R2R ELECTRIC: Germanium Pre-amp

It's no secret as to what the R2R Electric germanium preamp is based upon, which is an old and very rare Hofner preamp.  The Hofner is a full range germanium booster, which was once used as a way to plug guitars into old console radios way back in the day.

R2R ELECTRIC GERMANIUM PREAMP LAYOUT

R2R is pretty big on competent testing to get the sound that they want - not all components are equal, so it might be worth spending some time testing this to make sure you get the most out of what is a very simple circuit.  

He mentions selecting the AC107 as a low-noise higher quality alternative to the OC44 / OC71 that was used in Rangemasters - which makes sense.   Despite everyone wanting OC44s cause they were in rangemasters, they’re now a bit expensive and are sometimes very noisey.  

Side note:  An excellent podcast featuring Chris from R2R can be found here on the Fretboard Journal.   Chris talks about his journey and process. 

Boost / Volume pot:  I've done the layout based on photos, and what I assume is how R2R implemented the volume / boost control, which is the same way that it's done for a Rangemaster.  The stock Hofner has a 10k resistor to the collector, which has been replaced by a 15k pot on the R2R.  I'd suggest a 20k as an alternative, as 15k is an uncommon value.  Can always try 10k and see what you think.

Input resistor:  I would not stress about using an exact value for input resistor.  Stock is on the Hofner schematic is 37k, which is an unusual value.  R2R seem to have used 36k & 33k - so as long as it's close enough, you should be fine.  It is worth noting that the input resistor forms a voltage divider with the 10k resistor from the transistor base to ground.  Essentially these two resistors are reducing the input level by about about 80% before it is amplified by the transistor.  

Emitter resistor: The stock Hofner emitter resistor is 1k5, again some variance is OK, which can be tuned to match the transistor that you plan on using.  The layout below uses a 3k9, same as a Rangemaster.   The smaller resistor is more likely to produce a bit of dirt, the larger will be cleaner.  This would be a great place for a trimmer if you are that way inclined.  

The rest: The capacitor values and the 100k / 10k bias resistor seem to remain constant.   There is room to play with cap sizes, so if you don't have the exact value, there's no need to stress.  The input cap can be dropped down quite a bit before there's any noticeable change in response.  1u is barely any different to 4u7.  The 100u emitter bypass cap can also be dropped down to 47u with next to no change as well.  Same with the output cap, a bit smaller will not be noticed.

Drop down resistors: R2R have added 1meg input and output drop down resistors on the input and output of the circuit, which you can see wired to the stomp switch in the photos below.  This is not on the Hofner, but it is a common inclusion on circuits like this to avoid switch pop.  I'd recommend including it.

Board:  I’ve not listed the size of the board or any other info on the point to point layout - I’m sure you can work that out yourself.