Germanium Big Muff Pi. Diode trick to "increase" Ge Vbe.

[mac]

I always wanted to make a BMP with gainy Ge but the problem is that the low Vbe of Ge forced me to re bias the whole thing, even the interstage cap and resistor.
Recentrly a realized that adding a diode, and a big cap to bypass it if neccessary, at the emiter worked on a Jordan Bosstone.
I triedit on a BMP triangle version and it worked also. I used Matsushita 2sb176 hfe around 200, leakage near 200uA. I replaced the 100R resistors at the emiters of Q1,2,3 with 1N4001s. This diodes let me used the original bias. With others Ge I had to tweak the collector resistor to get 4V at Q1,3 but not much. Also the on resistance of the diode acts like the original 100R resistor.
Same for the recovery stage and Q2.
The trick is this:
In the silicon version the collector of Q1 is at near 4V so ic=5v/22k=0.22ma. The base sits at vb=5v/22k*0.1k + 0.6v=0.622v.
The current trhough the 82k is (9-0.622)v/82k=0.1ma.
A Ge transistor has a lower vbe, so to use the original bias, or something closer, the base must sit at near 0.6v. Adding a diode helps. Now the base is at vb=vbe + diodeVf ? 0.5 - 0.7v. With the proper diode one can get vc=4v without tweaking resistors.
Well, that is. I guess the trick can be applied to another projects. It is simple. It worth the time.

I breadboarded a demo today and it sounds nice. And voltages are within the normal Si ranges.



mac

[DDD]

The most Ge-based stompboxes are temperature-dependent. Adding a Ge or Si diode to the emitter circuit one makes the temperature influence on the stompbox parameters two times (approx.) more.
So pls be aware of it ;-)

[mac]

Thanks DDD, I forgot to mention that.
But to my surprise, I finger heated the transistors and I got only a 0.2v decrease at all collectors.

mac

[MikeH]

Next project:  Climate controlled pedal board

[mac]

Next project:  Climate controlled pedal board

I guess the best place to put a FF is inside the amp in thermal contact with the power tubes... 

mac

[Dragonfly]

I've done that in FF's before with decent results...a search of the forum should yield some results....i used germ diodes in a silicon FF though....

Nice job...hadnt tried it on a BM !

AC

[MikeH]

Next project:  Climate controlled pedal board

I guess the best place to put a FF is inside the amp in thermal contact with the power tubes... 

mac

That does remind me of something; I brought some of my DIYs to a friends practice for him to try out, but it was in the winter and they practiced in a garage.  Tried out the Ge fuzz face, and p-p-p-p-p-p-p-pfft.  Sounded like crap.  I set it on top of a space heater for about 5 min, and bam, it sounded like GOLD.