McTube: More mods and test results

[brett]

Hi
It's been a while since I built my Real McTube (see tonepad.com or search the forum for details of the McTube).
Over the last 12 months I've modded mine a fair bit and have finally arrived at tones that I'm really happy with.

First, I increased the B+ voltage from 120V to 240V by replacing one of the 12V transformers with a 24V transformer.  That gave more headroom (from almost none to almost a little bit).
Next came mods to clean it up, coz in standard form it's easy to get death metal tones, but hard to get modest overdrive tones.  The large amount of gain (60 dB !) also emphasised thermal noise and hum.

My final configuration is for a 470k plate resistor on the first triode (instead of 680k).  Also, a 4.7k cathode resistor on this triode makes the bias a bit more normal than the standard 15k value.  On the second triode, I changed the 330k plate resistor to 100k (and left the cathode resistor at the standard value of 4.7k).  Overall, gain is reduced by a lot, and this makes hiss and hum less noticable.

It now has a great range of tones.  With the drive control at 12 o'clock, it sounds a bit like a tubescreamer, but in a creamy, tubey way.  With the drive control at 3 o'clock, the tone is like a high-gain tube pre-amp.

I've gone from having significant doubts about the value of this pedal to really loving it.  I prefer the tones I get from the McTube and the clean channel on my amp (Marshall JTM-60) to the high-gain channel.

cheers

[swt]

cool!!. i've always wanted a tube pedal distortion...but i always end using the distortion and od in the tube amps i make. will you suggest this build or the matchless one?. what are your plate voltages?. tube choice?. thanks a lot for the report!!

[swt]

or the shaka tube??

[Pushtone]

Very detailed and specific notes, and a timely too. Thanks! 
Your mods sound just what I was hoping the RealMT would be.
Thanks for saving me some possible disappointment!

I wasn't going to beadboard this one because of the high voltage.
And now I'm thinking of just going with your mods.

snip...
First, I increased the B+ voltage from 120V to 240V by replacing one of the 12V transformers with a 24V transformer.  That gave more headroom (from almost none to almost a little bit).
Next came mods to clean it up, coz in standard form it's easy to get death metal tones, but hard to get modest overdrive tones.  The large amount of gain (60 dB !) also emphasised thermal noise and hum.

A couple of questions if you have the time...

Having confusing time locating suitable power transformers.
Filament current voltage, Plate voltage, center tap, what matters?
Could you recommend transformers to go with the 24v mod? 

What about this one? http://www.alliedelec.com/Search/SearchResults.asp?SearchQuery=227-0001&Submit.x=0&Submit.y=0


About the B+ voltage increase. I guess it goes with out saying but bear with me...
should the capacitors all be increased to a rating higher than 24 volts? (Duh)
What about the resistors? Is it safe to use all 1/2 watt resistors with a 24v transformer.

Glad to hear your happy with your build and mods brett.

[Pushtone]

Never mind, found the answer back on Fred's page

"If your wall-warts contain two diodes and a centre-tapped transformer instead of four diodes (no centre-tap), wire the filament portion of the power supply as shown below. (Full size, printable version of the image is available from the Resources section.) Note that the centre-tap of the step-up transformer T2 is left unconnected."


http://www.dogstar.dantimax.dk/tubestuf/mctconst.htm

Still hoping for a recommended power transformer. Anyone?

[brett]

Hi
Because of the low power consumption and need for small sizes, the best transformers come from wall-warts (500mA). 

To step up a 120V supply to 240V (which will give you less crunch and a more open feel), get T1 from a 12V wall-wart, but get T2 from a 6V wall-wart.  That way, your voltages will go 120V=>12V=>240V.  This is because the rated voltages are just ratios: 12V into a 6V:120V transformer provides 240V out.

The only special measure I took with the transformers was to keep the 12V AC lines short.  That's because they carry much more current than the high voltage lines, and therefore can induce much more hum if the wires go near sensitive parts of the circuit.

Just to be sure, I used 630V rated greencaps and 450V electrolytics in my 240V version.  Cost maybe $2 more.   Also, always check the voltage at Pin 4.  If it is more than 13.5V (as it was in mine), increase the value of R2 by a few ohms (mine is 18 ohms), so that the heater voltage isn't too high and shortening the life of your tubes.

I currently use a Sovtek 12AX7LPS.  But I'd recommend the JJ Tesla ECC83-S.  For extra gain (average 7%) the Sovtek 12AX7WB is an option.
cheers

[frickecello]

Check this out, the Nice Girl Hard Driver by Louise Goedhoop! its the best sounding tube pedal Ive built:

http://www.paginasprodigy.com/guitardoc/nicegirlharddriver.wmf

[Paul Marossy]

Cool. Thanks for sharing. 

[blindsjc]

I know that's a 2006 topic but it appears to be
a serious about mctube mods. So, I'm searching
for more gain on my mctube, I dont know how to
explain exact, some more particles on the sound,
some more fuzz. Any suggestion?

I used some brett mods in reverse way, and it
sounds good. I think that this effect can be good
for 70's metal like sabbeth and alternative dirty
music if we can get more particles in the sound.

[blindsjc]

Nobody helps anymore? Whats going on?
I think that this is my third post with no reply.