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Old 19th June 2008, 05:04 AM   #1
gary h is offline gary h  United States
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Default calculating transformer current draw question

Hi all,

I'm trying to design a PS for my new Aikido line stage. My (first of many) question is this:

JB's 6SN7 specs with a 300V B+ and cathode resistor values of 1k/input 470/output state current draw to be 4.5 and 7.2 mA respectively. To find a transformer current value do I just add these Ik values up? If so, how then do I calculate peak rating? How much overkill on transformer output is considered a reasonable margin? I notice that Hammond makes 300V transformers with a 6.3V secondary giving a wide range of current values...and then there's the impedence to consider.

As soon as I get my hands on a PC I plan to download PSU DII to help me figure how caps, chokes, etc. will effect this balancing act, but I feel I should have some transformers in mind before I do so.

Answers to these and the inevitable newbie questions that will ensue are greatly appreciated. I would also love to hear from those who have undertaken this Aikido PS project and could recommend designs and part sources.

Thanks,

gary

gary
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Old 19th June 2008, 06:24 AM   #2
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Poll..anyone interested in an Aikido linestage PCB group buy?


Tube Preamp Module and Power Supply PCB's
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Old 19th June 2008, 09:15 PM   #3
Merlinb is offline Merlinb  United Kingdom
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Default Re: calculating transformer current draw question

Quote:
Originally posted by gary h
Hi all,

JB's 6SN7 specs with a 300V B+ and cathode resistor values of 1k/input 470/output state current draw to be 4.5 and 7.2 mA respectively. To find a transformer current value do I just add these Ik values up? If so, how then do I calculate peak rating? How much overkill on transformer output is considered a reasonable margin?
First, assuming you're using an SS rectifier, then the ac voltage on the transformer should be roughly 300V * ^/2 = 210Vac. (but say 220V to 230V to be safe- it's easy to drop the extra voltage).

The circuit will consume: 300 * 11.7mA = 3.51W. Call it 4W to be on the safe side. (No need to calculate peak values, it's a class A design)

It's usual to use a transformer with a VA rating of at least 1.5x the power consumed, to allow for the crappy power factor produced by rectifiers, so 4 * 1.5 = 6VA.

So the tranny should have a current rating of at least 6 / 220 = 27mA.

So call it 30mA. (double if there's another channel to power)
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Old 20th June 2008, 02:08 AM   #4
gary h is offline gary h  United States
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thanks for the replies,

suppose I use a tube rectifier instead of a diode bridge, how does this effect the voltage drop from the transformer?

Judging by Bas Horneman's PSU schematic, do I understand correctly that it is common practice to bleed off some of the B+ DC coming from the rectifier tube to supply the filaments for the line stage tubes? Does getting another smaller transformer to supply the 6SN7 filaments have any advantage or just create unnecessary expense etc?

thanks,

gary
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Old 20th June 2008, 04:12 AM   #5
BudP is offline BudP  United States
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gary h

Download Duncan Amps PSUD2 power supply simulator. Be prepared to alter the transformer values once you have your load circuit working properly, then iterate the circuit load. This will provide you with realistic current draw numbers to allow you to spec the power transformer exactly. When using a FWB the power transformer can see a 1.9 form factor in DC to AC ccurrrent draw, so don't use DC current specs to size the AC power you need.

http://www.duncanamps.com/

Bud
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Old 20th June 2008, 01:14 PM   #6
Merlinb is offline Merlinb  United Kingdom
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Quote:
Originally posted by gary h
thanks for the replies,

suppose I use a tube rectifier instead of a diode bridge, how does this effect the voltage drop from the transformer?
Then you'll need to get a higher voltage transformer to account for that lost in the rectifier. If using an EZ81 say, you might get about 20V drop, so add that to the necessary transformer voltage (Current requirements remain the same).

As for your second question, do you mean this schem?: http://basaudio.net/diyhosting/myown...ido_psu_v1.gif

R4/R5 apply a DC bias to the heaters to avoid exceeding the maximum heater-cathode voltage ratings of the valves, see also: http://www.freewebs.com/valvewizard/heater.html
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