Higher gain Ge PNPs

[thermionix]

Say hFEs in the 150~250 range, generally too high for FFs and RMs.

What would you build with them?

[mac]

Ge Big Muff, check my gallery, there's a trick to use the Si schematics and pcb.

mac

[Kipper4]

Out of intrest.
Maybe there is a golden hfe range for a perfect fuzz. I don't know.
Why is it not possible to maintain the gain ratios and still achieve greatness no?

[thermionix]

Out of intrest.
Maybe there is a golden hfe range for a perfect fuzz. I don't know.
Why is it not possible to maintain the gain ratios and still achieve greatness no?

Dunno.  I've breadboarded some higher hFE ones, with less than stellar results.  Didn't spend too much time tweaking resistors though.

[R.G.]

First, you have to be sure it's real hfe, the AC current gain, not pushed higher by leakage.

Second, there appears to be a broad sweet spot, as mentioned, where the gain and idiosyncratic characteristics of germanium transistors as made back in the 1960s sounds good for some reason.

Higher hfe devices - if it's a real hfe - will likely sound more hard-edged, more like silicon. Maybe. It's uncharted territory.

Finally, when you have AC feedback, you have the feedback covering up and otherwise cloaking the gain characteristics of the devices doing the amplifying. The Big Muff fits in here. It has high AC gain, but feedback through diodes, so it's high gain to get to the diodes' forward voltage, then it's the on-resstance of the diodes making gain and letting you "hear" the diodes. Germanium doesn't help that much because it's largely hidden.

[thermionix]

Thanks RG, I'm using your "FFSelect" tester and method, subtracting leakage from gain.  I've just been accumulating some higher gain Ge PNPs that I don't know what to do with.  Freebies pulled from old equipment mostly.  All the classic Ge circuits I've looked into seem to want lower hFEs.  Yeah I know leakage is a factor too, but I was just looking for some ideas, then I can see if I have anything appropriate.  I've built a Si BMP, the Kit Rae supposed Gilmour ram's head version, and I love it.  I've never heard a Ge BMP, but I've seen threads here.  Would I be correct to assume that low leakage is ideal, given that Si has none?

[R.G.]

OK, you're getting decent information on leakage then.

It is a truism that you can't make pedals sound magically great just by rubbing germanium on them.   

The BMP is likely to sound much the same with silicon or germanium of similar gains. Silicon gains tend to start at 150-200 with "ordinary" devices like the 2N3904 and its ilk, and run commonly in the 300-500 range for a lot of them. The usual caveats apply - hfe changes over a wide range not only from device to device, but in the same device at different collector currents. So a high gain germanium would be somewhat likely to perform like a low gain silicon, other things being equal.

They're not equal where there is little or no emitter resistance to add local feedback to the transistor. Any significant emitter resistance will run the apparent voltage gain back down from Gm*Rc towards Rc/Re. Feedback resistance from collector to base lowers it further.

But it might be something you like. Since we still can't measure "tone goodness", it's certainly worth subbing in some germaniums to see if they're good to your ears.

Silicon still has leakage, but it's about a thousand times smaller than germanium for similar situations. Subbing germanium into a silicon circuit will require some thought about tweaking the bias a bit to run the device colder, allowing for the leakage to heat it up some, and then measuring and dealing with bias points when you see what you get. Yes, low leakage germanium is going to be the best way to go - if you have them.

[Rob Strand]

It is a truism that you can't make pedals sound magically great just by rubbing germanium on them.

[Plexi]

Like we talked in a similar thread (that I dont remember right now...), and mentioned the users here, the Ge transistors would be 'eclipsed' by the diodes..
I prefer swapp diodes comb than transistors: I've tried both in a wide range of BM's versions.

In another therms, some says that the low hfe (Si) transistors will give a 'smooth' violin like distortion (black russian).
Idk: not in my experience.

[Electric Warrior]

Say hFEs in the 150~250 range, generally too high for FFs and RMs.

What would you build with them?

What would make these hfes too high for a Fuzz Face or Rangemaster? Some Mullard datasheets from the 1960's list the OC44's hfe spec as "40 to 225". I've got an OC44 in yellow PVC tubing as used in Rangemasters that measures around 200. Sounds great and biases fine.

Maybe you should give the Fuzz Face another chance. Silicon Fuzz Faces used transistors with much higher gains than 250, so why wouldn't it work with high gain germaniums? You may have to tweak the bias, but that is true for any type. Drop them in to see where the voltages end up. Types with "average" hfe and leakage readings just tend to bias alright in this circuit without requiring any modification. There range of voltages that can work well is rather large, so most builders should be able to get away with using the stock circuit. I'd expect transistors with extreme specs to require re-biasing, though.
And if the Fuzz Face doesn't work out, the Tone Bender MKII hardly sounds any different with high gainers in my experience. I once bought some Valvo OC75s here that measure between 150 and 200. Extremely leaky, but interestingly they bias in the same range as the low gainers.

[Joncaster]

They're not equal where there is little or no emitter resistance to add local feedback to the transistor. Any significant emitter resistance will run the apparent voltage gain back down from Gm*Rc towards Rc/Re. Feedback resistance from collector to base lowers it further.

Which would be preferable for lowering the gain of a stage in, say, a fuzz face?

I've been lowering gain (in a hybrid fuzz) with Q1e resistance and through the Q2e to Q1b feedback network, fine tuning those two values, compensating with Q1c resistance changes.
Haven't tried the C to B resistance approach. Would it also add to the input impedence?
So far in my Hybrid tests, i like around 70hfe Q1 with less feedback OR 120+hfe with emitter resistance and more feedback).

There is something great about the higher gain transistors for a nicely balanced fuzz decay and just the right amount of rolled-off grit. But it takes a lot of fiddling.

My hybrid is almost ready for boxing...until i find another way to tweak and test!

[Joncaster]

Say hFEs in the 150~250 range, generally too high for FFs and RMs.

What would you build with them?

I'm really liking a higher gain AC188 (i think over 200...) on my Rangemaster breadboard (i have three small breadboards and a two footswitch box with fly-leads)
But went the other way with biasing, around 3V on collector, still tweaking the input network cause it's rolling off a bit too much top-end, and i'm loving a larger input cap (20nf) to make it more of a full range boost. Has a lovely drive and super clean and full volume rolloff.
Is that technically still a Rangemaster?

[thermionix]

Say hFEs in the 150~250 range, generally too high for FFs and RMs.

What would you build with them?

What would make these hfes too high for a Fuzz Face or Rangemaster? Some Mullard datasheets from the 1960's list the OC44's hfe spec as "40 to 225". I've got an OC44 in yellow PVC tubing as used in Rangemasters that measures around 200. Sounds great and biases fine.

Maybe you should give the Fuzz Face another chance. Silicon Fuzz Faces used transistors with much higher gains than 250, so why wouldn't it work with high gain germaniums? You may have to tweak the bias, but that is true for any type. Drop them in to see where the voltages end up. Types with "average" hfe and leakage readings just tend to bias alright in this circuit without requiring any modification. There range of voltages that can work well is rather large, so most builders should be able to get away with using the stock circuit. I'd expect transistors with extreme specs to require re-biasing, though.
And if the Fuzz Face doesn't work out, the Tone Bender MKII hardly sounds any different with high gainers in my experience. I once bought some Valvo OC75s here that measure between 150 and 200. Extremely leaky, but interestingly they bias in the same range as the low gainers.

Okay let's pretend I didn't include the phrase "generally too high for FFs and RMs" in my OP.

I already have a Ge FF that I like, not looking to build another right now.  RM serves no purpose for me at this time.  I've built one really good one, sold it.  I'm just not in a situation where I can play loud very often these days.

I've messed with the Mk II on the breadboard, I think you helped me with it, but I never got anything I liked too much.  Every time I got the collector voltages where they're "supposed" to be, it got a little gatey, a sound I don't care for.  IIRC Q3c down at 4.5 or so sounded better, no gating, but it was nothing more exciting than the FF I already have.  So I never built it.

[electrip]

One should not focus on gain/hfe,
you have to control the amount of overdrive in your system (fuzz).
So if your Fuzz Face Q2-Bjt is 'normaly' hfe=120@Ic0.5mA and Rc+Re=10k
and your high-gain-Q2 is hfe=240
then halve all resistors and overdrive is roundabout the same. 
Influence of leakage of Q2 is also reduced.

For the whole fuzz there are more things to consider but it is not impossible if you 'understand' the system of your fuzz.
(Input impedance, output impedance, interstage impedance ratio, biasing, choice of capacitator values)

electrip

[amptramp]

They're not equal where there is little or no emitter resistance to add local feedback to the transistor. Any significant emitter resistance will run the apparent voltage gain back down from Gm*Rc towards Rc/Re. Feedback resistance from collector to base lowers it further.

Which would be preferable for lowering the gain of a stage in, say, a fuzz face?

I've been lowering gain (in a hybrid fuzz) with Q1e resistance and through the Q2e to Q1b feedback network, fine tuning those two values, compensating with Q1c resistance changes.
Haven't tried the C to B resistance approach. Would it also add to the input impedance?

Adding a collector to base resistor will reduce the input impedance.  Suppose the input voltage goes up.  The collector voltage will go down, reducing the amount of current pulled up.  This is like a mixer where one input goes up and the other goes down, more current is required to change the voltage, both up and down.