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12AX7 heater filament resistance

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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.
 
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...

:D
 
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.
 
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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 said:
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.
 
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.
Source: http://en.wikipedia.org/wiki/12AX7#Twin-triode_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?
 
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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
 
dude reread the quoted text theres a 12AU7 and a 12U7 i got them mixed up. 12U7 is designed for low voltage.

and hes put a 12AX7 into a 12AU7 amp

which can be bad

is this a revised 12AX7?
the reason im asking is the plate voltages
on some tis 250v max on some its 300 and others 330 and ones like 12AX7A it could be anything

i may be wrong on this but if the circuit is depending on the resistance of the heater for part of the circuit, a small value pot for biasing might be a wise investment for that quarter you seen in the BK parkinglot
 
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Hi bacon665,
Well there are many tubes listed in the quoted section. I was trying to address a common problem seen often enough. I have even seen someone use a 12AX7 to drive a reverb tank. The 12AX7 kept losing emission and it arrived as a repair.

The original 12AX7 was superseded by the 7025 / 12AX7A. Every new manufacture 12AX7 is an "A" device because they improved the heater to cut down on hum. I just don't feel like typing the "A" all the time.

There are three rating systems in use. Absolute maximum, center design value and maximum design value (I think). The RCA tube manuals have an explanation of this, so it's easy to look up. These ratings simply point out guidelines so that a tube will last it's rated operating hours, and that can vary between different manufacturers. It is not wise to exceed these ratings. The absolute maximums are ratings that can not be exceeded at any time as damage to the tube may result.

and hes put a 12AX7 into a 12AU7 amp
Not great, but oh well.

The 12U7 and other mobile radio tubes are normally rated for lower plate voltages, like 250 VDC. With vibrator supplies, there are practical limits as to how much current you can run through them. This helps define the maximum supply voltages you may see in a car for instance. These are often "Loctal" tubes, they lock into the socket so they don't rattle free and fall out.

This particular tube is a nine pin, rated for only 30 VDC. Notice that this tube is intended to use the battery as its B+ source, so the normal plate voltage will run around 12 ~ 15 VDC. This device does not use a vibrator to step up the battery voltage to a normal 130 ~ 250 VDC.

i may be wrong on this but if the circuit is depending on the resistance of the heater for part of the circuit,
That would be unwise as this parameter is not well controlled. They do make heater - relay "tubes" for timing purposes.

-Chris
 
will measure the heater current today. Below is the schematic and B+ voltage is 250V.
 

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Just measured heater current of 12AX7. 150mA spot on. When plug in directly to RH preamp, the biases are way off the original value with 12AU7. So I guess, 12AU7, 12AT7 and 12AX7 cannot work by simply swapping the tube. I learn my lesson. Thanks for advise.

Regarding the glow issue. Why the new reissue Tungsol does not design it to produce the magic glow. There are only some small red dots visible when inspect carefully. The main reason for me to build a tube amp is so that I can enjoy the magic glow during listening. :dead:
 
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Well, you are welcome to choose your tubes for looks rather than audio quality! :D

Use a voltage regulator tube or two in the power supply. They look really cool! They are available in 8 pin and small glass envelopes. A wire mesh screen is highly recommended for these. Be aware that they will only tolerate a low capacitance in parallel unless you decouple them with a resistor.

Now you will have a light show and probably less noise in the output signal. That depends on the supply you are using now.

-Chris
 
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Hi bacon665,
Ahhh, shouldn't they be caged or shielded some how?

Mercury rectifiers are pretty, but tend to generate RFI +++. Noisy fellas!

I use #83s for the most part. Past that I go for solid state rectifiers. The HV ones (industrial) are costly as heck though.

-Chris

Edit:
That amp of yours is a thing of beauty to look at. Nicely laid out too.
What's the output? Around 5 W per channel?
 
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