I agree the hum can't be too bad, but as you pointed out it's certainly prevalent in what we are told is a pleasant sounding design. And striving to get lower distortion and flat response may not actually sound better, depending on what the aficionados are looking for.
Just adding my $0.02 CAD here... Hum is really annoying in amps of any kind. It's annoying to hear hummmmmmmm from the listening position during pauses in the music. Even if it's not so audible from the listening position it creates intermodulation products with the audio. That results in a fuzzy or muffled sound and is pretty obvious in an FFT of the output.
When I designed the
DG300B I aimed for hum & noise levels below 1 mV, RMS. I was able to achieve about 0.3 mV RMS by regulating the filament voltage and also the B+ voltage. I still sell the
21st Century Maida Regulator that resulted from that work. I also aimed for low distortion. The tube itself is very linear, but output transformers are not particularly linear devices. The ones for SETs are pretty bandwidth limited as well. You'll never get a ruler flat 20 Hz - 20 kHz amplitude response with an SET.
I ran the 300B pretty hot. 400 V (plate-to-cathode), 85 mA plate current, 5 kΩ plate load. I was able to get to 10 W (3 % THD) with 0.25 % THD at 1 W that way. Many use 360 V, 60 mA, 3600 Ω plate load for the operating point. That supposedly gives more "sweetness" (whatever that means). But you only get a few watt before the THD rises past 1 %. That's not my idea of a good time. That could explain the poor performance of the reviewed amp at 5 W, though. It would be more insightful with a measurement at 1 W.
1 mV hum & noise at the output of a solid state amp would be outrageous. The Hypex/Purifi amps are spec'ed to 100 uV RMS (0.1 mV RMS) and I find that kinda high. My current solid state amp (
Modulus-86) measures below 20 uV RMS, unweighted.
Whether you like a squeaky clean amp or not is a personal preference. Some like the "li'l something-something" that the 300B adds to the sound. And I'll admit. It sounds really good on relatively simple music. But if you push it with some more complex music like Joe Satriani "Seven Strings" and nudge the volume knob a bit, you'll probably not be so impressed by the 300B. But not everybody listens to Satriani with the volume cranked to 11. I sunk a lot of money into the 300B design. I had a lot of fun with that. But ultimately I find that a squeaky clean and powerful Class AB amp is the cat's meow - at least for me. We don't all have to like the same stuff.
I exited the tube market because each build requires individualized support. Parts, in particular transformers, are getting harder and harder to find. The Classic-Tone power transformer I used - the only off-the-shelf model that could provide all the voltages I needed - is no longer in production. So it became a huge time sink to support the tube circuit and the little money made did not justify the time spent.
Those who are interested in circuit boards for tube amps should contact George Anderson of Tubelab. Specifically his SSE could be of interest:
http://tubelab.com/designs/tubelab-sse/ Note that he's about to retire full-time and will close up shop by the end of 2023. So you might want to order soon.
Tom
