The 5-Buck Amp
single 50EH5 pentode per channel]
<A cross between a Silvertone,
a ground fault isolator, some dead computer monitors,
and an old obselete scanner (+ a few parts from the bins)>
|My First Tube Amp
For my first tube amp
project (I have built some Dyna kits) I choose something that was very simple and
had some nostalgic value.The genesis of this amplifier was a Silvertone record player
my parents got when I was a kid -- one of those with the speakers on the side and
the TT flips down in the middle. One of my 1st DIY in my early-teens was to convert
the single unit into a TT, an amp & a pair of speakers (double walled leaky box
using wood from a mandarin orange crate).
Well, the amp survived all my parent's moves and I ended up finding it. With the
help of Allen
Wright*, the circuit
was updated. Mostly it was a case of undoing the accountants cost-cutting measures.
*(Allen also came up with the name -- "... bizarre - a pair of $50 tube coolers
on a $5 amp!")
SE 50EH5 amplifier schematic
<clik images for larger pictures>
There were many parts donors
for this project. Few things were actually paid for:
- The output transformers and
the heater resistor were reused from the original amp
- A mains transformer was salvaged
from a ground fault isolator
- All caps were recycled from
dead computer monitors
- The chassis is made from an
aluminum extrusion from a Centronics parallel to SCSI converter box from an old Microtek
- Tube sockets came from a parts-bin
- The 5-way posts came off an
experimental set of mid-horns I built in the early 80s
- The fuse and power cord donated
by a dead Pioneer integrated amp
- Some resistors came out of
my bins, and I had to go out and buy 4
- The RCA inputs were on-hand
from a batch I got from ApexJr for some real cheap price
- And the $50 tube coolers were
rejects I picked off the floor after helping Bill
get an order off to England.
The amp is 8.5" W x 4" D x4.5" H (with tubes -- the chassis is only
1.75" high) 22.6 x 10.2 x 11.4 (4.4) cm.
This diminuative little amp
puts out on the order of 1 Watt.
I plan on adding wooden bottom
& end plates (Arbutus is on the top of the list right now) to finish it off.
Currently the tubes are the
Japanese ones that shipped with the amp -- I have a pair of NOS GEs to try once I'm
ready to start tuning it.
Initial listening tests are encoraging (Linn into NAD 7020 into 5-Buck amp driving
speakers). This 1W amp
plays these to a satisfying listening level, and gives at least a hint of what SE
is all about.
Note: after about a year the mains trafo failed. Doing the math i realized that with
the heaters on the trafo i was drawing WAY too much current. So i just hooked that
up to the AC (and made sure it was completely isolated and self-contained -- it really
should be done with a 2nd trafo) and replaced the trafo with another.
Amp during reverse engineering
The amp hadn't been plugged
in for 20+ years. When I fired it up with variac it hummed -- big time -- but it
still worked. (It sat in a seperate wooded enclosure when in use).
The chassis is a piece of Aluminum extrusion from a Centronics parallel to SCSI converter
box from an old Microtek scanner.
with 120 V --> 120 V isolation transformer
8" W x 4" D x 1.75" H
with existing amps Output Transformers and the tubes for an idea of their size in
relation to the chassis and Nokia 5120 for scale (and a little color)
||October 7 -- the original
amp is brought up on the Variac. It works, but hums big-time.
October 8 -- "breadboard" the new circuit -- it works! No hum. Even
with only FM source & a pair of modded (real) cheap 3" computer speakers
it hints at the beauty inherent in SE.
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