LA2a power transformer...

[alk509]

Hey guys,

How much current do your LA2a clones draw from the power transformer (not counting heater current, of course)? I want to cram two of them in one box with one power transformer/PSU and make a stereo unit... David J's book says the original had a 500V CT / 40mA max, so I was thinking at least 80mA to power two... but Hammond apparently doesn't make their 270CBX no more (100mA); they only go up to 70mA on 250-0-250V transformers...

Will a 70mA transformer be enough to power two units? what are you guys using in your clones?

Thanks,

Al.

 

[NewYorkDave]

This is easy to figure out if you can find a legible copy of the schematic, with voltages marked. I managed to find one that's at least halfway-legible.

Ignoring the voltage drop of the rectifier diodes (which is negligible in this case), the 250-0-250V winding will produce about 353V at the input to the first filter cap. All of the current drawn from the plate supply flows through the 4.7K resistor, and there's 275V at the output of this first pole in the filter. Using Ohm's law:

I = V / R = 78/4700 = 0.016A, or 16 milliamps.

The circuitry is class-A, so current draw will vary little. Even when you allow for a healthy margin of safety, 70mA is more than enough. Note that the output voltage of the plate winding might be on the high side if you draw much, much less than the design current.