SE171A Amp Remastered

My dad would've turned 97 today. To celebrate his birthday, the video clip above features a recording of his favorite violinist, Nathan Milstein, playing Presto from Bach's Sonata No. 1 in G minor for unaccompanied violin. While he was doing post-graduate studies at The Juilliard School in the mid 50s, he was fortunate to have attended concerts of the great violin virtuosos of that era - Jascha Heifetz, David Oistrakh, Nathan Milstein, etc.

He would later recount that Nathan Milstein's performance of the Glazunov violin concerto was the most unforgettable! Even if he never got a chance to add this violin concerto to his repertoire, he conducted the Manila Chamber Orchestra with me playing the Glazunov violin concerto in 1987. The music of the slide clip above is the middle movement from our performance.

Before I bore everyone reminiscing about my dad and our life as musicians, let's get on with the subject at hand. The amp featured in this upload uses a directly heated triode power tube that was introduced around the time my dad was born.

The audio adage "it's the first watt that counts" was minted during the audio renaissance spurned by Sound Practices in the 90s.

However, this amp doesn't even qualify to that standard since it was only designed to generate 790 milliwatts per channel as a single-ended amplifier. To my ears, those are precious milliwatts! 

I dubbed the amp "remastered" because it started life in the late 90s with a more complicated circuit. In my craft as a musician, I was trained to treat every performance as an opportunity to discover more nuances embedded in the composer's score. So I'd like to think that I actually built a better SE171A amp this time around.

!!!WARNING!!! 

The voltages in this project are potentially lethal! Proceed at your own risk!

IME, musical sounding pentodes like the EL34, EL84, 6F6, 6V6, etc. sound even better when connected as triodes. 

Since I've been enjoying the 6C6 pentode driver in the Radiotron SE2A3 amp, I chose its octal descendant, the 6SJ7 wired in triode mode as the driver tube for this project.

Another pentode worth considering is the grid-capped 6J7. With the proper adapter (as pictured), it is plug 'n play in this amp.

As the schematic shows, this is a simple 2-stage amplifier with a triode connected 6SJ7 input/driver tube choke capacitance coupled to a 171A triode terminated by a 5K primary Z single-ended Tango H5S output transformer. When budget permits, I'd like to replace the Hammond 156Cs with Intact Audio Nickel plate chokes and, at some point, I might also try 6C8Gs and 6N7s.

The power supply is a straightforward tube rectified CLC + split RC rail for each driver section. 6.3VAC light up the driver tubes and, as noted in the schematic, each 171A triode requires its own 5VDC filament supply.

In spite of the decent 10kHz rise time, the 100 Hz and 1 kHz square waves indicate less than ideal bass response. The lack of bass quantity is made up for in terms of quality. I actually hear clean and clear bass note definition in real world listening, which, IMHO, sounds better than many more powerful amplifiers with much larger output transformers that's just booming. 

With 1.7Vrms at the input, I managed to coax 2.543Vrms output, which translates to 808mW into 8 ohms before the onset of clipping. If these waveforms discourage meter readers from building the amp, so be it. Given the finite supply of 71A triodes, ideally, I'd rather that only those who love music enjoy them.😊

Because of its exceptional ability to provide low level detail, this amplifier doesn't encourage the listener to crank the volume up, which practically gives it headroom. OTOH, if and when it's pushed to its limits, you'll experience the most graceful clipping. Instead of flat out distorting, it'll just compress. Driving the proper speakers, it's just a beautiful sounding bonsai of an amplifier!

When the 171A triode was introduced, the heyday of the roaring 20s was at its tail end. In 1929, the Wall Street stock market crash caused a worldwide depression. Suffering people were easy targets for demagogues and their deafening proclamations for a better world. Eventually, the empty promises of these self-serving authoritarians driven by their unrelenting lust for power, would lead to the carnage and devastation of World War II.

As the saying goes, "those who don't learn from history are doomed to repeat it." Nearly a century thereafter, we are facing a similar situation. So after updating myself with the latest news and world events, which usually isn't very pleasant, I hook up this flea-powered amp, choose music for introspection and ruminate that hopefully, there are enough like-minded people who won't allow history to replicate itself.