A couple of months ago, I checked in a pile of amps for
repair/freshening up for a band that was going into the
studio to record another album. While reviewing this pile, I
found that I was in possession of the main recording amp for
the front man. It was a little vintage Supro that had
certainly been around and through the ringer. While it
definitely had a fantastic tone, it had noise and
reliability issues due to its age. I decided to surprise the
fellow and make a replica as close to the original as
possible. In my own amp designs, I try to add a dash of
originality into the mix, but for this project only a carbon
copy would do. I pondered the project endlessly, obsessing
on the creation morning, noon and night. A couple of days
after deciding on a game plan, I received a call from the
band's liaison asking me if I would mind making a clone of
that amp only with less noise and increased reliability. I
was now on the hook. Time to put my plan into action.
first step was to try and find a schematic for the little
A few were found in my files and on the internet, but they
didnít seem to be of this exact incarnation. Since my plan
was to be blueprinting and cloning this amp anyway, I
decided to dig in and make a schematic. I traced the
circuit, noting the apparent values (what was written on
them) of resistors and caps etc, as well as the type
(composition) along the way. All of the resistors were
carbon comp, and the coupling caps were mostly ceramic
discs. I then traced the circuit again, this time measuring
and taking notes on the actual values
and voltages of the resistors, caps, transformers, plate
voltages, cathode voltages, etc. The differences were
substantial. The tolerances on these amps were fairly broad
to begin with; on top of that, the values of the components
would certainly drift over the 40 odd years since this amp
was built regardless. Now that I had the blueprint made, I
needed to acquire the parts.
I placed a call to
to get transformers with the exact specs required. We do
quite a bit of business together and he has always been a
great help with projects like this (as well as with my DB7
trannys). Surprisingly, they had them on the shelf and ready
With those on the way, it was time to start punching the
chassis. I selected an aluminum box to be used as a blank
and laid out the measurements. My goal was to create a
replica in tone and feel only, so I took the liberty of
moving some of the component placement to cut down on the
noise. For example, I spaced the transformers out a bit and
took into consideration the proximity of the output tranny
and the first gain stage. During the construction, I also
implemented star grounding and ran shielded wire. For the
resistors I used NOS carbon comp where it made sense tonally
(input, plate load etc.) and a mix of carbon film and metal
film in other places. At times, I had to create values using
two resistors to try to match the values of the ones that
Ok, ok, I know that I said that I was making a carbon copy
clone here but I just couldnít bring myself to use ceramic
discs for the coupling caps. Instead, I used caps that we
have made to our specs for our amps by a high-end
manufacturer here in the U.S. They donít have the same
two-dimensional graininess as the ceramics. Instead they
have a very open and airy quality to them but shift to the
point of flattening out a bit when using some right hand
With the guts assembled and all of the voltages checking
out, it was now time for the speaker selection and cabinet
construction. I chose to make a separate baby head and
speaker cab for this project. I used pine for its tonal
qualities and tried to match the size of the original combo.
For the speaker itself I used an 8"
Weber VST Alnico
that was a direct replacement of the original. The cabinets
were then covered in a very funky black cowboy print type
Tube selection was also very important. I tried many tubes
from my private stash and finally rested on an NOS
6V6 for the output and NOS
grey plates for the preamp.
The end result was satisfactory but there was something
missing. The tonal characteristics were all there but it
just didnít seem to have the "magic." I gave it to the
client and told him to play/abuse it a bit to see how the
character would change when the amp/speaker had some time to
break in and then I would finish the voicing. A couple of
weeks later I picked it up from their studio along with a
wish list of how the tone was to be sculpted. I placed the
amp on my bench ready to start the voicing, plugged the head
into a very weak and old 10"
that I use as one of my test speakers and strummed a chord.
WOW! It sounded amazing! Hmmm, what happened I wondered?
I then plugged the amp into the cab that I had made for it.
While it still had the tone, the dynamics just werenít
there. The speaker that was in my test cab seemed to be the
missing ingredient! Fortunately, it was one of four that I
had pulled out of an old Hammond organ cab of mine. The new
speaker cab was taken apart, a 10" hole cut in the baffle
board and one of the speakers installed with hardly any room
to spare. I tried againÖ WOW! With this combination you
could play perfectly clean, sparkling chords and using right
hand dynamics shift into blistering, compressed distortion.
Truly outstanding! The
is a fantastic speaker -- don't get me wrong -- it's just
that for this application I needed a really tired, old,
crummy speaker to do its thing. The end result far exceeded
the greatest expectations of both my client and I. Certainly
well worth the time spent on a small pet project in a busy
shop. But hey, who needs sleep anyway?
is the owner, master luthier and chief designer of
Schroeder Audio Inc.in Chicago Illinois. There he oversees the daily repair
operations of the shop as well as designs the amplifiers and
effects that they manufacture in house.
Copyright © 2009 by Premier Guitar.