300b lovers

The evolution from the original Karna amplifier, built by Gary Pimm in 2002 and designed by yours truly, to where it is now, is a desire for rationality combined with reliable performance. The original was a proof-of-concept, the kind of thing I’ve been doing since Shadow Vector in 1975. I’m not so good at manufacturing engineering, which is where my collaborators come in ... Gary Pimm, Thom Mackris, and now, Don Sachs. These guys keep me on the straight and narrow and argue me away from my wilder flights of fancy.

The original Karna was based on a desire to get rid of all capacitors in the signal path, since I truly despise these things. But realistically, transformers are mostly problem solvers, so it’s better design practice to give them a problem to solve.

1) The input transformer accurately phase splits and offers ground isolation between preamp and power amp, if desired.

2) The interstage offers Class A1/A2 operation for the output section, a favorable load to the driver (compared to RC coupling), and balanced Class A drive for lowest distortion into a reactive load (the Miller capacitance of the 300B grids).

3) The output transformer multiplies current by about 28 times, so the peak currents the 300B offers (in the 200 to 300 mA range) now become many amps at the 4 and 8 ohm taps.

All three solve problems. The first interstage in the original Karna was mostly there for spite, to get rid of coupling caps once and for all, just to prove a point. But that required a low impedance tube, and I didn’t want to use a 6DJ8, which is woefully unsuited to power amplifier use. That left a family of industrial tubes that are reasonably linear (much more linear than a 6DJ8), the 5687/7044/7119 family. They worked reasonably well but I wasn’t entirely happy with the tone quality, but I was stuck, since the high plate impedance of the 6SN7 made it a no-go.

The founder of Tribute Transformers saw this as a challenge ... 15 K plate-to-plate ... and built four interstages with an 80 kHz bandwidth and perfect square waves. They’re in my Karna amps and Gary Dahl’s Amity amps now, and no, they weren’t free, both of us paid in full for them. So yes, it is possible to build an amp with two interstages and no coupling caps at all. But ... using a truly exotic interstage, a genuinely one-of-a-kind design from Tribute.

I wasn’t going to lay that burden on Don. We started with the greatly simplified Symmetric Reichert and improved it step-by-step, edging back to the original Karna, but certainly not using a four-box design with a quite exotic grounding system, aviation-grade Amphenol connectors, and four massive power supplies. You really have to draw the line somewhere.

Don had a lot more experience on the practical side, as well as real-world production experience with top-quality, and most of all, reliable power supplies in his well-regarded KT88 amp. So off we went.

I have never had a wooly or bloated 300b amplifier owned about 30 of the things not one had any bloat. All had a very wide frequency range when powering proper loudspeakers. Now if you don't use loudspeakers that are correct for SET or 300b amps they may have issues think that's where most of the bloated 300b comments come from. Since I see many SET or PP 300b etc users running speakers that are not best used with it.

It’s been fun writing about the zigs and zags that took Don, myself, and the team at Spatial to where we are now. More to come? Who knows? The preamp (with remote control thanks to Khozmo) and power amps are off to a good start.

Phono preamp? Ooof. Now that would be a challenge, not one I’d like to take. DACs? Definitely not. I leave those things to the pros. Speakers? Spatial already has a crackerjack in-house team, plus I have no clue how these dipole things work. Maybe they could teach me.

People ask me about 100-watt or even 200-watt tube amps. Designing around banks of KT90’s in massive parallelism, mostly likely in Class AB, is pretty much the polar opposite of the current project. Seriously, if you need Big Watts, just go Class D (with Bruno Putzey tech) or buy the Manley professional products. Both approaches are reliable and get the job done.

To Lynn's comments about the need for big power for your speakers, that Class D amps get the job done, I would humbly add that an excellent tube preamp such as Don makes in the path adds a measure of bloom and euphonic sound that so many of us desire.  I prefer to own speakers that mate well with tube amplification, but that is just me.  

The preamp, since it has balanced transformer-coupled inputs and outputs, breaks the ground connection between components, reducing hum, as well as RFI breakthrough and buzz. That’s why transformers are often used in noisy professional applications, and are used here.

This comes in handy for digital sources, since all ultrasonic noise (outside the 50 kHz bandwidth) is scraped off, as well as complete ground isolation. Rowland has been doing this for decades, and they're on the right track.