Angela Instruments

HOW TO: JE LABS PUSH-PULL CLASS A 2A3 STEREO POWER AMP PROJECT

VOLTAGES FOUND INSIDE TUBE AMPLFIERS CAN KILL YOU! IF YOU DON'T UNDERSTAND PROPER ELECTRICAL SAFETY PRACTICE, DON'T BUILD THIS AMP!

This amp evolved from my many years of tinkering, experimenting and collecting vintage tube gear and its related literature. It produces a solid 8 watts of pure Class A power per channel right before clipping. As long as you do not require head banging SPLs, in the past I have used this amp design successfully to drive LS 3/5As (I hear you Steve...), ProAc Response 2s, ARM1s, Raydeau Shaque Quatour eLex, Dynaco A25s and a couple of quarter wave pipe designs to satisfying levels.

No negative feedback is implemented as I have discovered that feedback only makes an amplifier look nice on paper but robs music of life and vitality. Also with directly heated triodes like 300Bs, 2A3s, 6B4Gs, 50s, 45s and etc....it's quite unnecessary due to a triodes inherent lower plate resistance compared to pentodes and beam tetrodes.

I am not an electronics engineer. I do not claim any special expertise nor innovation in the design of this amp. Basically, I trusted my ears and experience. The input/driver stage is inspired by my favorite and classic Mullard 5-20 circuit. This topology was used in the Marantz 2, 5, 8, 8B and 9, as well as in the Eico HF60, 50, 35, 87, 89 and many other classic tube amplifiers.

My design uses half of a 6SN7 as a voltage amp direct coupled to another 6SN7 in a long-tailed driver/phase inverter, capacitor coupled to the grids of a pair of 2A3s in push-pull, cathode biased for Class A, loaded by a high quality vintage 7K primary output transformer pulled out of a junked Fisher 500C receiver that Steve M. was trashing...OK, I offered a couple of 10 notes to discourage him...We are not actually cultural vandals here at Angela Instruments. Anyway, I prefer the use of octal based preamp type tubes whenever possible due to their larger plate structure, as they sound warmer and more dynamic to my ears. The Art Deco look is also a plus!

The power supply uses IN5408 high speed diodes in a voltage doubler, feeding a pi filter. This arrangement is again similar to the Marantz 8B and Citation II. In spite of the budget nature of the design, I used a combination of selected carbon, metal oxide and wirewound resistors matched to 1% tolerance. Facon paper in oil coupling capacitors, Kimber hook up wire and silver solder. The aluminum chassis is finished in silver Hammerite textured lacquer, ala vintage Western Electric theatre amplifiers. The transformers are finished in vintage '20s style black wrinkle paint. The input jacks are good quality gold plated RCA types. The output terminals are presently wired to the 8 ohm taps, with additional 4 and 16 ohm taps available under the chassis; see schematic for details.

I may have gone over the top in terms of the amount of labor and parts quality I spent in building this amp at a 'modified Dynaco ST70 price'. In my honest opinion, my 2A3 amp sounds better than a lot of circuits I tried on an ST70 chassis short of actually reconfiguring the unit to use directly heated triodes. It is my motto to share my joy in music making whether as a violinist or as an audio DIYer to music aficionados. ENJOY!

Joseph Esmilla, March 5 1997.

Upgrades and Tweaks!

For the last 4 years I have been using a more "deluxe version" of this circuit built on a pair of
Heath W4 chassis using the original choke, power transformers from a Scott 99 integrated and
Peerless 16309 OPTs, LCR caps (recently upgraded to Cerafines), paper in oil caps for coupling
and generic mix of "selected" resistors (metal oxide wirewounds for the plates and carbon for the
grids). I tried other "vintage iron" from my stash that worked just as well, such as the W4 Chicago OPTs and Acro TO 300. If you prefer new iron, check out the PP line from Audio Note, anything from 5K up. Just remember that a lighter load (higher primary 2) = less distortion, better damping and about 1 W less power while the 5K standard recommended load offers the most efficient transfer of power. If you are an experienced tube DIYer proceed reading a few of my suggestions.

If you Plan to use this as a bass amp in a multi-amp set-up, try the Audio Note 4.3K UL and
DEFINITELY use fixed bias. A couple of years ago, I built this circuit on an ST70 for a friend
and realized that anything lower than a 5K load makes the sound too mushy with cathode bias.
With fixed bias the bass is tighter, with more definition and speed while the sweet midrange is
still there but just a tad bit analytical for my taste!

Tube rectifiers do make a significant difference in sound and I prefer the extra lush quality it
provides. I use a 5V4G in my amp but also tried the GZ37 and 5U4 which are, perhaps, a bit of
an overkill in mono block configuration. But definitely a MUST if you opt for a stereo chassis.
My power supply design is probably going to elicit controversy among tech-types, by taking the
driver stage supply from the output of the choke' and dropping the B+ after the choke through a
"big resistor" for the output stage. Admittedly not the most elegant approach, but it has worked
reliably on my amps.

The Angela Universal power transformer is probably the best bet for a mono block design. With
its dual secondary HT winding, you can build two separate supplies for the output and driver
stages, at least 400-450 volts of raw supply for the driver stage for maximum voltage swing and
anywhere between 300-340 volts B+ to the center tap of the output transformers. I never tried
this type of power supply configuration but have seen similar designs from JC Morrison.

Otherwise you can copy my "goofy approach" and choose from the Hammond line. You need a
secondary winding for HT anywhere between 700-760 VAC CT rated at least 175 ma. for mono
or 350 ma. for stereo. While you are there pick a choke, between 5-10 Henrys with at least a DC
current rating of 150 ma. per channel. And do not forget 2.5 V @ 5-6 amps filament transformers for the 2A3 heaters or use 2 X 2.5V @ 2.5-3 amps if you want separate bias for EACH 2A3.

Cerafines are definitely worth spending for although you can do pretty well even with the cheap
LCRs. As for Black Gates, copper foils for coupling, Riken and/or Tantalum resistors, go for it if you can afford them and don't tell me!

My present set of Chinese 2A3s have been running at maximum plate dissipation since I built the
amp. They are rugged, cheap and do not sound bad at all. Save those NOS RCAs and single
plates for SE applications! For the 6SN7 input/driver tubes, my favorite combination is a 5692
for the voltage amp and GE 6SN7GTBs or brown based Raytheons for the drivers. Audiophile
neurosis is best left to "glossy magazine" readers. Honestly I have used Sovteks in the past and
found them quite acceptable.

My audio buddy, Ding, is actually building a 3008 version of this amp. Last time I saw him, the
copper chassis was already punched out for his Tango XE 60 PP, with a nice maple wood trim
for a base carved by an Italian craftsman. I am not holding my breath though since it took him
almost a year to finish wiring his Audio Note Kit 1......