Current measurements and spec'ing a custom power trafo.

[sircletus]

My question is a two-parter, I suppose.

Part 1:

I've got an LA-2A in front of me, and I'm curious as to how to measure the total current drawn by the circuit.  Why am I curious?  Well, I noticed the power trafo is spec'd to deliver 40mA of current at 500V.  But if I add up the total plate current (as read from datasheets) drawn by all of the tubes, I get the following:

1 x 12AX7 = 2.4mA (1.2mA current per triode)
1 x 12AX7 = 2.4mA (")
1 x 12BH7A = 23mA (11.5mA per triode)
1 x 6AQ5 = 30mA max plate current

Total = 57.8 mA for a single LA-2A.

So how is a trafo spec'd to deliver 40mA able to deliver enough current to the circuit?  Obviously, the circuit isn't drawing that much current.  So...am I adding typical plate currents incorrectly?  Am I going about this in completely the wrong way? And, of course, how do I measure actual current drawn by the LA-2A? Disconnect R29 and measure across?

Part 2:

Ultimately I'm diving into this stuff so that I can eventually create a spec for a custom power transformer for a project I'm working on.  But I need to know how to estimate current draw first.  I know what tubes I'll be using, but obviously just adding up "max" or "typical" plate current draw according to datasheets isn't the way to go...

 

[Butterylicious]

The original schematics give all kinds of voltages so calculating expected currents is simple.  Exanple:  Total current flows through R29.  The voltages given are 350V into R29 and 215V out.  350-215=135V,  135/4700=.0287 or 29mA  You can finger each individual valve purdy much the same way by calculating voltage drop across it's plate resistor and dividing by the resistor value. 

Using max ratings from the tube data sheets and summing the values will certainly guarantee your designed PSU should be ample, most likely overkill.  The textbook way to calculate current is to draw load lines on your curve sheets.  The fast way is to buy TubeCad & point-n-click.

 

[sircletus]

Excellent information.  Should be able to handle it from here.  I'll likely be picking up TubeCad.  It looks like a highly efficient way of doing this.  Thanks!

 

[Butterylicious]

see attached V1 analysis:

 

[PRR]

> add up the total plate current (as read from datasheets)

Data-sheet numbers are "SHOW-OFF" numbers.

Yes, a 6AQ5 can be run at 30mA. Even 50mA. Or 3mA, though it may be silly to to use a 30mA tube for a 3mA chore.

IIRC, the LA2-A's 6AQ5 works with a ~~300V supply and a 10K resistor. If the tube were a dead-short it could only pass 300V/10K= 30mA. A dead-short isn't interesting. Resistor-loaded tubes usually split the supply voltage between resistor and tube. Toss a dart at 50:50 split. 150V across tube, 150V across 10K resistor, 15mA. (If it indeed runs 215V then current is even less.)

Why not run 30mA? Well the night-light element only needs a few mA of drive.

Why not use a smaller tube? 12AX7 won't pass many mA. You could just about do it with 12AT7, both sections parallel, but that is/was nearly the same cost as 6AQ5. There's many ways to do it. Maybe their distributor was over-stocked on 6AQ5. (And it may have been prototyped with 6V6, a nice fat octal, just to be sure of enough oomph. When testing showed the 6V6 was loafing, the 6AQ5 is a small low-rating 6V6 and much cheaper.)

When imitating well-documented plans like LA2-A, simply figure the currents from the voltages and resistances shown on the plan. As Buttery did for R29. Even allowing +/-20% slop (often true on old plans) it only needs a 35mA HV supply.

If inventing from-scratch.... well, as a designer you should have at least a cocktail-napkin idea how hard your devices are working. For initial breadboard and smoke-test, use a larger power supply. In the old days I'd grab an old stereo hi-fi, toss the power bottles, and know I could pull 150mA easily. Voltage usually had to be dropped, resistor is simple, V/R tells the current. From this I could look for a more right-size PT for the job.

 

[lassoharp]

Something you rarely ever see for tube PTs is a graph of output voltage vs current draw.  This is useful for letting you know how much voltage output to expect if you're PT is being ran at 40ma and is speced to deliver 500V @ 200ma.  It will be higher - possibly enough to warrant more dropping resistance in the supply design.

 

[sircletus]

Something you rarely ever see for tube PTs is a graph of output voltage vs current draw.  This is useful for letting you know how much voltage output to expect if you're PT is being ran at 40ma and is speced to deliver 500V @ 200ma.  It will be higher - possibly enough to warrant more dropping resistance in the supply design.

Exactly.  This would be immensely helpful.  I know there is a relationship; if you're drawing less current than the power trafo is spec'd for, voltage rises.  By how much?  I suppose that depends on the trafo...

 

[sircletus]

> add up the total plate current (as read from datasheets)

Data-sheet numbers are "SHOW-OFF" numbers.

Yes, a 6AQ5 can be run at 30mA. Even 50mA. Or 3mA, though it may be silly to to use a 30mA tube for a 3mA chore.

IIRC, the LA2-A's 6AQ5 works with a ~~300V supply and a 10K resistor. If the tube were a dead-short it could only pass 300V/10K= 30mA. A dead-short isn't interesting. Resistor-loaded tubes usually split the supply voltage between resistor and tube. Toss a dart at 50:50 split. 150V across tube, 150V across 10K resistor, 15mA. (If it indeed runs 215V then current is even less.)

Why not run 30mA? Well the night-light element only needs a few mA of drive.

Why not use a smaller tube? 12AX7 won't pass many mA. You could just about do it with 12AT7, both sections parallel, but that is/was nearly the same cost as 6AQ5. There's many ways to do it. Maybe their distributor was over-stocked on 6AQ5. (And it may have been prototyped with 6V6, a nice fat octal, just to be sure of enough oomph. When testing showed the 6V6 was loafing, the 6AQ5 is a small low-rating 6V6 and much cheaper.)

When imitating well-documented plans like LA2-A, simply figure the currents from the voltages and resistances shown on the plan. As Buttery did for R29. Even allowing +/-20% slop (often true on old plans) it only needs a 35mA HV supply.

If inventing from-scratch.... well, as a designer you should have at least a cocktail-napkin idea how hard your devices are working. For initial breadboard and smoke-test, use a larger power supply. In the old days I'd grab an old stereo hi-fi, toss the power bottles, and know I could pull 150mA easily. Voltage usually had to be dropped, resistor is simple, V/R tells the current. From this I could look for a more right-size PT for the job.

Thanks so much for the info.  Great stuff.  I have very little experience with designing tube gear, though I've done several low-voltage solid-state pieces.  PRR, where in Maine do you live?

And Buttery, I'm pretty much convinced at this point that TubeCad would make an excellent low-cost design aid.  Thanks for that image.

 

[PRR]

> PRR, where in Maine do you live?

A workingman's town outside of fabulous Bar Harbor.

FWIW: I think that Radiotron and wanton plagiarization is a better design technique.

 

[sircletus]

> PRR, where in Maine do you live?

A workingman's town outside of fabulous Bar Harbor.

FWIW: I think that Radiotron and wanton plagiarization is a better design technique.

Was just in Acadia today.  I live in ever-so-thrilling Bangor.