Big Muff Pi MOD - Transistor to Tube - 12ae7

[Will1001001]

I just modded a Big Muff Pi. I took out the 4 transistors and replaced them with 2 12ae7's. For those who don't know the 12ae7 was designed to run from a car battery at 12 volts, they also allow for voltage fluctuations as expected from a car's electrical. I also had to replace 2 stock resistors with jumpers to get the gain up. It sounds great. In case you are wondering, I tried to swap the tubes to 12ax7's and it sounded horrible, so even though the 12ae7 is a lower mu tube, it still has better gain at this low voltage. I hope this helps other people who are trying to make a low voltage tube pedal.

[llmdcll]

That's really cool man I've been building for a while and I'm just now geting into tubes how did you wire the tubes to replace the transistors? And I'm assuming the 12au7 will work as well right?

[amptramp]

Good to see an update of a useful circuit!  Car radio tubes have achieved a niche popularity for some things:

http://www.sophtamps.ca/mambo/index.php?option=com_content&task=view&id=25&Itemid=37

Here is a listing of tube types:

http://www.angelfire.com/electronic/funwithtubes/12v_tubes.html

Tips and projects here:

http://www.junkbox.com/electronics/lowvoltagetubes.shtml

The nice thing about these builds is you can use tubes with the relatively small capacitors used for solid state, so component sizes are smaller.

[PRR]

> a lower mu tube, it still has better gain at this low voltage

Low voltage means low-low current and very-very low power output.

(We don't usually compute "power output" in small-signal stages, but it IS work to smack the next stage, even if only nano-Watts.)

Low Mu is better conductance, more current.

In particular: if you have retained the transistor-value resistors of the BMP, a 60K-100K impedance 12AX7 will be essentially "off", the 12AE7 will be passing some useful current.

Another thing: tubes have "contact potential" which is an uncertain error of bias. Often around 1 Volt. If the supply is 12V and the Mu is 100, the computed bias is much less than the contact potential variation. The tube may not turn on at all, or may gush more than you expect. There's an obscure old "rule" that Mu should not be greater than B+ (ie, that we do not expect 1V certainty of bias).

> 12au7 will work as well right?

Probably not. The 12AE7's sections have very much better conductivity than 12AU7, and are also processed to reduce contact potential. (That is why the maximum plate voltage is 16V-- any more might strip-off the delicate surfacte treatments.)

[llmdcll]

That's cool I'll have to look more into that tube I wanna build more tube circuits and this seemed like a really cool project. I've been reading more about the safety precautions for dealing with high voltages and I've been interested in the tube projects at low voltages to get started out. Plus I love big muffs now I went from never playing one to falling in love with them so this thread seemed cool. I did think it was funny that it was old and no one had responded so I didn't know if anyone was gonna reply haha