McTube idea

[brett]

Hi.
I'm about to build a real McTube.  http://www.tonepad.com/getFileInfo.asp?id=43
Instead of the 2 internal 12V transformers, run back to back (120VAC stepdown to 12VAC => 12VAC stepup to 120VAC), I was planning on using a wallwart (12VAC 500mA) instead of the first transformer.  The advantages I envisage include less space/smaller box, no fuses, and no cable clamp.  But I wonder whether running 12VAC at about 400mA from the wall to the box isn't a recipe for hum or some other problem.  I'm not worried by the small voltage losses in the longer 12V wires, as reports indicate that a low B+ voltage is advantageous in this circuit.

any suggestions?  just try it?

[JimRayden]

Been there, done that. I guess you would be just fine unless you wrap the 12VAC supply cable around the tube itself.  Just keep the AC and audio signals apart and you'll be fine. Oh, and also when connecting AC wires inside the box, make sure you either twist the +/- wires or tape them together. They tend to neutralize each other's magnetic fields, resulting less hum.

Good luck!

--------
Jimbo

[MartyB]

   In Fred's original article he mentions this is an option, so I guess he tried it.  Don't know how it affects hum issues.  My builds were all internal trannies - one less wall wart.  It'd be tough to get this into a box the size of a 1590BB anyway.  UI've used an old covox speech synthesizer box, an a/b printer switchbox and a military surplus junction box.  They were all roughly  2.5 x 5 x 7 inches.  RIP Fred.

[brett]

Hi.
Thanks for that.  I've read the McTube V2 article, not the original, so I missed the comment about using a wall-wart.

Another option that I just thought of is to use the standard stompbox 9VDC supply (not a small battery - you'll need about 400mA of supply for this, mainly coz the heaters need 300mA).  Run the heaters in parallel  using DC.  Then, to get B+ use a 555 timer to convert the 9VDC to a 9VAC square or sine wave.  Then step up using one of those little 8ohm:1kohm audio coupling transformers.  They're rated at a massive 350mW (we only need 50mW here), and as long as their insulation is ok, they would be great.  The 8:1k transformer has a voltage step-up factor of about 11, so 9 V would get you to about 100VAC, which rectifies to just the right B+ voltage.  Hmmm....

cheers

[gez]

Then, to get B+ use a 555 timer to convert the 9VDC to a 9VAC square or sine wave.  Then step up using one of those little 8ohm:1kohm audio coupling transformers. 

Better to step up with a mains transformer.  The load on the primary is going to be greater than you think and you could well exceed the power rating of an audio transformer (though I haven't seen the schematic, just playing safe here).

Plus, the 555 might need buffering with something that can source a bit of juice (MOSFETs for example).  One approach I've seen recently is use a power amp chip as a square wave oscillator and feed it to a step-up.  Another approach is just use an AC wall wart with a step up - wall wart to heaters, stepped up voltage gets rectified and supplies the plate(s).

[bancika]

What about safety issues, when you use two trannies you can connect safety ground to chassis, but when you connect just 12V input you can't. Can that be a problem?

[brett]

Hi.
The 12AX7 only needs about 0.3 to 0.5 mA at 130 to 140V.  The input current on the audio tansformer would only be about 5mA.  I'm fairly sure that it could handle that.  It is the voltage that would worry me.  It's designed for tens of volts, not hundreds of volts.

In any case, I'll be building the wall-wart and one mains transformer version.  The audio transformer idea is more theoretical.

Even with a wall-wart, there's no safety earth.  However, here is Australia almost all houses, and many buildings have earth-leakage protection, which is a really good safety system, unlike earths and fuses.

cheers