THE ENGINEER'S AMP
designed by Pete Millett
built by Raymond Koonce
Here are some photos of the amps I built recently for my friend Ray
Gallagher and myself. I decided to build two of these at once to
see if I could get some economy of scale in the work. It helped
to build two chassis as once and the wood sides were a no-brainer.
I did get slowed down however, by the different paint jobs. The
red amp is painted with "Chinese Red" lacquer in cans from Home Depot.
The brown amp is painted with a custom-mixed automotive enamel also
from spray cans. I'll just say here that the lacquer is *much*
expensive that the auto enamel. The enamel was easier to sand and
polish than the lacquer, probably because it's a harder paint.
The circuit for these amps was designed by Pete Millett
(www.pmillett.com), a friend of mine. He provided the big red
circuit board that you will see below. The chassis is
1/8" thick aluminum that I have bent and welded at a local metal shop.
They do a very nice job and the chassis is sturdy enough to hold
you might load it with. I do all of the machining myself. I
cut the tube socket holes with a Unibit on a 1/2" hand drill.
It's a great tool as long as you use a good lubricant and take your
time. The remainder of the holes are done with regular drill bits
The wood side panels are Honduras mahogany
finished with clear lacquer, wet sanded and polished with
automotove wax as a final coat.
These two amps are provided with a remote control module by Tent Labs
in The Netherlands. The remote provides four inputs,a line out,
remote volume/mute and power on/off functions.
I provided a small relay to handle the mains power switching. The
remote answers to a universal remote with a Philips television code.
I have installed a standby switch and a small control
transformer to power the remote. You can see the control module
at the bottom of this photo, the display panel at the top and the blue
relay to the right of the control module.
The amp uses Edcor transformers, manufactured in New Mexico. They
make good quality iron at reasonable prices. Pete had them
custom-design the power transformer
for this amp and when they do it once, it becomes a stock item that you
order by part number. They make the transformers when you order
them, so the
delivery time might be a little long, so plan ahead for that
The long screws with the acorn nuts are not ornamental. I wanted
to keep the top of the amp free of fasteners, so I devised a method to
attach flat-head machine
screws to the chassis and I place upper and lower nuts to fasten the
circuit board. To avoid undue stress when inserting tubes, I
installed long screws and nuts that
are in contact with the bottom plate when it's in place, thereby
transferring any stress from the upper connections to the 1/8" aluminum
Here's the bottom cover in place, with ventilation holes and rubber
I used some generic Chinese speaker binding posts on the red amp, but
on the brown one I used some WBT clones that I bought
at the European Triode Festival, much better, IMO. The only other
difference between these amps is the coupling caps I used.
I put some generic poly caps in the brown amp, and in the red one I
used some ultra-voodo Vitamin T Mojo Tone caps. I honestly
can't hear a difference.
When you start on these amps, there's a real mess of wires from the
transformers to tie off, but once that's done, you're almost finished.
Here's a close-up of the Tent Labs volume control module. It's a
really nice piece of equipment. It provides four inputs via the four
relays below the blue capacitor. The
volume control is in 64 steps done with the other six relays and the
precision resistors. When you adjust the volume, you hear the
relays clicking. When I installed the module in
the red amp, it wouldn't switch the relay. I removed it and
took it with me to the ETF and Guido was kind enough to replace a
faulty SMD resistor for me on the spot.
Here's a shot of the brown amp. The black thing on the chassis is
a heat sink for a couple of regulators. It makes a nice "racing
A side shot.
And the rear. This amp has the better-quality speaker terminals.
And the red amp.
When Edcor ships their transformers they are painted a bright blue
color. I disassemble the transformers and repaint the end bells
to match my project.
When I reassemble the transformers, I use threaded rods and acorn nuts
for appearance. I cut the threaded rods to length as needed.
If anyone would like additional photos or further information about
these amps, I'd love to hear from you. You can email me at firstname.lastname@example.org.
Copyright 2012 Raymond Koonce all rights reserved.