Bass guitar preamplifier in the Ampeg SVT tradition
Wiring jacks switches, tube sockets, and potentiometers
|For me, making a turret board and placing the
components are much easier tasks than wiring the controls,
the tube sockets, and putting all the sub-assemblies in the
box. together. I'm not going to elaborate on the particular
order in which you should perform each of these tasks.
The illustrations below should give you an idea how to
proceed with some of the more complicated wiring tasks.
Important details about grounding and
isolation of the jacks, switches, and potentiometers
I followed several recommended noise prevention practices in
wiring the front and rear panel controls. All the 1/4"
input and output jacks are Cliff UK types, so the ring
terminals are isolated from the chassis. All grounds
to pots, switches, and jacks are wired directly to the
ground turret for the source or destination stage for the
signal. The only chassis signal ground connection is
made from the ground lug of the input jack. The ring
(ground) terminal of the output jacks have a high frequency
AC ground to the chassis through 1000 pF ceramic caps.
This effectively kills RF noise entering through the jacks,
as well as high frequency ground noise from the mains.
I have found that a properly sized RLC filtered IEC power
line entry module is essential for keeping mains line noise
out of the circuit, especially with a toroidal power
Instrument input jack and pad switch
The tube socket holder is 1" x 1" x 1/16" aluminum angle stock from the hardware store. The holes for the sockets were made with a step bit. I solder flying leads to everything, except for the grid connections that will come on shielded cable later. I do the filament leads first. You can see them running along the bottom. Because I used regulated DC for the filaments, I did not twist the leads tightly, which would be standard practice for AC filaments.
Ultra-low selector switch
Connecting the tube sockets and ultra-low tone selector switch to the main board
I solder most of the connections between the main board and the tube sockets and controls on the underside of the main board. The top of the board will appear very neat and clean, but connections under the board will be harder to service later. Good solder work is important to making the project reliable - I continue to learn the hard way! It is critical to trim the each flying lead to the correct length for where it will be soldered to the board. I like to use solid core wire for all these flying leads because it stays where you put it. The switches, pots, and tube sockets form a fairly rigid assembly that is installed and removed from the chassis in one piece! I install and remove the assembly from the chassis bottom plate many times during the assembly process, keeping each component aligned with chassis and control panel holes
Bass and treble tone stack
|The bass and treble controls are of
the "James" type, and were wired with the
potentiometers attached to the front panel. I
might have preferred putting the tone stack on the
main circuit board, but I ran out of space.
Some builders may want to glue the capacitors to the
potentiometer bodies to minimize vibration pickup.