600mW single-ended battery supplied power amplifier

Jim de Kort


This project is a continuation of the first 71A Puppy Power amplifier. Below I have placed the schematics of the new driver using a 31 instead of a KC3 and the new power supply.

I use a 10uF capacitor over the batteries to lower impedance of the entire supply. The 31 uses a 2V/5Ah SLA (Sealed Lead Acid) battery, the 71A uses a 6V/4Ah SLA with a 3.9 ohm resistor to drop the extra 1V.

Okay, what's the benefit your wonder. Why go through all the trouble. Well... I had to do something to keep me busy. The power supply was already being used on my new 26 preamp, might as well hook up the power amp and see if the same happens here.

Switching from the KC3 to the 31 had a big influence on it's own. The KC3 was very lovely; a warm sound, detailed, alive. The 31 was a bit thinner, not as warm but a lot more open and free sounding. In short it is a compromise, but isn't everything? The 31 also has a 'small' filament, meaning suitable for battery supply. At 2V/130mA it was a perfect replacement for the KC3 (2V/250mA). A few weeks ago I got a pair of amorphous OPT's from Tribute to test against the OPT's I had. You can read more about this on my Puppy Power 1 project. Just to give an idea: WOW...

Hooking up the 31/71A amp to a battery supply was a big impact. No more background noise, no more crap from the 230V mains. Total and utter silence, black as night. The sound so detailed and 'free'. Ahhhhh.... what a joy... This is the way to go with these tubes, like they are in their elements now. Sweet music.



Signal section, about 9V input sensitivity.

 


The B+ 'supply'



Top view that reveals the layout of the mono amp.

 

The battery supplies version uses the same chassis as my first 71A amps, but has some connections running to the two high voltage battery supplies I made recently (yegh, nothing to look at yet!). The caps and chokes on this supply are not used, what a waste ;)