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Author Topic: 12AX7 Current Draw?  (Read 3040 times)
Posts: 255

12AX7 Current Draw?
« on: February 01, 2011, 06:55:25 AM »

Hello all,
So I'm in need of a nice simple tremolol. A while ago, I breadboarded frequencycentral's vibracaster, and i really liked it. It seemed to fit the bill. Problem is, I've a pretty packed pedalboard already, running off a boss powersupply. I would imagine that between the heater and the anode, a 12AX7 would have a pretty high current draw. How would I go about checking or does anyone know what it's like as it? Also, as an alternative, does anyone know of any really simple but effective tremolos that would be a good alternative?
Posts: 3599


Re: 12AX7 Current Draw?
« Reply #1 on: February 01, 2011, 07:47:57 AM »

...but leave the extra "L" off the end of tremolo.   Wink
Posts: 6738

Paul R. - Maine USA

Re: 12AX7 Current Draw?
« Reply #2 on: February 01, 2011, 10:47:57 AM »

> I would imagine that between the heater and the anode, a 12AX7 would have a pretty high current draw.

"Between heater and anode"? Or "heater and anode combined"?

I think you know this, but someone else may not:

Most post-1928 tubes, the heater is Insulated and "NO" current flows to other electrodes.

Someone could be imagining that there is high current from CATHODE to anode. Nope.

Back in the last century, white-shirted junior engineers made many tests and plotted many curves JUST TO answer questions like this. Don't let their work go in vain.

Frank's tube datasheets:

GE 1953 sheet, Page 3:
at 100V - 2.5mA
at 12V - 0.5mA

Contrast this with heater demand of 12V at 150mA: the plate current is never "pretty high" compared to heater current. (This is why tubes suck at low voltage.)

If you use both sections of 12AX7, at 12V, 0.5+0.5= 1mA.

The "combined" load is 151mA.

However the schematic for Vibracaster which I found shows 12AU7. That is a different tube, more conducty. It could pull 2mA per section, for a total 154mA.

BUT the Vibracaster plan shows 220K and 100K plate resistors. The MOST the plate-circuit could pull is 12V/100K= 0.12mA in the second stage, 0.054mA or half that in the first stage. And if it pulled that much the plate would be slammed to ground, no signal would pass. So on average it must be less. Near 0.1mA (for ANY tube used). With this correction the total heater+plate demand is 150.1mA.

"150mA" is nominal. Depends on the heater wire. They try to hold it to spec, but there's tolerance. For a similar tube, the hi-reliability data shows 99.6% of tubes will fall between 142mA and 158mA at 12.6V exact. So now the maximum is near 158.1mA.

There's also current for the Vibracaster's two transistors. Each has a 10K resistor in series, and they are direct coupled so as one current goes up the other current goes down. So two 10K resistors with maybe 6V each is two 0.6mA loads, 1.2mA.

No LED is shown on the plan I found.

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