how to calculate current requirement

[chicago_mike]

Being new to making my own tube designs, Im looking at some small transformers for pedals and preamps.

Lets say i have an idea that uses 1 to 2 12ax7s, and I want to run them around 150 or so volts, whats my average current requirement my tranny will have to supply?

Thanks.

[Merlinb]

For a single 12AX7 triode operating at 'normal' voltages I generaly factor in about 1mA anode current, so 2mA for both triodes in the envelope.
In your case, with 150V supply voltage a value of 0.5mA should be enough. Therefore 2mA total for four triodes. Then add a bit for safety.

Oh, and the 6V, 300mA or 12V, 150mA heater, for each valve of course.

[chicago_mike]

Ah, I thought it was a small amount for the tubes, not the heaters, Thanks!

[RMS]

I thought this might help you out, though I just saw your post now; its been a while since I have visited here. But I am now working on an audio project so......

Here is the calculator:
Power Transformer mA Calculator

[Merlinb]

Originally posted by RMS
I thought this might help you out, though I just saw your post now; its been a while since I have visited here. But I am now working on an audio project so......

Here is the calculator:
Power Transformer mA Calculator

I think there's an error with that calculator. If you select "diodes" (I assume that means silicon?) it gives you a rather low DC voltage.
e.g., if you select the 250-0-250V transformer it tells you that you have 325Vdc, but it should be closer to 350V...

[Geek]

Uhhhh.... two 6L6 power tubes requires 198.6 mA @ 390V?

That calculator is on crack.

Cheers!

[Tweeker]

I dont see why 2 6L6s at full swing in class AB couldnt pass 200ma or more, depends a whole lot on your assumptions.

The RCA datasheet puts a pair in AB2 at 360V with 22.5V fixed bias as consuming 205ma max signal (88ma idle), add screens and preamps. Calculator is conservative due to scenarios such as this. AB1 at higher voltage is similar.

STC 807 datasheet puts pair in AB2 at 400V as 240ma max signal.

[Merlinb]

Originally posted by Geek
Uhhhh.... two 6L6 power tubes requires 198.6 mA @ 390V?

That calculator is on crack.

Cheers!

I think it assumes you're biasing the thing to max dissipation regardless of the HT, then it adds a bit to allow for the power factor.

[Geek]

OK, I was thinking class-A

[RMS]

Originally posted by Merlinb

I think there's an error with that calculator. If you select "diodes" (I assume that means silicon?) it gives you a rather low DC voltage.
e.g., if you select the 250-0-250V transformer it tells you that you have 325Vdc, but it should be closer to 350V...

Yes, you are right, it was calculating a little too low. I did some more actual testing and have come up with a better ratio I think.

Thanks for testing it out.

The bias calculator I should note is for Class AB and is the bias at idle current at 70% max plate dissipation.