Quick real mc-tube question

[cakeworks]

Sorry to clog up the forum once more but I have a little real mc-tube question (i'm gonna try build one today) I'm looking at the tonepad layout http://tonepad.com/getFile.asp?id=43 And I'm wondering what the what's with the led and resistor just left of the step up transformer? what's it for and how would it be wired?
oh crap, just realised that the tube is powered by the 120V coming from the transformer... would this mean I would only make the first transformer 240V (australia) rather than both?

thanks

[brett]

Hi
That LED indicates when the circuit is being used (ie not bypassed).  For Australian 240V supply, to get the standard McTube voltages use a 240:12V transformer to step down and a 24:240V transformer to step back up.  Because the second one is only getting 12V in, it will only put 120V out.

Have fun, it's a great build.  PS you can also use 2 x 12V transformers and use 240V to get B+.  It gives less distotion and more gain.

Don't forget how dangerous this project is.  EVERY time you work on it after powering it up, you must discharbe the B+ caps by shorting B+ to earth (they're conveniently next to each other on the signal board).  On mine, it takes ages (an hour?) to dissipate the B+ on its own.

cheers

[d95err]

Don't forget how dangerous this project is.  EVERY time you work on it after powering it up, you must discharbe the B+ caps by shorting B+ to earth (they're conveniently next to each other on the signal board).  On mine, it takes ages (an hour?) to dissipate the B+ on its own.

Shorting will probably wear out the caps and electronics if you do it a lot. Put a resistor, e.g. 500k in paralell to the main filter caps, and you'll see B+ drain in a few seconds or so when you turn it off. That's what's usually done in (DIY) tube amps.

EDIT: Looking at some tube amp schems, a 100k resistor would be enough (the lower, the faster it will discharge the caps).

[brett]

Yeah. that's right.  Use a resistor.  In the McTube you can spare at least 2mA, so use 140V/2mA = 68kohms.  140V x 2mA = 0.28W, so use a 1/2W resistor.
My DIY amps have 220k 1W resistors in them, and drain reasonably quickly. 
But I DO short them as a safety thing.  Caps cost $5, life is worth....?

[cakeworks]

Hey thanks guys. Random question: I have bought a pre-sensitised pcb for this (god knows why!) and I don't have any OHTs so am i able to just draw the schem straight onto the film covering the copper?