Transformer Specs question

[TurboGuitarMelton]

Hi,


Just trying to clear up my understanding of spec for transformers.


Lets say we have a 325-0-325 @70ma transformer. The 70ma is the current that the transformer supplies right? How do we calculate the total current draw of a circuit? I've tried to figure this out a few times looking at tube data sheets but I can't seem to make sense of it.


Does the bias of the power tubes increase current draw? For example: 1 6v6gt @37ma and 1 12ax7 @~1ma. So that would be around 38ma total current draw? Just a guess.


Thanks!

[PRR]

A guitar amp normall uses both sections of 12AX7 so that will be 2mA.

The "70mA" is a load the transformer will deliver without undue heating. It can deliver zero. It can deliver 100mA in short bursts.

[shooter]

So that would be around 38ma total current draw

That's my mythology, I start with the PA tubes, and datasheet typical Max I.
You can also calc an approximation by having a "guess" of plate B+, known OT primary impedance (in ohms), use ohms law to calc I.  Like PRR said, I just assign each 12XXXX tube as 2mA (not ea 1/2).

filaments are the biggest - need to know value!

an extra step I take;
once I get my I guess, I calc an equivalent R (including watts) make sure I have one on hand - or close, then I use that as my PS dummyload.  that way I'm confident the PS is working before I stick it on $150 of tubes

[jjasilli]

325-0-325 @70ma means: i) the tranny will deliver the stated voltage @ 70mA of current draw; and ii) it's rated for 70mA.  As PRR said, if you cause it to draw more current, that's at your risk; though more current in short bursts should be OK.  That's because the issue is heat.  Heat takes time to have a bad effect; and intermittent heat allows for cooling-off periods.

The specs further imply that if less current is drawn through the tranny, then secondary voltage will be higher (and vice-versa).  This is because trannies are typically poor at voltage regulation.  I.e., they cannot maintain a consistent secondary voltage as current draw varies.  This can be overcome by building a robust, physically large tranny.  But pragmatically, no one does that, especially for guitar amps.

That said a quality tranny can be over-spec'd up to 50% current wise (at your risk).  This will enhance voltage sag under signal conditions, which is a possible design feature for guitar amps.

To spec a tranny go to the power tube chart.  The preamp tubes draw so little current that they're not even a rounding error.  Get the current draw for i) plate + screen, ii) under full signal conditions, iii) in the column for your target plate voltage.  Make sure to account for all the power tubes.  Their combined current draw will inform you how to spec your PT for voltage & current.