The Junk Box Preamplifier

by Bill Werzner

A couple of months ago one of our customers at Vintage Sounds was in need of a not too expensive pre amplifier that could amplify sound from two of his older model turntables to be fed into an older amplifier.  The problem being, was that the output from his record players was not strong enough to produce a signal for sufficient volume.  This was a problem that I had toyed with in my mind for several years. In our business we deal with vintage equipment and I could foresee a problem like this at some point in time. The homebrew preamp he brought in for repair, and I won’t go into details, was beyond repair. So, I decided to experiment starting with a schematic I found while surfing around on GOOGLE one evening. 

Schematic for a Junk Box Preamp                                   by Bill Werzner, Vintage Sounds, Houston, Texas

 

PARTS

C1, C2, C7     47 uF  250 V. electrolytic       D   Full wave diode RB 157          R9,R11   150 K Ohm  0.5 Watt 
C3   22 uF   50 V.  electrolytic                       G   1 Meg. Pot. / switch SW        T1  Power transformer  P-T442     
C4   470 pF  200 V. ceramic                          R1  1.8 K Ohm 1 Watt                  117 V. pri. 12.6 V. & 117 V. sec.               
C5, C6   0.022 uF 200 V. ceramic                 R2  39  K Ohm  0.5 Watt              Antique Electronic Supply
C8   0.10 uF  250 V. polypropylene             R3  27  K Ohm  0.5 Watt              Tempe, AZ   
C9   1.0 uF   250 V. electrolytic                    R4,R10 1.0 Meg.  0.5 Watt          V1 12AU6,  V2  12AU7 tube
CW1  250 K potentiometer                         R5, R8  1.5 K Ohm 0.5 Watt        Note, 6AU6 tube used if T1 has
CW2   1.0 Meg. potentiometer                   R6  47 K Ohm  0.5 Watt               a 6.3 V. fil. Supply & use 12AU7 
CW3   25 K potentiometer                           R7  33 K Ohm  0.5 Watt               pin #9 fil. tap for 6 volt supply.

                                                                    

Voltages:  B+ at:  C2  150.5 V.,   V1  plate  93 V.,  V1 (pin 6) 27 V.,  V2A  plate  140 V.,   V2B plate  84 V.   

The author of the article described the preamp as a knockoff version from a Marshall amplifier. With an inexpensive small power transformer, two common tubes and a hand full of common components, this was the chance I had been waiting for. I built this unit on an aluminum bud box one evening at my work bench. Notice how the input signal is fed directly to the grid of the 12AU6 tube. CW1 adjusts treble, CW2 bass, and CW3 is for mid range. When I fired it up with headphones connected to the output, I was amazed at how well it performed when I fed a really weak audio signal into it from a bench radio – I think you will be too. The author of the original design coupled a 22 uF cathode bypass capacitor in parallel with R8, but removed it as the output was too loud!  Be careful when you hook this one up – you might blow a speaker cone or knock your neighbors out of bed.