HOFNER: Germanium Pre-amp

I only came across the Hofner Preamp quite recently, despite seeing some other builders making point-to-point style germanium preamps based on this circuit - the prime example being the R2R Electric preamp.

There's a good write up here, but in summary: Back in the day before guitar amps were affordable, and TV was a thing, pretty much everyone had a big valve radio in the house.  Many had an input for turntables, but most people didn't have a turntable as they were expensive.  So Hofner made a preamp to allow early electric guitars to be plugged into a valve radio, via this additional and often unused input.   

The preamp boosts signal and helped fix the massive impedance difference, turning the family radio into a primitive guitar amp.   

Worth noting that old record players had a much higher output than the current ones, hence the need for a boost.  The output was so high, that when you look at old schematics for valve PA systems, the P.U. input skips the first 12AX7 gain stage, and goes straight to a mixer, so guitars really needed an extra boost to make it work.

HOFNER PREAMP SCHEMATIC

It kicks out a good 20dB of boost and has a relatively flat response

  

I'd be very surprised if the boost is as clean as it looks on LTspice - I'll have to breadboard this and check it out on the scope.  I'm sure there would be some dirt.

MODS

I guess the first and most obvious is adding a volume pot.  A 220n output cap is pretty healthy, so a standard A100K volume pot will work just fine.   Note the R2R electric method, which is similar to a rangemaster.  This is probably a better way to do it. 

Reducing R1 increases the input level, as it normally forms a voltage divider with R2.  Things will get a little fuzzy and overdriven.

C1 can be reduced to treble booster territory. 

C4 can be reduced for a bit of low-end roll-off.

TAGBOARD & VERO LAYOUTS

The original was constructed in a very similar fashion to the Rangemaster - a simple point-to-point circuit in a box.