Hello my fellow Star Wars Fans,
Here is a fun amp that has been percolating in build for over three years. Pre-pandemic I was originally going to build a generic follow-on to my previous 6AM4/EL84 SE amp, this time refining the circuit to add voltage regulated screen grids on the output tubes. And then I watched the first season of the Mandalorian on Disney+, and I knew I had a new theme.
First, the skinny on the circuit, and then we can talk about the aesthetics.
The amplifier circuit itself is rather conventional: Single ended 6BQ5/EL84 output stage, operating in pentode mode, driven by triodes with a moderate level of negative feedback. Rather than the ubiquitous 12AX7, I went with a rather unknown gem, the 6AM4. This little ‘acorn’ triode delivers similar gain characteristics as one of the triodes inside the 12AX7, but it was specially designed and ruggedized internally to support High Frequency (for the time) Radio signals. It is a hard one to tame for audio applications however and will exhibit oscillation and feedback issues without careful wiring and isolation as it was originally designed for grounded grid operation. Certainly worth a read of its data sheet if you have time – very smooth and symmetric gain behaviors deliver crisp signal amplification.
The other piece of the puzzle that sets this amp apart sonically is the fact that the screens on the output pentodes are each individually powered and regulated. There are lots of opinions out there on the relative merits of this approach, but I have found that the extra circuitry and hassle of ‘dialing-in’ the voltages is well worth the sonic reward.
Quantitative amplifier performance specifications are all shown in the photos, but I will list them here as well. At 1 Watt RMS output @ 1kHz into an 8 ohm resistive load, THD is right at 0.5%. At the rated power of 3 Watts, THD sits around ~1.4%. As the amp pushes into soft-clipping at close to 4 Watts, THD still remains stable and low at just over 2%. For comparison, I also have plotted the harmonic spectrum for the amp at 3 Watts of output if it were to be operated with no feedback stabilization. Frequency Response and phase error for both channels is excellently matched, and the audio power band is wide and flat, despite the moderately sized output iron. The -3dB bandwidth spans from 33 Hz all the way to a HF roll off point somewhere beyond 30 kHz. As for hum – there isn’t any. Careful power supply design and wire routing saw to that.
Qualitatively, the amp sounds much larger than it is. Harmonic content is just right and gives a sense of air and soundstage without any hints of a sharp edge. That single ended mojo sound is there, just not overdone. Bass is not overwhelming, but it is tight and not flabby. With efficient speakers you will be amazed at how much musical energy 3 watts of output power delivers. While the amp was designed to pair with single full range drivers, I had no trouble running my old three-way Pioneer ‘eviction notice’ speakers to party level volumes with this amp. As with most of my builds, this one is designed around my soft spot for female vocals, and it doesn’t disappoint. Ofra Haza goes from ethereally haunting singing Love Song, to downright sultry and decadent in Temple of Love with the Sisters of Mercy. That said, the debut song selected for this amp in front of others was ‘Deutschland’ by Rammstein. Delivered through a pair of modest Dayton Audio 8” full range drivers in an open baffle setup the end result, was in a word – Tempestuous. Absolutely the best crowd reaction I have ever had with any of my amps – including DHTs. I attribute a good part of that to the synergistic match between the characteristics of the 6AM4 front-end and the 6BQ5 output, although it could also have been the free booze I plied the listening audience with… With no ego I can say that this humble assemblage of iron and glass delivers a magical balance between precision and art that few others can match.
Now for the aesthetics. I was originally planning for a steam-punk theme of some sort, but it just wasn’t going anywhere past wire-wheel polishing the power transformer and choke. Inspiration struck when I binge watched the first season of the Mandalorian. I knew then and there that I had to build an homage to that amazing universe.
An armored steel body painted to match Din Djarin’s original armor from season 1 that was experiencing structural integrity issues. There are some blaster burns, as well as some damage from other ‘kinetic’ engagements. One puncture on the top deck has been patched with tantalum foil to prevent a radiation leak. Unlike those over-confident Imps, the thermal exhaust port on this amplifier incorporates a pure Beskar steel shield plate. No pesky teenagers will be exploiting a weakness there.
The front panel is of rare Ironwood from Mustafar, full of unpredictable Sith energy absorbed over the tree’s millennial long lifespan before being smuggled off planet.
The front-dial indicator monitors… something…important? I can’t read Mandalorian writing. The magic eye tube acts as a power on indicator, and it also modulates with the output signal intensity.
The rest of the build fare is my standard offering – top shelf electrical components, safe grounding and fusing, careful and intelligent wire routing, stainless steel hardware throughout, and lots of bench burn-in and tuning before delivery.
This is the way.
Foxtrot 9 Audio 