Real Mctube

[bamwels]

Hi there,

I want to start building the Real Mctube preamp an I have some questions:

I am from Europe, so I have 220v not 120v. If I change the 1st transformer to a 220->12v and the 2nd to 24->220v it will be ok?

The 2nd question is about the layout. On the schematic it appear a R10 and a LED1 but on the layout it doesn't. Between points 4-9 and 9-5 is a resistor or is just a wire?

Thank you!

[duck_arse]

from this page at that site:
http://www.dogstar.dantimax.dk/tubestuf/mct-user.htm

Using the Real McTube II with 230VAC
Several people have inquired about how the circuit should be modified for use with 230 Volt AC ....

Regarding voltage: for 230 volt operation, the step-down transformer must be a suitable 230 --> 12 volt unit, at 500 milliamps. However, do not use an identical transformer as the step-up transformer T2; otherwise you'll have twice the B+ that the circuit calls for ....

For the second transformer you can either use a 115 --> 12 volt unit as specified, or use a 230 --> 24 volt at 200 milliamp transformer. Either option will give the proper 140 VDC or thereabouts for proper operation.

--- and ----

Duncan Eliot writes (21 August 2002),

"Thanks for introducing me to the wonderful world of tubes. Just a quick note on 240v operation after reading your feed back section. I found the simplest solution was to use a set 12.6V CT trannies. 240v into T1 with heaters running off the 12v side and then just feed 6.3 volts using the CT back into the second trannie.

in your case, treat 220V as 240V as 230V. so, yes, is the answer to the first q.

pins 4//9//5 are the valve heaters. they need to be wired as per the circuit diagram; there is a note on using twisted filament wires on the "building" page. the led and resistor are  hanging off wires/switch, with the led mounted on the front panel. I can't even see a layout diagram.

[bamwels]

http://tinypic.com/usermedia.php?uo=a6BMdyuA%2FVzfOfdH%2F7u7SIh4l5k2TGxc

Is this right?

[duck_arse]

those two short lines w/ the little peak in them are the circuit drawing way of representing the filaments, or heaters. the heater is a coil of wire, heating the valve kathode so it will emit electrons (under the right voltage conditions). so, you have to wire the filaments in the valve to the power supply and ground, using heavy gauge twisted wires.

and yes, they have neglected to include the led and resistor. you'll need to add a wire to the same track as R2//C4//pin4, and one to the ground trace.

is this your first pedal?

[bamwels]

First tube based pedal, and I want everything to work well from the first time

One last(hopefully) question: what does the LED does to the circuit? Is there just for on/off indicator?

I think I don't understand very good what you mean :"you'll need to add a wire to the same track as R2//C4//pin4, and one to the ground trace" .... can I just take out the LED? If I take it out I have to do something else?

[duck_arse]

the led is just there for indicator, yes. you can leave it off, makes no difference and serves no other purpose.

have you been across to the valvecaster/tremcaster threads here? lots of pics and ideas, but it's verrrrrrrrrrryyyyy long.

[bamwels]

Unfortunately no, but I will.

Thank you for the information!

[GibsonGM]

You might want to check out the valvecaster, etc, and try one at a lower voltage first.  It's safer, and you also get a better feel for what these circuits actually sound like. They are NOT screaming, high-gain metal circuits!  The do sound good!  And they definitely get you going and learning, so that later you can make one of those if you want to!    Great way to start learning about tubes, and the ones at 12V or so are very mod-friendly.

Same parts as the McTube except for the transformers, too, so you'd be going 'in order' towards a higher voltage build...

[bamwels]

I bought all the components and i started to test the output DC voltages. For the first transformer I've got 15.6V. It's too much? I should have 12.6V?

For the 2nd one I've got 145V DC. I guess that for the 2nd transformer the voltage is proper, but I don't know if is ok the first too. Any thoughts?

Thank you!

[GibsonGM]

Yes, it is high, provided you have everything connected up (both transformers).   

If it is high, and you are ready to run the heaters, you could put 5 or 6 power diodes (1N400x) in series to drop that down near 12.6. Your heaters will last longer if you remain within a volt or so of the ideal voltage.


 

[PRR]

> transformer I've got 15.6V. It's too much? I should have 12.6V?

UN-loaded?

Transformers sag under load. Small transformers sag a lot, as much as 20%.

You can run a "12.6V" tube at 15.6V for many hours, so I would go ahead and heat a tube, see if the voltage comes down below 14V.

I suspect at FULL load you'll be very close to right.

[bamwels]

Thank you for your answers! It was 15.6 unloaded. I connected the tube and the voltage was 11.8V DC between pin 4 and 5.

I also measured the voltage right after the C3, before the 2W resistor and loaded I have 13V DC.

Now...11.8V is too low? I should change the 2W resistor from 12ohms to a smaller value?

[PRR]

> 11.8V is too low?

Very acceptable for small tubes. They have a ton of reserve emission. You might drop the 12 Ohm resistor but it isn't urgent.

[bamwels]

Thank you!

[alfafalfa]

> 11.8V is too low?

I think you can reach your 12.6 by making that 12 ohm resistor a lot smaller.

In a preamp I made with an old transformer designed for 220 v ( nowadays over 230 V !) it took only two resistors of 0.33 ohm  to bring down the voltage from 7.15 to 6.3 V. I use 5 watt resistors however.

So bringing down that 12 ohm resistor may get you to a better voltage.
Just go in steps to get to the right value.

My 2 cents
 

[bamwels]

Changed the 12ohm with a 10ohm resistor and now the voltage is ~12.6V DC.