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Si boost into Ge stage

Wed Sep 12, 2018 7:09 am

I want to put together a circuit that runs a simple Si booster into a Ge gain stage, something like the Fredric Accomplished Badger...


It's for a friend who wants a stock Rangemaster sound in there plus this slammed desk tone.

I'm gonna try something on the breadboard today but I'm looking for any tips. Obviously my RM circuit is PNP....so I'm wondering if I can just build a PNP si gain stage in front of it, maybe with a 2n3906. Is there something else I could try? An opamp? Another PNP ge before? Another PNP ge after the RM? I've read that if you just stack two RMs you just get hiss, the gain is just too high.

Re: Si boost into Ge stage

Wed Sep 12, 2018 9:16 am

How much boost do you want to push into the Ge stage? You don't need to shoot for the moon, just shore it up a bit, right?

You could just go with a simple common emitter amplifier built with an NPN and shorten up your collector load resistor enough that you're not wildly overloading the Ge stage. With a following PNP Ge this would offer you the option of just connecting the two stages with a straight length of wire, as you can easily bias the first stage collector signal into the nominal operating range of a PNP stage. Gain could be a cap coupled bypass pot on the emitter resistor of the NPN. You might be at the mercy of the elements somewhat, unless you weave a feedback path thermal drift may swing stuff around on a day by day basis, which can be both adventurous and annoying.

I just wrote that while drinking my morning tea, so I'll probably read it and laugh later, apologies if I'm being overly something or other. :lol:

Re: Si boost into Ge stage

Wed Sep 12, 2018 9:29 am

The Ren and Stimpy(cuff) phat phuk might contain some treasure for your quest. It's a pretty good Ge boost/gain with low parts, and you can give it a gain knob by replacing the 2k trimmer on the Ge.. anyway it sounds good, and it would be easy to breadboard.

Re: Si boost into Ge stage

Wed Sep 12, 2018 10:35 am

Great guys, thanks!

The Fat Fuck looks interesting, I thought I would need a P channel Jfet for a circuit like this but apparently not. I don't properly understand the difference between NPN and PNP, this is the problem.

A problem I now have is that I sat on my resistor tray while it was perched on the corner of my coffee table, sending all my sorted resistors out into an unsorted pile on the floor, and a few into my underpants. :thumb:

Re: Si boost into Ge stage

Wed Sep 12, 2018 10:54 am

Ben79 wrote: I don't properly understand the difference between NPN and PNP, this is the problem.

Let's think of the BOG standard common emitter stage powered by a single pole power supply with earth/common connected to the negative leg of your battery/power supply. So I'm talking about *everything* in 0 volts and +9 volts.

NPN has the emitter referenced to ground (so imagine a couple hundred mV worth of positive voltage between the emitter and the emitter resistor [if any], and the base is your PN junction drop above that...let's just simplify and call that one volt on your base.

PNP is inverted, so your emitter is hanging off the positive rail and your collector points towards ground. In an identical circuit the base of your PNP stage will sit at 8 volts (+9 minus 1).

You can size up your emitter resistors to nudge the emitter (and consequently your base voltage) toward center point of the rails, which, will act in tandem with the collector resistor to provide a working environment for your transistor. So, it's possible to limit the full excursion of any given stage to inside a voltage window. Being that the NPN window is generally closer to the +9 volts, and the PNP base circuit operates in its most linear fashion in the same window you can get away with direct coupling the two stages. Mind you, I do this stuff while watching a scope, so you're either flying blind with your ears out or using math to dial in the best range of values - which hinge on the transistors used..

But ultimately, the only difference between NPN & PNP is the way voltage is referenced in the circuit.

Re: Si boost into Ge stage

Wed Sep 12, 2018 5:36 pm

Thanks Curt, I'll read that a few more times and try to digest it.

Re: Si boost into Ge stage

Sun Oct 28, 2018 2:57 pm

this is a very difficult idea to execute. I applaud anyone with enough patience to spend the time developing a working prototype. the biggest problem with putting a Ge stage after and Si stage is that the Si stage will be loud and linear such that it will immediately clip all the time at the Ge stage. all the tweaking we can do to get the dynamics and EQ just right can only happen after we tackle the first problem. now add to that we do not know the volume or the impedance of the guitar signal connected to the input. yikes! there must be some attenuator knob up front or some attenuator knob after the Si stage. if you get it too hot you get garbage slewed square wave output. if you get it too low, you get noisy Ge gain stage plus all kinds of non-linear crap. we want the Ge to be non-linear because this is a guitar pedal. however we are always wrestling with the same problems that plague Ge PNP transistors.

PNP or what?
I think the entire pedal will be negative supply (posative ground for people who still talk like that) since your Ge stage is PNP. there is no added difficulty in designing solid state Si gain stages with a negative supply. you may have to use PNP silicon or the appropriate mosfet jfet etc.. the opamp designs don't change at all.

the poor mans solution is a clean boost into a fuzz face. this has been done before a lot. I just really hate these setups because there is no dynamics, no life, no tone. it is a waste of a $350 analogman sun face that sounds fucking amazing as is.
anyone can say this is a good idea but it completely depends on who is building it. very few people can execute these ideas.

Re: Si boost into Ge stage

Sun Oct 28, 2018 3:39 pm

eatyourguitar wrote:the Si stage will be loud and linear such that it will immediately clip all the time at the Ge stage.

Not necessarily, one can tailor the gain at any given silicon stage through supporting components.

Re: Si boost into Ge stage

Mon Oct 29, 2018 8:04 am

Well thanks for the input guys.

In the end I went for something pretty simple but with lots of control; a Rangemaster into a Germanium Giant. The Rangemaster got a switch for input cap between stock 0.005, 0.022 and 0.1 - this makes a huge difference to the sound. That goes into a Germanium Giant which is just a PNP Ge gain stage and a couple of clipping diodes. The mods I made were a 0.047 input cap to tighten up the sound a bit and I put the clippers on a switch instead of a pot. I used a back to back OA5 ge and a 1n4148 si with the Ge having its anode to ground. I also had the bias pot on the control surface as a gain control. The result was all kinds of boost, overdrive, distortion and fuzz - I really dig it into my 7 watt EL84 head.

You can see it here... https://www.facebook.com/TranSisterAudio/

Germanium Giant info here.... http://tagboardeffects.blogspot.com/201 ... giant.html
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