Germanium Jordan Bosstone and some mods.

[mac]

I adapted the original JBT to fit Ge transistors, and find some mods while tweaking.



The original uses a Si complementary darlington pair. The current across the 150k resistor is aprox 0.6v/150k=0.004ma. Since ib1 is small then the voltage drop across the 560k pair is aprox 0.004ma*2*560k=4.48V, and the emiter of Q2 is at 5.08v aprox.
Also ic1 is, neglecting ib2, 0.6v/18k=0.033ma.
Since Ge Vbe is smaller than Si, Q2's emiter should be lower in a Ge version. And leakage could make things worse.
To have the same currents I added Si diodes so as to keep the same voltage drops and currents I estimated above. And bypassed the small resistance of the diodes with big caps to increase gain.

I tried some Ge transistors in the range hfe: 50 - 150 and leakage: 20ua - 200ua. I had to adjust the 150k to values between 100k - 120k. The less leakage the more I got closer to 150k. I tuned Ve2 to 4.5v.

As always temperature is an issue. When Q1 is finger heated Ve2 falls fast. Q2 is happy no matter if it is cold or hot. Making the 150k an external 250k pot is one solution.

Mods: (also works on the Si version)
1) Making the two 560k a 1M-B pot changes the freq response of the filter. Gain is reduced as the pot is turned up or down. More treble when the cap is on the base side, and more bass when it is on the collector side.
2) Add a pot, say 100k-A, from the junction of the 47pf and the 18k on Q1 collector to the 18k on Q1 emiter. As the pot is increased the gain is reduced. I find this mod useful with the Si version. The cap from the union of the 560ks to gnd not only increases the gain but the bass content. At full gas it sounds muddy, imho. So reducing the final gain a little avoids this situation. But this is my personal taste.

I'm not an enemy of Si, the JBT sounds good with Si, but I prefer the sound of Ge. This is why I adapted the Big Muff, Bazz Fuzz and other circuits to use Ge.

mac

[John Lyons]

Nice one! I'll have to give this one a shot.
Thanks for posting in these uncertain times!!!

John

[Stompin Tom]

Thanks! I'm looking forward to hearing this one... I just need to find a npn Ge... got a pile of good unused pnps.

[brett]

Wow
I have wondered about this many times and you've done the hard work.  Glad to hear that it sounds good.
Have you tried it without the 1N4048s on Q1 and Q2 ?
cheers

[mac]

Wow
I have wondered about this many times and you've done the hard work.  Glad to hear that it sounds good.
Have you tried it without the 1N4048s on Q1 and Q2 ?
cheers

I tried without the diodes a month ago. I had to reduce the 18k on Q1 collector to almost 1K, and tweak the 150k until I found bias. Sadly I did not take notes at the time because I redisigned the whole thing until I got something similar to the Bazz Fuss but with this complementary darlington pair.

Thanks! I'm looking forward to hearing this one... I just need to find a npn Ge... got a pile of good unused pnps.

IMHO, the npn should be low gain, hfe near 50 - 70, and the pnp up to 150. I tried with a matched pair, 2n388 and 2sa49, hfe: 55, leakage<40ua. Who needs more?

mac

[markm]

Nice one! I'll have to give this one a shot.
Thanks for posting in these uncertain times!!!

John

+1!
This thing looks cool! 

[mac]

My only complain about the JBT is that it gets a little dark when reducing the guitar volume. Not bad, but not like the FF. Maybe tweaking a cap here and there...

mac

[Mark Hammer]

Very smart, and well-explained.  Thank you.

If you have not tried it yourself, Aron suggested increasing the value of the .022uf input cap a few years ago, and I find it produces a very interesting and nice change in tone, almost like an octave divider on the lower strings/notes sometimes.

[mac]

Very smart, and well-explained.  Thank you. icon_smile

If you have not tried it yourself, Aron suggested increasing the value of the .022uf input cap a few years ago, and I find it produces a very interesting and nice change in tone, almost like an octave divider on the lower strings/notes sometimes.

I did not try increasing the input cap, but increasing the cap to gnd from the divider. It brings some delayed octave down notes sometimes, and more gain. Could be nice for a bass fx. 

------

I wonder if this could work on another discrete Si based fxs, so as to convert them to Ge? Going to try with a Big Muff.

mac

[John Lyons]

You could aways put in a treble bleed cap across the wiper and top of the pot to brighter up the lower gain settings. 100pf of so.


John

[mac]

You could aways put in a treble bleed cap across the wiper and top of the pot to brighter up the lower gain settings. 100pf of so.

Do you mean the guitar pot? I've done it. But I test things without the cap.

mac