Push-Pull 6A3 power amplifier
Jonathan Noble
The Tales of Brave Ulysses
My first exposure to Vacuum Tube audio came in my high
schools years when I was fortunate enough to purchase an old pair of Quad 2 valve power
amps (with originals Mullard KT66 tubes). I paid just R50, folks that's approx. 8 US
dollars for one mean audio classic! Well when I got them home and all rigged up I was
truly amazed to hear such beautiful sound. I have been a valve man ever since. Until
recently, however, I knew nothing at all about triodes. Now that I have completed the
building of my first DIY triode amp the Quads are on the shelf and my lounge is filled
with a truly glorious three dimensional sound.
It was through surfing the net that I hit upon all the wonderful exotic sites (like this
one) devoted to horns and triodes. Reading about others DIY projects began to make me
really excited. I just knew that I had to build one of those wacky SET designs. Shortly an
opportunity came my way in the offer an inexpensive pair of Fisher PP OPT's. Single
ended must surely be the best, but a PP triode configuration seemed a good economic choice
for my first DIY experiment.
Initially I intended to build a regular PP circuit. That was until I discovered Michael
Paquettes PP2A3 amp design (see his web site: ) which uses an Interstage transformer for
coupling and phase splitting. I fell in love with the symmetry of this arrangement. My
little 'economic' experiment just got a bit more expensive; it has been growing ever
since.
I have called my amp Brave Ulysses, he is very brave as he has only eight watts. To date
Brave Ulysses has been built almost three times over, and with each re-build the sound
gets better. My circuit is worked quite closely on Michaels layout, only I have done some
thinking about the power supply. Here in South Africa it is quite expensive to import the
exotic parts that triodes demand. So my ideas have developed around what is available. You
can see for example that my power supply uses three chokes, real overkill! This is an idea
used by Susumu Sakuma. I have discovered that chokes are relatively inexpensive
to have wound, that is here in Johannesburg, and that many chokes allow me to use low
value pp caps that are easily available. It also means that Brave Ulysses weighs 42 Kg for
the stereo pair, that works out as a ridiculous 5.25 KG per stereo watt!!
Recently I got lucky with the offer of some Leak TL 12 .1 iron and PIO caps for a real
bargain price, so I have had to accommodate this into my chassis layout. I have not as
yet, however, mounted the Leak power tranny, that is going to require a bit of future
surgery.
A 6J4WA driver swap is planned for the near future!
There are a few features of my amp layout are I think are quite successful, may I point
out the following:
The use of a separate chassis plate for the driver stage. This will allow me to mod the
driver without messing up the chassis as a
whole.
The floating of the chassis plate above the timber box. It looks great in the flesh,
although quite tricky to build. The chassis plate is powder-coated aluminum
and the
box is made from MDF. The MDF was treated with artists oil paint mixed with woodock (a
natural wood varnish) for a way cool modern finish.
My mono-blocks are designed to right next to the passive pre section in the middle. This in
effect makes for an integrated amp design in three separate boxes, which is just as well
as I don't think I would be able to lift the 42 Kg all in one go!
My pp6A3 seems to be a great match for my newly constructed 92 db/watt sensitivity ARIEL
loudspeakers. It is only eight watts, but this single digit power proved enough to rock
the pants off a 'good quality' 120 watt MOSFET setup that I compared it with.
Regards from Africa's biggest triode fan.
Jonathan Noble
email: 013noj@cosmos.wits.ac.za
Schematic of the signal and supply section
Link to Cuno Snoeren's site about Ariel speakers
