Lighting 12ax7?

[Zben3129]

What do I need to do to get a 12ax7 to light up, just so I have some sense of success? I have a transformer that reads 14vac unloaded. Also, for pin numbers, if you were to look at the bottom of the tube with the pin gap at 12 o'clock, would you go clockwise or counterclockwise?



Thanks

[R.G.]

Looking at the bottom of the tube, pins toward you, gap up, the pin on the right of the gap is pin one. Proceed clockwise.

With the tube out of the socket, all pins except the heater pins are open circuit.

The heater pins on a  12A*7 are pins 4, 5, and 9. 9 is the center tap. For 12V use, apply 12V between 4 and 5. For 6V use, short 4 and 5 and apply 6V between pin 9 and the shorted 4 and 5.

Hook up your transformer to pins 4 and 5 and measure the voltage. If it's between 13.8 and 10.8, you can use the transformer to light the tube. Note that this MAY still blow out the transformer, even if the tube likes it. It takes 300ma to run the heater this way. I would not run a tube with more than 13.2V on it for more than a few minutes in the interest of long life as high voltages wear out the cathode faster.

[Zben3129]

Hmmmmm I can't seem to get it to light up. I have it now as...

12.6vac+ ----> Pin 4
12.6vac- -------->Pin 5

I can't get the pin into the socket, either. Do you have to push them pretty hard? or should it just pop in. I can't imagine not having it in the socket would matter.



Thanks

[aloupos]

You need to apply some surprisingly firm pressure.  The tube won't break. 

Are you really just looking to light it for the sake of lighting it? 

There's some great tube related projects at ax84.com -- if thats your interest, check it out. 

[Zben3129]

I'm so afraiiiddd 
Im more afraid slicing my hand to shreds if it breaks.

Only tube I got (105 in the mail though )

[R.G.]

Hmmmmm I can't seem to get it to light up. I have it now as...12.6vac+ ----> Pin 4 12.6vac- -------->Pin 5

Did you happen to measure the resistance between pins 4 and 5 to be sure you got the right pins?

Im more afraid slicing my hand to shreds if it breaks.

So put on glove, or use a kitchen hot pad to prevent the cuts.

[Zben3129]

Yeah I got it in, and I used gardening gloves


Anyways, I still have no lighting, with a 12vac supply and also tried with a 9v battery as in the tube boost+overdrive thread, neither produced a single photon. So, I proceeded to breadboard the tube boost+overdrive, and I get absolutely no signal through at all.

EDIT: Also, what should the resistance be?


Thanks


Zach

[Zben3129]

Pin Voltages:

1:  6.5v
2: -.11v(?)
3: .02v
4:  0v
5: 7.97v
6: 7.19v(?)
7: -.04v(?)
8: 0v
9: Not connected to anything, but reads 4.05v

[Zben3129]

OK got signal through it, still no light, and it is an extreme deboost with some crappy sounding distortion



Anyone?

[theundeadelvis]

Connect pins 4 and 5. Pin 9 is the heater tap.

[theundeadelvis]

Also, some tubes don't glow very bright. Might turn the lights off.

[dxm1]

Pin Voltages:

1:  6.5v
2: -.11v(?)
3: .02v
4:  0v
5: 7.97v
6: 7.19v(?)
7: -.04v(?)
8: 0v
9: Not connected to anything, but reads 4.05v

You're remembering to switch your DMM to volts AC when measuring pins 4 and 5? Pin 9 is the center tap of 4 and 5, so it _is_ connected if 4 and 5 are...

[Zben3129]

I had the DMM on DC for 4 5 and 9 as I am using a battery to run it. Also, bu not connected I meant the wire is free, not physically connected.

[Valoosj]

This is how I light my tubes. No fear of a broken or blown up tube!

[R.G.]

Pin Voltages:
1:  6.5v
2: -.11v(?)
3: .02v
4:  0v
5: 7.97v
6: 7.19v(?)
7: -.04v(?)
8: 0v
9: Not connected to anything, but reads 4.05v

OK, you do indeed have the heaters wired correctly, since the pin 9 centertap is half the pin4-pin5 voltage.

The tube will not light up with only 8V across it. You need to get it up to about 11V minimum to see any glow. That will be a very dim red. It will be a dim orange at 12V.

Until the heaters are running at a minimum of 11-12V, there will be no signal coming through the tube. The glowing heater is what makes electrons boil off the cathode so any current can go through. Heat the heaters!

[Zben3129]

Got It! Used a 12vac tranny and shes working. Now I just gotta pop it into a circuit on the good 'ol breadboard

[petemoore]

Valoosj
  What kind of circuit uses that color tube?
  Lighting circuit...lol.
  If it works for B it works for Me.

[Valoosj]

I just widened the tube socket and stuck a 5mm bright blue led under/in the socket.

[km-r]

^ clever, i wanna do that too!! 

[petemoore]

  It's good for blues.