8-Watt S5 Electronics Integrated Tube Amplifier
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This a tube amplifier I built from a kit (purchased from
Antique Electronic Supply for $140). The kit was created by
S5 Electronics. The kit included only
the barest of the amp (all the components including the tubes are there, but for things like
the power switch, power cord and enclosure, you'll want to go beyond the kit).
The only "enclosure" to speak of included in the kit is a piece of
wood (bread board). So besides building the kit itself, I created the enclosure you see here.
As for the amplifier itself, here are some specs. It is an integrated amplifier meaning that it
has both a pre-amplifier stage and power amplifier stage in the same box. Furthermore it is
stereo meaning (of course) that the amplifier circuits are repeated twice. Pro equipment is often
mono (mono-blocks, requiring two of course) and the pre-amp and power amplifiers are often seperate.
There is a single stereo input (you can see the cords running into the front) volume attenuation
(also on the front, the volume knob) and a single pair of outputs (the speaker wires running
out toward the rear on left and right sides). It uses four 11BM8 vacuum tubes (valves) to deliver
8 Watts per channel. Eight Watts may sound anemic but as I intend to take this to my office at
work, 8 Watts is perhaps even excessive (remember, these are "tube Watts"). In fact,
my wife finds it quite loud when brought up to 100% in our living room.
The amplifier is arranged in a push-pull configuration - something I'm only slowly coming to
understand. There are three transformers in the amplifier. A single power transformer powers both
the circuit (over 200 Volts throughout) and the tube filaments. The power transformer is the
largest of the transformers and so pokes up out of the enclosure behind the tubes (it has a metal
cover over it). The other two transformers are the output transformers - one for each channel.
Physically, they are smaller than the power transformer and fit nicely within the enclosure.
The enclosure was enjoyable creating. The sides are 1/2-inch birch plywood cut on the miter so
that the corners of the box don't reveal the end grain (ply in this case). It was finished with a
simple coat of tung oil. The top surface is an 8-inch square of aluminum sheet - cut from a
"door kick" (plate) I picked up at a hardware store. A 3/4-inch Forstner bit punched out the holes
for the tubes and with a nibbler I was able to open up a rectangle for the power transformer.
Finally, the aluminum was painted with black enamel.
The bottom two-thirds of the amplifier are open and, with the tall feet, this allows a certain
amount of convection to cool the interior of the amp. Large capacitors were soldered in inverted
(on the bottom side of the circuit board) to allow me to prominently display only the tubes on
the top of the enclosure.
A retro-styled knob I had around was popped on the front of the amp for volume control. Also,
in order to cover the power transformer, I used the bottom of a cast aluminum project case
flipped over and bolted to the top aluminum sheet. Holes (to allow some convection) were drilled
out of the backside of the cover. |
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How does it sound? Well, I think it sounds excellent. My one complaint is that there is a slight
60 Hz hum that is persistent - even with the volume turned down (at least it doesn't get louder
when you turn it up - this way the music drowns it out). My understanding is that this is most
likely due to the AC current running through the filaments in the tubes - specifically, a sort
of capacitance between the filaments and the cathode (?) that leaks into the output stage of the
amplifier. One way to minimize this is to twist the wires that supply current to the tube
filaments. I did this up to the point where the wires enter the circuit board. The circuit boards
copper traces therefore may be the source of the 60 Hz leak. I may pull the wires off the board and
try to run them directly to the filament pins of the individual tubes - twisting them all the
way.
Or I may not, since the hum isn't obtrusive and doesn't bother me.
What a fun, small, dense little amp. It reminds me of, I don't know, a hot little lunch box with
a brick inside. And I like how it looks too.
The thing gets hot though. I measured the temperature of the enclosure top and saw it climb
fairly quickly up to 140 degrees Fahrenheit before stabilizing. After several hours the power
transformer too begins to
get fairly hot. Also, as someone else noted on
Audio Asylum, the tubes, when first powered on light up briefly like light bulbs. Quickly they
drop down to the more familiar orange glow of a vacuum tube. In fact I think one or two are worse
than the others, so it is probably tube specific. All in all, I loved it. I would highly recommend the project to beginners (an easy first kit) as well as to the more experienced kit builder (it is affordable enough that you will enjoy building it just to have it around as a conversation piece). Like me, take yours into work ... run your iPod into it. |