300b lovers

The Blackbird amps are running the 300b tubes at about 400V B+ (plate - cathode voltage), and just a wee bit under 80 mA. About 31.5 watts at idle. They sound wonderful and should last a very long time. The tubes each have their own independent regulated filament supply and are right at 5 volts. I have measured a lot of them and it is always 4.99 - 5.01 volts. All the supplies are regulated so the tubes stay at that operating point. It seems to be a sweet spot for the 300b tubes, although I suspect that dialing the operating point up a bit for the Euro super tubes would be an option. The supplies are run very conservatively and could easily handle the extra current.

One of the maddening things about the original Karna amplifier was sonic variability. Some days, it would be a glimpse of Heaven, and other days, nothing special. It was always extremely transparent ... that’s the nature of the circuit ... but the tuning came and went. Most of the time it was quite good, but every now and then, it was extraordinary.

By contrast, SET amps are usually consistent, any time of day, due to massive 2nd harmonic masking all the high-order harmonics. Tuning a SET is straightforward ... which parts complement the dominant 2nd harmonic most gracefully.

One of my experiments was bringing out the filament circuits on a separate power socket. I bought the Audio-GD AC regenerator, and powered the filaments with that. Sure enough, the variability went away, and I discovered quite small variations in filament power had a big effect on the sound, overwhelming any other tuning decision, including tube swaps. But the Audio-GD liked to run its fans, despite the easy load.

So I was open to Don’s approach of regulating everything, using the proprietary regulators he’d been using so successfully on his 6L6 and KT88 amps. Sure, why not? I’d already been splitting the high voltage B+ supplies between input+driver and output section (to prevent crossmodulation between sections), and using high-quality regulators for a balanced drive for the filaments and heaters made a lot of sense.

(I should mention regulating filamentary tubes is not trivial, and the usual 3-pin low-voltage regulators introduce unacceptable colorations. Filament and cathode nodes are extremely sensitive to coloration, and balanced discrete circuits are required.)

Sure enough, variability gone, vanished, like the morning mist. No change in sonics depending on the time of day. No annoying and objectionable regulator coloration, which is the bane of high-end audio ... that obnoxious grainy transistor sound, coming out of an otherwise good tube amplifier. Not a trace of that, thankfully. Reliable, too, which goes with good regulator design.

This alone justified re-naming the amplifier. I suggested Blackbird (because Red-Winged Blackbirds are a common sight in Colorado), and to my surprise, the name appeared in the Pacific Audio Festival show guide. So Blackbird it is.

Yes, these amps sound very good after 10-15 minutes and are very consistent.  They sound the same every day.  I have the preamp and amps plugged into a Puritan supply, which does clean up grunge in the AC line.   Basically, you turn them on and they sound quite good as soon as they can play music in 30 secs or so, but in 15 minutes or so the tubes are all warm and then they are pretty much on song until you turn them off.  They may sound a wee bit better after 30+ minutes, but really 15 gets you almost all of it.  Plus they run quite cool.  You can leave your hand on the transformers or anywhere on the top panel even after the amps have been on for an hour.  No, it is not a class D amp that barely warms, but they run quite cool for a class A tube amp.

This has been an interesting thread.  I didn't start it, nor did I expect it to have this long a life.  It is very clear that there are many ways to build amps and preamps.  I just want to restate my goal for this project.  I am semi-retired.  I wanted to build the best sounding preamp and amp combination that I could.  A setup that I can happily listen to the rest of my days and that will drive a reasonable subset of the speakers out there.   Within reason, I didn't care at all what it cost.  Once done it will be produced by a partner and priced according to what it actually costs to build plus enough to cover their labor and margin to stay in business.  It will be sold directly to customers though with no distributor markup.  But it costs what it costs to build because the main goal of the project was to produce the best sounding preamp and amp that was possible without have some four chassis amp or gold plated case and $50,000 starting point.  The thread has shown why we made the design choices we did and where it led us based on a combined (gasp) probably 70 years of experience with amplifier design and builds between Lynn and I.  He is the historian as well, and I have rebuilt literally hundreds upon hundreds of pieces of classic tube gear, so I guess that makes me sort of a hands on historian.  

At any rate, I thank everyone for their civility and free discussion of audio ideas that has made the thread enjoyable.  I hope to meet a few more of you next year at the Pacific Audio Fest when I will be there with actually production versions of these for you to hear.  Trust me, they are FAR better than the prototypes shown this past June.

I want one