Hi there,
On temperature sensitivity and other quirks with old germanium transistors, I’ll say I have quite a few of these old germaniums in various shapes and casings. Most of them are more of historic interest than anything else. I actually have one or two of the spetz transistors- those have a little rubber hood over their glass casing and the story behind that is from the childhood of transistors and there even guitar amps built with these glass containers that made the amps light sensitive and so the phototransistor came about and also the rubber hood. I have had the pleasure or not to repair a number of Hagstrom amplifiers many of them with transistors. I actually slaughtered a preamp of a Hagstrom amp that was supposed to compete with Fender Twin Reverb and about the size of one such Fender too and with two 12” speakers but it was not worth repairing so the preamp became a fuzz pedal- a fuzz face type with a buffer ahead. Funnily I sold an old Farfisa organ the vibrato of which inspired me to make the MGMV, to a friend who wanted to slaughter it to make fuzz pedals- there were three AC128’s per oscillator per note and then some such as for the vibrato.
Oh yes those old AC128’s only some worked as some had too much leakage saturating the transistor and some even had a peculiar thing called whiskers from the casing protruding to the inner core making them intermittent and then some or most had the pop corn noise so called because it behaves a little like popcorn in an intermittent noise and many of them were so heat sensitive that you could change amplification by holding the case with your bare hands while others you could do the same thing with a hairdryer or a lamp.
When I took electronic class-on my own- there was this section of designing circuits with OC transistors at the last pages of one of the tube manuals and then icbo-the leakage from collector to base would be important to count with and sometimes the leakage alone would be enough to bias the transistor and the final circuit would appear not to work………unless you considered the leakage.
with the advent of silicon transistors icbo would become something that could be ignored in design for all practical purposes and only under extreme conditions ever to be considered
On beast deserves mentioning and that’s Hagstrom’s entry of a Backing Elvis model in a 4x12 combo at 130W’s and that was driven by the largest germanium power transistors I have ever seen about three times the size of a 300W silicon transistor.
These things could deliver a lot of current over the 2 Ohm speaker load of the 4x12 but would not take more than 40Volts so this amp had a power supply regulator making sure there would be no more than 40V across the trannies but this regulator had a tendency of dying itself so I asked my local submarine engineer and he produced two replacement heavy germanium transistors and also gave me permission to use his floating power supply to safely protect what appeared to me transistors worthy of being in a museum. There was no way of determining if these transistors worked other than connecting them in the circuit-they actually measured as if they had a short to any terminal from any terminal- but behold they worked and the amp eventually could be fired up after some really heavy restoration and it was totally worth it as that may be one of extremely few of those anyway not so many made amplifier that is still to this day working.
Oh yes when a couple of AC128’s and the like transistors could be found that were reasonably stabile and reasonably not so noisy and that further had some amplification enough to make fuzz pedal I can say the sound would then be great and yes still it would be very good to have access to a fridge in warm countries and especially if you want to play fuzz guitar on the porch in the midst of the Sun.
Aleksander Niemand told me something fun about SSSR transistors in that when the way to make silicon transistors was discovered it was consider such an advance that this was kept out of commercial electronics and so the commercial electronics were instead made with the older germanium transistors and so there would be massive amounts of commercial intended germaniums that might leak out into the world.
The transistor chosen for Sparkle Face is of a later generation when they actually got good enough to compete with silicon but alas too expensive with their golden legs and many of these transistors have indeed been slaughtered for their legs.
However they are still available since they were used in computers, the very early ones, and so crates of these were stashed as replacements-that is to say no magic just that these more modern types have much less temperature dependency on the order similar to many silicon.
have fun
BJ
So yes.. doing this sort of project is a fantastic springboard.. I recently made a green russian and triangle muff in the same enclosure where you can patch the pedal order how you like and put something else in between if you want..creative, not so hard & lots of fun!! Yeah, I like fuzz.. 
A few years ago I helped Gary Hurst, the inventor of the Tone Bender in 1965, come back from the dead and remake the MK I, MK 1.5 and MK II. The MK I is pretty insane but so much fun!!
