Build a Tube Preamp for guitar with tone control


I decided recently to build a tube preamp for my home studio. Please note, however, that this is not a circuit for the novice electronics builder. The circuit itself is simple, but unlike most semiconductor projects, this one uses voltages near 300V. The danger of electrocution is present and neither the author can assume responsibility for protecting you.
If you are not familiar with good construction and safety practices for high – voltage electronics, do not attempt to build this circuit.

So how does it sound?

This preamp won’t make the sound of an overdriven Marshall stack, nor is it a heavy metal distortion unit, but it does produce tonal colors ranging from a slight “warming” to soft distortion. Turning down the drive control produces a somewhat brighter sound with no distortion? Turning the drive all the way up produces distortion that really bites on the bass strings and softens out on the higher notes. Driving the unit hard also produces a bit of a compression effect. With my two – finger power chord playing style, I find the overall effect quite pleasing.



Circuit description

The preamp circuit, based on a design in the 1975 RCA Receiving Tube Manual, uses a 12AX7 tube as a two stage, voltage gain amplifier.
This “generic” circuit is similar to what you will find in most guitar amp rpeamps.
The 12AX7 (6N2P and ECC83 are equivalent tubes) is probably the most common tube used in the music business. Almost all current tube effects use this tube, as do most vintage tube amps.
The 12AX7 was designed for use in audio applications requiring exceptionally low hum and noise levels. It is a high gain, twin triode device, having tow identical sections sharing a common heater filament.
The guitar or other input feeds J1 and goes to the grid of the first tube stage, which produces a voltage gain of about 30.
This boosts the guitar’s 40 mV (nominal) input up to about up to about 1.2V.
Coupling capacitor C2 picks off the amplified signal while blocking the high voltage plate supply. The signal then goes through the tone control circuit (R5-R9 and C3-C6) before feeding the second stage’s input. Potentiometer R10 is a voltage divider that sets the drive.
The second tube stage provides a gain of ten. The output signal couples through blocking capacitor C8 to the load resistance (R13 and R14). Control R14 varies the output signal level at J2, up to a maximum of about 1.5V. this should feed a relatively high impedance stage; 600ohm mixer inputs are not recommended. Resistors R4 and R12 are bias resistors that allow the tube cathodes to develop a positive potential with respect to ground. Capacitors C1 and C7 filter any ripple appearing across these resistors.



POWER SUPPLY.

Transformer is a dual secondary type rated at 250VAC center-tapped for the tube’s plate supply and 6.3VAC for the tube filament.
The output from a full-wave rectifier is 0.7 times the input AC voltage, so about 185V appears across C10 and under load, about 140V appears across.
Any transformer that produces 150 to 275VAC should work fine.
You can also use a transformer with a 12VAC filament winding by grounding ground pin 9 of the 12AX7 and running the 12VAC into pins 4 and 5.


PARTS LIST

R1, R5, R13------------47K 1/2W
R2----------------------1M 1/2W
R3, R8----------------100K 1/2W
R4---------------------2.2K 1/2W
R6, R9, R10-------------1M potentiometer
R7---------------------10K 1/2W
R11--------------------68K 1/2W
R12--------------------15K 1/2W
R14--------------------50K potentiometer
R15--------------------47K 1W
R16------------------220K 1/2W

C1, C7------------------25mf 62V
C2, C8-----------------220nf 400V ceramic disk
C3----------------------20nf 400V ceramic disk
C4, C6------------------1.2nf 400V ceramic disk
C5---------------------220pf 400V ceramic disk
C9, C10-------------33-47mf 400V electrolytic

D1 – D4------------------1N4004
F1-----------------------0.1A fuse
J1, J2--------------------phone jack
T------------------------2x250V 50ma + 6V 1 A transformer

V1-----------------------12AX7 or ECC83, 6N2P
S1---------------------- -switch
Misc---------------------9-pin tube socket