The road to nirvana |
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My journey into Photography, Horology, and Audiophilia |
300B "Assault" Stereo Amplifier
I called this 300B amplifier Assault because it literally is, an assault on my wallet.
This is my first attempt to make a transformer-coupled SET amplifier and I thought of going all-out when it came to the major components. To begin with, my Lundahl LL1648 has been practically used as a prototype power transformer as it was never permanently used in some of my amplifier projects.
I also have a pair of Lundahl 1660S/10mA which was used in a PP EL-34 prototype which while successful, did not materialize. A couple of email exchanges with designer Per Lundahl confirmed that the LL1660S can indeed be used in SE-SE configuration using a modified ALT-T (20mA) or ALT-S (10mA) wiring.
I bought a pair of WE 396A which was intended for pre amplifier duties, but I decided that they be used as drivers for this project instead. I also have a pair of Raytheon 2C51 which is an electrical equivalent so I pretty much have a reserve. The driver sections will be configured in parallel and will be biased to 2V by an LED. The estimated current is around 8mA per section, for a total of 16mA so the LL1660S will be configured in ALT-T (2 : 4.5 ratio).
The recently purchased James JS 6113HS will be used as the output transformer, utilizing the 3.5K tap. This means that my Tri-Amp, the AK-300 will take a back seat, for now. Since the 6113HS has a max DC current of 70mA, I am aiming for >8W in fixed bias configuration, but my first iteration will be cathode biased.
The filament supply is going to be initially AC and I plan to implement a DC current regulator if I'm not happy with the noise level.
Making the chassis is my greatest weakness, so I will be contented with an aluminum top plate an an ebony wood base. As much as I would like a chassis found on most China-made amplifiers, I don't have access to those right now.
During the assembly, I made a couple of mistakes, but was fortunate to have spotted them early. The first was the PSU capacitors, the big 47uF Nichicon Muse, were soldered with the wrong polarity. The second was the wiring of the 0D3. I made it a series regulator, instead of shunt. I felt like a newbie.
Another problem that ate up my time was that the left 300B always have a wrong, very high bias, resulting in no sound. Measuring the cathode resistance, always yielded the right value and I thought of pulling my hair. In the end, I found that the hum balance potentiometer that I used, was faulty. I replaced it with a huge TDO 50-watter hum balance pot which is over-kill for the requirements, but at least it works, as always.
All in all, it took me more that a week to assemble, test, and troubleshoot the amplifier. But the results were rewarding. This is the best sounding 300B I have every built. It's fast and dynamic, it has grunt and balls, and I like it for being able to rock! A friend who heard it at home, christened it "Stormbringer" borrowed from one of Deep Purple's composition.
I don't have Tung oil to wipe the ebony base with, so I ended up giving it a dose of Virgin Olive Oil.
Here is the current schematic.