrat 30th June 2009 06:02 PM

measure the current with a meter - if you don't have 150ma, then that tube probably has a problem.

tomchr 30th June 2009 06:08 PM

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

bacon665 30th June 2009 06:24 PM

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)

oldmanStrat 30th June 2009 06:57 PM


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:

and they show a Va max of 300V.

bacon665 30th June 2009 07:46 PM

Sorry for the mistake

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.

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?

anatech 30th June 2009 11:48 PM

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.


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