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Its FUN to build a 2A3 amplifier....

Posted by drlowmu (M) on January 22, 2011 at 09:25:39

Show and tell:

It is always so much fun, and so special to me, to build a SE 2A3 amplifier.

This was orginally going to be a VAIC AV-32B amplifier, but when I tested one channel, it just couldn't power my inefficient speaker load as well as a reference quality solid state amp I owned, so, I am converting this chassis to become a 2A3 amplifier, direct coupled. The (necessary) highly-efficient speaker comes later, in 2011.

Top photo is the outer chassis, with four 5V3 rectifier tubes, a pair for each channel's B+ filter. The driver tube (half a Telefunken 12AX7, and the finals tube, a JJ 2A3-40) are not installed.

The middle photo is the underside, and it is better than 70% done. Notice, the use of multiple paralleled Kimber Kable TCSS wiring, to maintain reasonable transfer function. This is audible!

The B+ filter is a combination of Henry Pasternak's 2006 Flywheel, and Dennis Fraker's power supply concepts from 1989. L1/C1/L2/C2 for each 2A3 channel is 40 mHY at 700 mOhms (Signal CL-2-4), 20 uF (Electronic Concepts) film, 2.2 HY at 7 Ohms (the small green chokes), and finally, 45 uF GE 97F oil.

The bottom photo are my two (reworked) Hammond 2A3 2.5 VCT filament transformers. The outer ends were rewired with a couple paralleled runs of TCSS (green color) and the center tap return to ground is triple 21 AWG teflon coated silver wire. This center tap's lead length is purposefully kept short, under 2 1/8th inches, and each center tap will use two Mills MRA-12s to return the center tap to star ground. It is a good practice to have the shortest (UNcluttered path, no pots!) return of the 2A3's cathode to ground.

This amp, being done on a stereo chassis, forced me to make many (too many) compromises in wire lengths, but I thought it would be fun to post these teaser shots, none the less.

The DCR across half of the (custom) power transformer's center-tapped secondary winding is below 8 Ohms. The primary's DCR is under 0.8 Ohms.

Cheers,

Jeff Medwin

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