Following the trails of The JE Simple 45/2A3 and Steve's EZ 2A3 Project is another variation on the theme of a 5691 input/driver. The voltage amplifier is a normal plate loaded triode section 5691/6SL7 direct coupled to the other half configured as a cathode follower for low impedance driver stage, capacitor coupled to the grid of the 2A3 output tube.
Due to the 140V present at the cathode of the driver stage, make sure that the 6.3V filament center tap (marked "C" in the schematic) is connected to the voltage divider network at the power supply to avoid exceeding the +/-90V heater to cathode rating of a 5691/6SL7.
This amplifier produces around 4.5W per channel before clipping with an input sensitivity of.7V rms with the 2A3 output tube running at about 17-18W plate dissipation which is not a problem with USA made bi-plates, nor with better China or Russian examples. From experience, they sound better under these operating conditions. We don't recommend using costly, rare and fragile vintage 'single plates' in this circuit.
The chassis was fabricated 'freestyle' by me, Mr. Steve in my drive-in garage shop in East Bethesda, Maryland using nothing fancier than a drill press and a few chassis punches. I had the finished chassis coated with Polane T textured epoxy paint; I masked the area near the cathode resistors and ground points on the inside of the chassis. Joseph Esmilla and I collaborated on the concept and some of the parts choices but he deserves the credit for the fundamental circuit design work, wiring and testing.
So, how does this amp compare to other 2A3 designs? If a smooth, natural sound is your goal, it's hard to do better than this for the bucks. The power this little amp puts out, especially in the bass, surprises most listeners. No low power triode amp really 'kicks ass' or truly 'rocks' heavy metal style, but most adult listeners will find this little amplifier plenty loud in normal sized rooms. If you want a more exciting, more detailed sounding 2A3 experience, try our SRPP version. We encourage you to compare either of our 2A3 project amps with 'store bought' 2A3 amps costing much more; you'll wonder why they cost so much, since these projects cost so little to build.
Remember, this is a FREE PROJECT, not a 'kit' and we offer no 'package' of parts. Many parts (Hammond transformers, good tube sockets, Sprague and Jensen capacitors, tubes, etc.) useful for building this project are, of course, available from Angela Instruments. Check the web site pages recommended below and you'll find 'em listed with prices there, probably for much less than you'll find 'em elsewhere.
1. Chassis? Why the 1444-32 Hammond
aluminum 10"X17"X3"
and matching 1434 Hammond aluminum bottom plate, of course. Aluminum is
easier to work with than steel. Be sure to include lots of ventilation
holes on the top of the chassis, especially around the power tubes and
rectifier tube. Also cut some ventilating holes in the bottom plate and
use rubber feet or other bumpers to raise the bottom of the chassis
above
the floor slightly. Be sure to remove the rough around any chassis
holes
with sandpaper, especially those holes that plastic insulated wires
will
pass through. You'll notice that I used Radio Shack rubber gromments on
many of the chassis holes. See our CHASSIS pages for current price.
2. Power transformer Hammond 272HX. Mount this with the laminations at
a right angles to those of the output iron. I also try to keep the
power
transformers in my projects as far away as possible from the input
stage
of the amp. I installed the power switch, fuse assembly and AC inlet
port
on the back end of the chassis near the power transformer. See our
HAMMOND
TRANSFORMERS pages for current price.
3. Choke Hammond 193G. You'll note that the laminations of the choke
are
also at a right angle to those of the power transformer. See our
HAMMOND
TRANSFORMERS pages for current price.
4. Filament transformers Hammond 166L2, these mount underneath the
chassis
in our version of this amp. Note that the green/yellow center tap wires
are capped off with wire nuts. See our HAMMOND TRANSFORMERS pages for
current
price.
5. Output transformers Hammond 125ESE. Yes, Hammond says the 125ESE
wasn't
designed for critical high performance hi-fi but we know it won't stop
you from trying it anyway and it sure didn't stop us. Actually, I
encourage
you to use more serious output iron if you can afford it but you'll
probably
have to build on a more substantial chassis to handle the extra weight.
6. AC inlet port by Bulgin UK. See our CONNECTORS pages for current
price.
7. The On-Off switch we used can be found for sale on our CONNECTORS
pages.
We use this same switch in almost all of other web site projects.
8. Fuse assembly , vintage Fender style, hard phenolic plastic. Takes
full
size 2A SLO-BLO fuses, not those nasty mini ones. You'll find
this
part listed on our FENDER AMP PARTS pages.
9. Volume control pot, 100K audio/log taper stereo cheap piece of junk
I bought at Radio Shack! It works surprisingly well and cost next to
nothing.
The shaft of this pot is sized for USA knobs! WOW! An ALPS 100K will
probably
work a bit better if you've got the extra bucks to burn. This amp has
plenty
of gain on the input side, so you can use a line source like a CD
player
straight in with no preamp.
10. Knob was a Fender '52 Tele chrome dome, set screw fixing to solid
shaft
pots. See our FENDER TELE PARTS pages for current price.
11. Input jacks? Any good RCA jack with isolation washers will work
fine.
I used some Vampires I had laying around but the Kimber RCA input jacks
we sell on our CONNECTORS pages are nicer to work with.
12. Speaker terminals? I used Radio Shack banana jacks but I don't
recommend
that you do the same. Like many (but not all...) of the parts
I've
tried from Radio Shack, they really suck; they're about a cheap as you
can possibly make 'em. Too cheap, I say. I wrecked two pairs of these
fucking
things trying to tighten them down with only moderate wrench torque. I
don't honestly think higher quality (meaning, not inferior
quality...)
banana jacks will 'sound bettter' than Radio Shack junk but they're
probably
worth the extra bucks just to avoid the hassle.
13. Most of the polarized electrotytic type capacitors were Sprague
Atoms;
one 20uF/500VDC, one 100uF/450VDC, two 40uF at 500VDC, and two
100uF/100VDC
(between one end of the cathode resistors and the ground lug
nearby...).
See our SPRAGUE CAPACITORS pages for prices. You could also use
ANGELA/SCR
caps, Black Gates, ELNA CERAFINE or other high quality capacitors.
14. We got really crazy here and used a Nichicon Muse 100uF/100VDC
polarized
electrolytic instead of a Mouser cap costing $.20 less. See the
location
of this cap near the ten lug terminal strip back by one of the filament
transformers. See our LCR CAPACITORS pages for price.
15. Most of the resistors used in this project are Riken Ohm; see our
schematic
for values and ratings. For a couple of odd resistors rated higher than
1 watt we used miltary wirewounds; if you can find N.O.S. carbon comps
they'll work just fine, as will metal oxides if you can't. Cathode
resistors
are RCD 880 ohm finned aluminum type we use in our other web site
projects.
See our RESISTORS pages for prices.
16. Angela/Jensen paper in oil signal caps were used on the front end
but
we won't call the 'upgrade police' if you use SBEs or other plastic
caps
instead. See our various signal capacitor pages for prices and
selection.
17. Octal tube sockets are all Angela ceramic silver types. Plastic
sockets
will work fine if you want to save money. The 2A3 sockets are Johnson
style
wafer types from China. See our various TUBE SOCKETS pages for prices
and
choices.
18. Hum balance pots, anything between 50-100 ohms works fine. We used
a pair of Clarostat 100 ohm 2W wirewounds. Just stick your ear close to
the speaker as you turn each pot to the position with the least hum.
See
our POTENTIOMERS pages for prices.
19. Wire? Kimber braided PSB for the input between the pot and jacks.
The
rest of the internal wiring was done with various colors of Kimber 22
gauge.
See our WIRE pages for prices.
20. Terminal strips, ground lugs. One ten lug terminal strip is used;
see
our CONNECTORS pages for price. You'll also need some ground lugs to
fit
under the transformer mounting screws near the cathode resistors; buy
the
Ground Lugs Dujour package listed on our CONNECTORS pages. Also near
the
ten lug terminal strip find a common phenolic five lug (the center lug
is continuous with chassis ground...) terminal strip, available from
Mouser
and other 'generic' parts suppliers. Near the input tubes you'll need
to
use a couple of three lug (the center lug is continuous with chassis
ground...)
cheap phenolic terminal strips, also from Mouser.
21. Most of the fasteners used in the prototype are stainless steel,
available
from industrial suppliers and better hardware stores (NOT Home
Depot...).
22. Tubes? I used N.O.S. Sylvania double plate 2A3s, one of my favorite
triodes, for the power tubes. If you can't find some of these beauties,
cheap 2A3s from China will get you by. The amp was voiced with JAN
Sylvania
5691s; that's my pick for this position. For rectification I used a
GZ37
but other rectifiers with similar specs should work fine. See TUBES FOR
GUITAR AMPS AND HI-FI as well as our TRIODE ZOMBIES CORNER for current
stock and prices.
Sorry but time doesn't permit us to answer
technical (or nontechnical, for that matter...) inquiries about our
various
FREE web site proejcts. If you want to build these FREE projects you're
on your own. Remember, D.I.Y. means DO IT YOURSELF. Good Luck!
Schematic
And Photos
The Entire Contents Of This
Website,
Including This FREE Project, Are Copyrighted © By Angela
Instruments
2006
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