SRPP/Germanium Push-Pull Fuzz revisited

[Steben]

Updated version of my germanium push-pull:
Have to take some time experimenting more with this...
Two versions:


FIRST version: 47k/820k/47k biasing brings the bases at about +/- 0,45V. (could try 42k's for 0,4V). the cap across 820k creates shortcut for AC signal, allowing both Transistors to amplify a signal half.
As battery dies out, the biasing dies out too a bit.

SECOND version: schottky/820k/schottky biasing brings the bases at about +/- 0,4V. If this is exceeded by the input, the diodes clip and the circuit is less germanium working. With a single coil guitar signal this won't happen. But when battery dies out, the biasing doesn't change.

I guess first version is most versatile, allows big signals, without loss of Ge tone and gets some dying battery "sag".

[petemoore]

  Well it looks different enough [to what I've have/tried] and very interesting...
  I might just have to throw V1 down on a piece of perf  to see what it sounds like.

[Steben]

Question: is there a beneficial soul out there that is so kind to put this in a spice/simulation program?
It is not a hard nut to crack.
I'm not sure about the 2k7 resistors at the emitters.

[amz-fx]

Shouldn't there be a connection at the base/820k junction on the second drawing?

I suspect you will get better action with smaller emitter resistors. I would try 470 and then work down from there.

To put it into Spice you need a good Ge transistor model and those are scarce.

regards, Jack

[petemoore]

  'Wierd config for Ge's'
  Looks a little like a Mu amp of sorts, has it been tried?
  No large value resistors though...

[Steben]

Shouldn't there be a connection at the base/820k junction on the second drawing?

Correct it is. Guess I was drawing the "dots" but missed a spot  . I started out without "dots" as connection, you see...

  'Wierd config for Ge's'
  Looks a little like a Mu amp of sorts, has it been tried?
  No large value resistors though...

I'm dying to try it out , but last weekend was a tough one in time management since I threw a surprise party together for my girlfriend.

Push-pull is better than Mu-amp as a guide. Each transistor gets a half wave to amplify. As in a class (A)B power stage, but with connected collectors instead of emitters. The one and only thing I want to achieve is to get a symmetric gain in the squashy non-linear-germanium way as in the RM.

The input impedance is small yes, but that's because of the BJT's. A FET buffer in front should be quite appropriate.

[petemoore]

  So, it looks as cool or cooler than yesterday to me.
  Nice catch there Jack.
  I'd say one of us should try it out...
  A couple things.
  Since the + half of the input is going to PNP and the - half to NPN [or vice versa...[?] I think:
  Gain would have to be controlled outside the transistors 'or
  Gain of the transistors would do well to be matched 'or
  There might be enough headroom that it won't matter so much which is the gainier device.
  :To get ~symmetrical output.
  Gain isn't overrated, but Tone is underrated...or put another way, one can easily enough attain plenty of gain.
  I do like my GE deviced SB units,  this one, being a departure from any of the tried topologies for Ge implementation is at the top of the list of things to do when more is figured out, or get enough various Hfe, [low leakage I'm thinking] NPN's and PNP's in Ge form, to figure it out.

[johngreene]

This is what I get, made a couple of substitutions though.


--john

[petemoore]

  Looks like when I tried to cut a side piece to a cabinet with a hacksaw, not exactly square, but a rough square shape/
  Is taking voltages an option?

[Steben]

Hey John! that's amazing!

Some remarks: You used silicon diodes, right? Their 0,6V Brings the trannies more in "linear" zone.
And just maybe resistors instead of diodes (version 1) keeps it more dynamic design?
What input voltage did you use? Since it looks like the outer limit gain is produced. A volume pot in front of the circuit should definitely be there, you see.

Again: a saucerful of gratitudes

[Khas Evets]

John,

What spice program is that? Does it have germanium transistors in the component list? It may be time for me to switch programs.

[Steben]

Updated comments...
Mmm, looks pretty hard to get symmetrical soft-knee. It seems the lower tranny starts driving when the signal goes up (away from the soft-knee). Push-pulling kills te objective... I wanted it the other way around... Yet for squary lovers this is an ideal effect of course this way.
Still, in the square wave world it is softer than anything else, thanks to the GE capacitance...

I guess it needs decoupling the inputs from both trannies like in a more SRPP-muamp style.
working on it...
Thanks for pointing me in the right direction...

[Steben]

What about this.
Spice masters still enough patience to endure my requests? 
As I see this one, it is a buffer-push-pull. The lower tranny works as deliberately misbiased amp, resulting in (even mor) squashy response as in a RM at negative wave half. The positive wave half is inverted and then buffered by the second tranny which goes in squashy zone too. The point is achieving soft-clipped waveform without real to-rail clipping.

the upper 4k7 may be better if smaller, the other one bigger, to compensate the signal going to second tranny.

[Phorhas]

I'll give it a try on the breadboard later today

[Steben]

I'll give it a try on the breadboard later today

That's very friendly. It could be nice to have some potentiometers around. the lower 4k7 as a 10k pot for example.

[Steben]

Second circuit won't work either... sigh. Have a spice program on my own now.
Phorhas, don't try it...

I think Tim's "Cinnabar" is the closest to what I want to achieve, but I think of it with germanium...

The first push-pull fuzz above works better with the emitter resistors omitted and 4K7 resistors at the collectors instead (output taken between two 4K7 resistors). Really!

[Phorhas]

sorry buddy

[johngreene]

John,

What spice program is that? Does it have germanium transistors in the component list? It may be time for me to switch programs.

I'm using Orcad Capture/Pspice. It doesn't have germanium transistors with it but I was able to create a model using the Pspice model creator and data sheets found on the web.

--john

[bioroids]

Second circuit won't work either... sigh. Have a spice program on my own now.
Phorhas, don't try it...

Don't trust blindly the spice simulation, specially if you dont have much experience with it  it's easy to make mistakes. A good idea is always check the simulated circuit in the "real world", until you get a good understanding on the models limitations.
For example, remember the guitar pickup is not the perfect voltage source as modeled by spice (not even close).

Keep trying this stuff, looks promising, or at least different 

Luck!

Miguel

[johngreene]

Updated comments...
Mmm, looks pretty hard to get symmetrical soft-knee. It seems the lower tranny starts driving when the signal goes up (away from the soft-knee). Push-pulling kills te objective... I wanted it the other way around... Yet for squary lovers this is an ideal effect of course this way.

it's worse than that. Look what happens when I apply a signal with decaying amplitude.



It works that way with Schottkys, silicon diodes, and another pair of transistors as diodes.

However, when I use 47Ks the gain increases tremendously and it looks like this:

I reduced the input signal level from starting at 500mV to 50mV and increasing the decay rate 100%.

--john