Real McTube Questions

[skumbeg]

I just built the Real McTube using the layout from Tonepad.  It sounds great, nice distortion sound for guitar and very low noise and hum.  I would love to get as much clean gain as possible and was wondering what quiescent plate voltage and current I should bias for.  Right now the first stage (pin 1) is 104V and the second (pin 6) is 45V with the B+ of 135V.  Should I try to aim for half the supply voltage with tubes??  I will probably remove R9 and change the pots to 1M as I read others have done in previous posts.  Can I replace R8 with a jumper?  I also read that some have replaced the 12V xfmr with a 6v.  Can a 6V transformer withstand this?
Thanks for your help.

[Dragonfly]

  I would love to get as much clean gain as possible .....

i would start by swapping to a 12AU7, 12AY7, or 12AT7 to get more clean gain...the 12AU7 is the lowest output of the 3 (IIRC)...they should just "drop right in"....

AC

[skumbeg]

I will try that first before I make any changes to the circuit.  Just in general though with tubes do you want the plate to be biased at half B+ for max signal swing without clipping? 

[brett]

Hi
From memory of messing with the McTube:

I thought a 12AT7 didn't sound very good in the McTube.  I guess that the bias point wasn't quite right.  I didn't try a 12AU7 for some reason.  Looking back, I should have, because they often sound better to me than 12AT7s in circuits designed for 12AX7s.

You might decrease the extreme amount of gain by reducing the plate resistors to about 100k each.  That has a big effect.  The cathode resistors will then also have to come down in proportion.  

Another approach, that I found useful, is to increase the B+ to 260V.  Don't forget that this change greatly increases your chance of injuring yourself.  Assuming your supply and plate caps are rated at 400V or above, you can replace the step-up 12V transformer with a 6V one, and get a B+ of about 260V +/- 20V.   After this change, you'll need to reduce R5 from 15k to about 4.7k, so that the bias point is OK.  Pin 3 on the tube (cathode A) should be about 1V.  If it's close to 2V, decrease R5, it it's less than 0.5V, increase R5.
cheers

PS The plate voltage idea is more-or-less correct.  But the non-linearity of tubes sometimes mean that you might be better biasing the grid about halfway between cutoff (about -2V in the RMcT) and saturation (0V).  Also, you don't want too much gain.  Hence my suggestion of dropping to 100k plate resistors.

[skumbeg]

Cool.  I will bring down the plate resistors to somewhere between 100-200k and adjust the cathode resistors to get -1V on the grid.  Can I jumper R8 or make it a much smaller value like 1k?  I don't mind the high gain and distortion I just want to get as much clean gain as possible before it starts to break up.  If there is too much gain maybe I will drop it drastically by removing C7, C9.  As far as the 6v xfmr, the build fits so nicely in the box and the caps fit so perfectly on the tonepad layout that I will probably just build another one with the EH xfmr and make it the clean version.   Thanks for your help, any addition input is greatly appreciated.  Thanks to tonepad for the great layout.