300b lovers

The Karna Mk II/Blackbird is bit by bit evolving towards the original Karna, but without the madness of a four chassis design. Having a separate chokes and power transformers for #1 B+, #2 B+, and the filament supply gets really heavy and awkward. Don’s monoblock approach is much more sensible, and more important, he has real-world experience of what is reliable in the field.

I design things as a thought experiment, just to see how it works out. About one design in three is a flop, and gets abandoned. You have a sound in your mind, and wonder if the real thing will sound like you imagined. Sometimes it does, sometimes it doesn’t. You never know in advance.

The Raven and Blackbird were, and are, thought experiments to explore what minimum intrinsic distortion would sound like. Zero loop feedback, and zero local feedback, with all cathodes fully bypassed. Balanced, but not differential, with passive transformers doing the summations and cancellations.

It is not SET, which require skillful arrangement of various colorations and very, very careful component selection. But it still requires careful selection of components because there is no feedback to minimize and wash out colorations. If XYZ tube has a certain sound, well, that’s what you’ll hear. If XYZ cap is imposing a coloration on the cathode circuit (which is a very sensitive circuit node), yes, it will be audible.

A big difference between the solid-state world and vacuum tubes is capacitance. Capacitance with tubes is essentially linear, aside from Miller capacitance, and even then, the delta in the capacitance is very small (no more than a percent or so). The transit time through the circuit is constant, regardless of signal. Part of making the transit time constant is passive (not active) phase inversion and summation.

Solid-state capacitance is known for varying with current and temperature, so it pays in transistor circuits to get the (nonlinear) capacitance to the lowest value possible ... if it can’t be linearized, get it as close to zero as possible. Modern transistors are much faster than previous decades, so this really helps. Build a very linear video circuit, and many problems are solved.

Lynn,

Could you expand further on your balanced, but not differential comment? What is your philosophy, what have you observed?

I can appreciate the desire to chase the sound quality rabbit to fruition, but it seems likely this will result in more boxes and a bit higher cost. No doubt there is a market for this. Maybe there are two markets? Two versions at different price points?

A bit of background on cost to the consumer: if a company isn’t charging Bill of Materials (BOM) cost times four, they won’t be around very long, one or two years at most. This rule-of-thumb has been true since the Fifties (for hifi manufacturing in North America).

Not true for cars, of course, since that is a hyper-competitive, extremely price-sensitive industry that has enormous capital barriers to entry. In electronics, the Chinese are able to shave it down to two to one, most likely due to a wide range of hidden subsidies that favor exporting.

So a smart DIY’er can indeed get serious high end for medium (not low) cost, partly by pricing their labor at zero. But even a very experienced DIY’er is going to find that building a Blackbird from scratch is the same as the price of a good used car, setting aside labor and debugging time. I know several people who got stuck halfway through building a Karna and wanted many hours of my free help completing it. No, that’s not how it works. You want a Heathkit, go buy one. If you can design and build an amp from scratch, more power to you! Have fun! Be glad you don’t have to use a slide rule any more, like the bad old days.

(Yes, I have used slip-sticks. They are no fun. You’re lucky to get 2% precision, and you have to do the calculation twice because it can be off by a factor of 10 or 100.)

Back to circuits. A differential and balanced circuit are not the same. A differential circuit has a current source or high-value resistor in the common cathode (or emitter) circuit, which is why they are called "long-tailed pair" in the literature. This forces differential operation, but has a limitation because the two tubes (or transistors) are effectively in series. If one device cuts off (impedance goes to infinity), then the other device is hard-limited to 2X the quiescent current. It can never go further, because the long-tail or current source hard-limits total current to both devices.

By contrast, a balanced circuit, without a long-tail or current source, can turn on the "on" device as hard as it likes. That can be as high as 5X the quiescent current or even more. It effectively slides over into Class AB if it needs to, unlike a differential circuit, which will hard-clip if too much current is demanded. The phase splitting is done by transformers, not a long-tailed pair.

By the way, if you are looking for value, you really should audition the Valhalla from Spatial. It took on every other high-bucks big-name tube amp at the show and came out ahead, often by a good margin. It is a seriously good amplifier at an absurdly low price.

Now, if you are looking for 100 to 200 watts of tube power ... hate to break it to you, but paralleling arrays of power pentodes does NOT improve the sound. Rule of thumb for PP tube or transistor: no more than two devices if you care about quality. Once you start paralleling arrays, there’s always just a bit of mismatch to trip you up. And that’s just DC matching, which is completely separate from matching transfer curves (AC matching). That’s a lot harder, and there’s always the issue of tubes drifting apart as they age.

The other issue with arrays of pentodes is the grid capacitance for the power tubes is multiplied, which then requires high-current cathode followers, or separate power tubes as drivers. This gets into No Fun territory as the design complexity multiplies, all just to squeeze out a few more watts.

From my perspective as an amp designer (not as a consumer or reviewer), the Sweet Spot in tube amps is from 3 watts (Class A SET) to 60 watts (Class AB PP pentode). These are all simple circuits with an emphasis on sound quality and reliability.

If you MUST have 200 watts, consider combining a modern Class D amp with a preamp like the Raven. The new Class D amps don't have the irritating and fatiguing Class AB sound, while a good tube preamp lends the sound some charm and likability.