The RCA BA-31C would have been found in broadcast consoles of the day (late sixties), as a microphone preamp module.  

I think it may have been the starting point for the Hudson Broadcast, as they look quite similar and I’ve heard some references made to RCA.  If it is what they used, I can see why they did – it’s very tweakable, with some nice options for setting gain as well as high and low-end roll-off.   

The original runs at +/-30V.   I was aiming for a 9v single rail supply to keep it simple, and of course without expensive transformers.

RCA BA-31C MICROPHONE PREAMP – ORIGINAL SCHEMATIC

RCA BA-31C – MODDED FOR GUITAR

I tried to keep things simple, while still maintaining the structure of the circuit.  

Note:   This is still a work in progress, still some fine tuning and changes in progress.

GAIN

Like a Hudson Broadcast, it runs from clean boost to an overdriven fuzz.  It’s not super fuzzy like a fuzz face, but you can hear it.

After experimenting on the breadboard, I decided to use an A1K pot for R6, and set R10 to 22k – 68k (anything a bit bigger than the original 6.8k resistor is fine, so long as it still has some feedback).  Decreasing R5 also provides a bit more dirt.  

FREQUENCY RESPONSE

With a few capacitor value changes the frequency response is easily adjusted.  

Increase C5 to roll-off high frequencies, although this is only needed at the highest gain settings, on lower gain settings, the tops are already reduced.

Decrease C3 to roll-off lows after Q1.  1u is pretty flat, can go down to relatively small values (22n as an example).  

MODDED RCA BA-31C ON THE BREADBOARD / SCOPE

So it works….  this is using a B5k pot for R6, and I think a 100n on the output as it was sitting right in front of me at the time.  Will change over to 1u.

PNP is a germanium 2SB457, NPN is a BC108.   It really isn’t at all fussy about what germanium is used for Q2. 

 

caution: this starts at low volume and increases – watch your ears / speakers.

VERO LAYOUT – THE BALMORAL BROADCAST

Initially I thought I would end up with a few switches for bass and treble – but for now I decided to keep it simple.  Think I will most likely land on gain, volume, and a fat switch (which will change the coupling cap value between Q1 and 2).  For me it doesn’t get bright enough to need control on the treble – if anything it’s a bit low on treble on cleaner settings.

At low gain settings, it has a slightly rolled off high-end, with is nice and bluesy, maybe a bit of a jazz tone.  I did try it with a boost in front, which works pretty well, although it can use a bright cap across the boost level control.

This layout is a little different to the schematic above, as I decided to change the output cap.  I’ve called it the Balmoral Broadcast – due to some similarities to the Hudson, and I live in Balmoral.

 

Note:  not final versions