Looking for a single-ended hi-fi amp that really ROCKS? You'll notice that this cathode biased amp shares the same general layout and chassis as our single-ended 2A3 project but the output stage uses a single triode-strapped EL34 per channel (see the 100 ohm resistor connecting lug 3 and lug 4) rigged to the the 2.5K ohm primary tap of a Hammond 125ESE output transformer. You can also use KT88s, 6550s, KT66s, KT90s, EL37s, 350Bs and other popular octal based output tubes with no modifications, although you may want to experiment with different output transformer primary impedances to optimize/personalize this stage. The sound of the EL34 is a bit more 'forward' than a true directly heated triode but still smooth and rich with of plenty of single-ended mojo. This amp is pretty easy to build, offering lots of opportunities to experiment with the sounds of different types of parts (e.g. Angela/SCR Metalized Polypropylene Caps, Sprague Atom conventional electrolytics, etc.). One of our goals was to offer another project that could be put together by almost any experienced tube hobbyist with inexpensive, non-exotic 'off the shelf' parts. A good circuit, well constructed, will shine no matter what parts you use. Yes, this notion is guaranteed to infuriate brainwashed High-End tweak parts purists. So what?
Note that our experience prototyping this project illustrates rather boldly the possibiliy of variation from 'cookbook' specs of inexpensive 'off the shelf' power transformers. Yes, the 272HX is supposed to put out 300VAC per side on the high voltage secondary but we actually measured 322VAC with 120VAC out of the wall, 311VAC with 115VAC in. Also, the 272HX will run a bit hot in this circuit, even though the load is well within capacity; this is 'normal', believe it or not. If this troubles you, use N.O.S./Mil. Spec. iron with similar specifications if you can find it.
I've tried lots of different output
tubes in this amp and my favorite
so far is the "Siemens" EL34. Sounds like a 'mini Marantz'! Save those
old Mullards for a guitar amp that needs 'em.
Wiring the power tube sockets? From the schematic and the pictures
below
you'll note that lugs one and eight are linked together and that a
short
length of yellow wire connects to the one end of the 880 ohm 25W
cathode
resistor along with the positive leg of the 100uF100V capacitor. Lugs
two
and seven are for the EL34 filament. The brown wire from the primary of
the output transformer connects to lug three, the PLATE, along with one
end of the 100 ohm 1/2W resistor. The other end of this 100 ohm 1/2W
'triode
connection' resistor connects to lug four; "G-2" of the EL34. Lug 5
connects
to one end of the 1K 1/2W resistor that runs over the terminal strip
where
the other end of that resistor meets one end of the .22/600V signal
capacitor
as well as one end of the 330K 1/2W resistor. There is no connection at
lug six. As you look down into the chassis with the amp turned upside
down,
note that the right side power tube socket is where the two twisted
green
6.3V from the power transformer connect to lugs two and seven. A
twisted
pair of white and black wires run from lugs two and seven of the right
side power tube socket over to lugs two and seven of the left side
power
tube socket. Another pair of twisted black and white wires runs from
lugs
two and seven of the left side power tube socket over to the filaments
of the 6SL7s.
VOLTAGES FOUND INSIDE TUBE AMPLFIERS CAN KILL
YOU!
IF YOU DON'T UNDERSTAND PROPER ELECTRICAL SAFETY PRACTICE, DON'T BUILD
THIS AMP!
6/9/2003 JE
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