BK Butler/Chandler Tube Driver current draw?

[SpufuZ]

Im about to build a power supply and want to make sure i dont order a transformer that dont have enough power.
The power supply will feed one EHX Big Muff and one BK Butler Tube driver.

The Big Muff should be around 8mA but i cant find any info about the Tube Driver.

Im going to pick a much larger transformer than i need in order to be able to add more circuits to the power supply.
But how do I translate mA in to VA!?

EDIT:
After a quick google search it seems like the formula for calculating VA is as simple as: VA = V x I.

But my transformer will have 12VAC out, regulated to 9VDC for the Big Muff.
Should i calculate from 12V or 9V?

//David

[kurtlives]

Dosen't the TD use a Bi-Polar PSU?

The tube and op-amp wont be drawing more than 10mA combined.

The heaters are where the current is being sucked. I forget how the BK wires the heaters but they will either be using 150mA or 300mA.

[SpufuZ]

Take a look here
http://www.generalguitargadgets.com/diagrams/tube_driver_sc.gif
and you will se how the power supply is managed.

Since total current will flow through R18 wich is rated 1W.
The maximum current draw would be around 80mA before R18 burns up right?

1 / 12 = 0,083

[kurtlives]

Ya correct. Obviously max current is never reached so 1W is dissipated. A good designer will go to about half of a part's rating (I go higher myself).

So anyways, your looking at a low current draw.

The schematic does not show the heaters. The layout does. The heaters are being used in series, 150mA then.

[KazooMan]

There are many BK Butler Tube Driver schematics floating around.  I got the one I followed from the ....org website.  Here it is.



As was mentioned above, the heaters are what will be drawing the most current.  Note that in the schematic I posted the heater supply does pass through a 10 ohm (power not given) resistor.  In the schematic in the GGG link the heater supply IS shown, it is right across the secondary of the transformer.  It does NOT pass through this resistor.  Confusing stuff!

In the numerous notes in the discussion for this build there are comments quoting an old article where a 200 ma transformer is mentioned.  Elsewhere, a 500 ma transformer is recommended.  I used a 500 ma Hammond and it works just fine.  Probably major overkill for the job at hand.

[SpufuZ]

That looks very good!
Yeah thats right about the heaters. Did not think of them! 

My tube driver will be sharing space with a big muff in a 1HE 19" unit.

Ive searched through my stuff here and found a 230V pri, 12V sec. transformer.
Its 8VA wich will be about 666mA. So i think it will be enough.

[PRR]

Heater is nominal 12.6V 0.15A, and may be AC.

The DC supply is 12V AC doubled to roughly +/-18V DC, 36V total.

What sucks DC power?

Opamps. Look up TL072; it is around 5mA.

Tubes. The datasheet is NO help because we are so far away from "typical" voltages. Also one sheet shows cathode bias (common to both cascaded tubes, an unstable affair), the other is zero bias; one shows 68K plate resistors and the other shows 47K. Exact analysis is hit-or-miss, but worst-case analysis is easy. Assume the tube "could be" dead-short. Then the maximum current is 36V/47K or 0.75mA. Times two is 1.5mA. The actual current must be less. Do we care? Let's see how bad things look.

36V (total DC suply: 5mA opamps, 1.5mA tubes, 6.5mA total.

To get back to the transformer AC rating: first double this to account for doubler-action, 13mA. Then multiply by 1.6-1.8 to account for AC-DC conversion stress. 22mA.

Total AC current is 150mA + 22mA or 172mA.

Transformer VA rating is 12.6V*0.172A = 2.17VA.

You can hardly buy a transformer this small. The 12V 200mA part is in excess of the required 172mA and will be fine. The 500mA may be easier to find and/or cheaper.

> Probably major overkill for the job at hand.

Yes, but, it's like: you need an 1/8-inch hook to hang your guitar, but there's a 1/4-inch hook in your spare-screws bin, so that's what you use.

Notice that with over-kill, a heavier guitar or a mistake in computing the opamp or tube load does not mean a bent hook or a burnt transformer.

> the heater supply does pass through a 10 ohm (power not given) resistor.

Little transformers sag. Actually a "12V" trannie is wound for 14V-16V no-load, and sags near 12V at maximum load. Since you won't have a custom exact-right transformer (values of 200mA, 500mA, and 660mA are mentioned here), the transformer is UNDER-loaded and you wind up a little above 12.6VAC at actual load. 13V or 13.5V is not worth fretting about. At 14V or more I would consider reducing the voltage. (At 15V the heater life would be similar to a light-bulb, a thousand hours.) The heater is just a hot resistor, and the easiest way to waste-off some voltage is with a series resistor. At 150mA 0.15A the 10 ohm resistor loses 1.5V, which is probably a good value for many situations.Build it, measure voltage AT the heater, and fudge this resistor to stay above 11 and below 14V.

[sgmezei]

Hey folks I was looking at building one of these. I have looked through the forums trying to find some good layouts with no luck. If anyone could send me a link for the butler or recommend a different nice tube preamp/overdrive pedal it would be most appreciated. ~Scott~

[KazooMan]

Here's a link to a long thread on another forum.  This is where I got my information.

http://www.freestompboxes.org/viewtopic.php?f=13&t=1171&start=0&hilit=butler+tube+driver

Read through the entire thread before you dive in.  There are many good ideas and modifications presented along the way that you might want to learn about. 

As mentioned in the original posting in that thread this is not a beginner project.  Two major issues are (1) working with line voltage and (2) the double sided pcb. 

I have hand my fingers in tube amps and a ton of other electronic gear so item (1) did not concern me.  If you haven't worked with line voltage you should be very careful and perhaps seek some advice from a friend with some experience.  Put this pedal in a metal enclosure and mess up the wiring and you could have a real shock hazard on your hands.

The double sided pcb was a challenge.  Getting the two sides in proper alignment was not easy.   It took me a few tries before I got a board I was satisfied with.  You also need to watch out for places where there is supposed to be a through board connection and for a few places where connections must be made in the proper sequence so that the pads don't get buried (I recall one that is under the tube).

[KazooMan]

Oh my, this forum won't allow me to post a url link to the "other" forum.  

http://www.....org/viewtopic.php?f=13&t=1171&start=0&hilit=butler+tube+driver

You will need to replace the three dots between www. and .org with the name of the other forum.  It would have something to do with "no cost" stompboxes.

Hope I don't get banned for this.

[sgmezei]

Thanks man, it is most appreciated