12au7 heater problem

[fossilshark]

I built a little headphone amp using a 12au7 and im running it off 12v from a battery. One channel works perfectly and the other one only has a tiny distorted signal coming out when checked on the O'scope. One heater is glowing normally and the other one is not, i have pin 9 disconnected with 4 and 5 going to + and - of the battery. What does the problem sound like?

[kagliostro]

Are you sure about connections ?

Have you tried one other tube ?

Franco

[fossilshark]

Ive checked all my connections and it is heating up a little but only at the bottom of the heater. Could it be the polarity of the voltage? Ill try testing it with a 12ax7

[kagliostro]

Measure the current consumption

Franco

[HotBluePlates]

... One heater is glowing normally and the other one is not, i have pin 9 disconnected with 4 and 5 going to + and - of the battery. What does the problem sound like?

Either a wiring error or a bad tube.  Easy to verify by swapping with any 12A_7.

[fossilshark]

Popped in 12ax7 with no change, i measured the voltage on the heater (4 to 5) and i got 11.8, i measured them in respect to the center tap (5 to ct and 4 to ct) and got 5.8 so the voltage is there. I can measure current at the moment because i blew the fuse in my multimeter, ill have to get one soon. Any ideas?

[shooter]

i measured the voltage on the heater

pull the tube and do the same check from the "top-side"

[fossilshark]

I got 11.9 on pins 4 and 5, on 4 respect to ct i got 11.9 and 5 respect to ct i got 0, is this odd?

[HotBluePlates]

... measure current at the moment because i blew the fuse in my multimeter, ill have to get one soon. Any ideas?

You know not to measure current the same way you measure voltage, right?

To measure current with a typical multimeter, you have to unsolder a wire and connect the meter in series with the circuit.  You also need to move meter lead sockets on many meters.  Remember to move them back when you're done measuring current, as failing to do so will likely pop an internal fuse.

All of the above is because the meter acts like a near-short-circuit when measuring current.  If you are careless, you could damage the meter or the circuit being tested.

... One heater is glowing normally and the other one is not ...

I got 11.9 on pins 4 and 5, on 4 respect to ct i got 11.9 and 5 respect to ct i got 0, is this odd?

Does the tube light up now?  You are not connecting pin 9 to anything for either socket, are you?

[fossilshark]

That was voltage measurements lol. Im going to wire the heaters in parralel  (on 6.3v) and see what happens

[HotBluePlates]

That was voltage measurements lol. ...

Understood, but you will blow meter fuses if you set the meter to read current, while trying to probe a circuit the way you do to measure voltage.  I had guessed you might have found that out the hard way (like I did a long time ago).

... Im going to wire the heaters in parralel  (on 6.3v) and see what happens

If you have 12vdc, why would you want to hook up the heaters as for 6.3v operation?  If you're unlucky, you could burn open the tube's heater...

[fossilshark]

I got an idea. Is i measure continuity between each heater and the ct i could tell if i blew a heater right? Also i blew the fuse on my meter (had an ice tea spill on my backpack) so as of right now i can only measure voltage, resistance, continuity, and farads.

[fossilshark]

I was putting the cereal in the fridge and the milk in the cupboard when i remebered i already checked the circuit with another tube. Wow... ill try heating it off another 12v source

[pompeiisneaks]

You can measure continuity on the tube itself between pins 4-5 or 5-9 etc.  You can also test continuity on the heater line, just touch one probe to the entry point of the first tube and touch along all the other tubes heater connections to ensure you're getting good connection.