RealMctube on 240v

[luke691]

Hey, Im planning to build the Real Mctube 2 according to the tonepad layout, however would like to use it as a clean-erish pre-amp.
Living in Australia Im unable to get 120v transformers as the layout suggests, and was hoping instead to run the cicruit on 240v, to increase the headroom (which I want anyway). Any ideas as how to do this.

Also would it be possible to run the Real Mctube off of a 12vac plugpack, I may need to do this as Australian schools (I'm 15) wont allow us to work with mains voltage (there still unhappy about me using a step-up).

Thanks for the help.....

[brett]

Hi
the 12VAC plug pack works fine.  Just use a small 12VAC:240VAC transformer inside the box to get 240V out.

Don't forget:
*The 240 VAC that you'll be dealing with, and the B+ of about 350V, are EASILY able to kill you*

If you still want to go ahead, you'll need to re-bias the grids, too - because they are set to be mis-biased (causing distortion) for 120VAC.

If you are up to a bit of technical learning, you could investigate bias and "load lines" for the 12AX7 tube.  Otherwise, check out the values for the plate and load resistors used in cleanish amps, and copy them.  Personally, I think the old Marshall 18 watter pre-amp would be hard to beat.  The 18 watt "lite IIb" (I think?) schematic is available at 18watt.com.
cheers

[andrew_k]

Don't worry about the 240v, just buy a 12v AC power pack form Jaycar /altronics/rsaustralia or where ever.
THe circuit wants 12v AC, it doesn't care what's on the other side of the transformer.

[edit] Here you go - http://jaycar.com.au/productView.asp?ID=MP3020&CATID=27&keywords=&SPECIAL=&form=CAT&ProdCodeOnly=&Keyword1=&Keyword2=&pageNumber=&priceMin=&priceMax=&SUBCATID=521

[frankclarke]

I built my McTube at 240V or so, nothing wrong with that.

[luke691]

Thanks Frank.
Did you make any modifications to the circuit?? Did it make any diference to the sound??

Btw nice site, just checked it out...

[Jimmy-H]

Hi Luke,

Maybe it's an idea to make the vanilla overdrive from Tube-Town. http://www.tube-town.net/diy/lov/lov01-vanillaoverdrive-eng.html
It can be used as clean boost to mild overdrive.
And it works on 12 volt DC

Whatever your build is, success with it !! 

[brett]

Or check out the excellent thread concerning 12AU7 and 12U7s and others at 9V.  (Most of the thread is now about using 12V)

[luke691]

I've seen the thread (looks like fun) but I already have the 12AX7 and was aiming to use it as a clean amp, ive already made a couple of boards, using a roland engraver.

Ill definetly make the 9v AU7 or the Vanilla eventually, still heaps of stuff on the list.

[luke691]

Brett, I checked out the 'Lite IIb' at 18 Watt. It uses a single 120k, resistor to pins 1 & 6, would this be appropriate, if not, any idea of what would be??? Thought I'd make sure so I didn't destroy anything....

Also I was thinking of a power stage and realised I had a couple of LM1875T around, could I possibly power this from the same plugpack???

Thanks again..

[brett]

Hi
yes, that single 120k plate resistor (pins 1 and 5) and an 820 ohm cathode resistor (pins 3 and bypassed with a 47uF cap would make a good clean valve boost with just a hint of distortion when pushed hard (check the pin numbers, as I'm not sure about them).  Use 12V DC for the heaters.

The tube will use a fair bit of power (about 500mA), so you won't be able to power anything else with the plug pack

You'll need a simple buffer between the tube stage and any power amp stage.  Take any common BJT (2N5088, PN100, 2N2222A, etc).  Bias the base to half the supply voltage with 2 x 220k resistors, and use a 10k emitter resistor.  (check out the AMZ-FX buffer page for details).

Also, the LM1875 only puts out 5 W with a 20V supply (it needs 16V for a minimum 1W), but puts out 25W with a 50 V supply.  IF and only IF you will be careful and safe with 240V AC, you could use a 240:35 or 240:40 500mA transformer and a simple power supply (diode bridge and 2 capacitors) to get your 50V.
have fun