300b lovers

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.

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.

This statement is false. The devices are not in series, else Kirchhoff's Law would prevent the second device from conducting if the first were in cutoff... At any rate if one device is in cutoff, the other will be in saturation which is the limit of any device's ability to conduct!

Quick recap: actually, vacuum tubes are far from saturation when set to normal bias points. Look at a 300B, or any other power tube. Normal quiescent bias is set between 60 and 85 mA, if Class A operation is desired. If Class AB is desired, 35 to 40 mA is more typical. With 400 volts from cathode (or filament), that's a steady-state plate dissipation between 14 and 34 watts, well within the 40-watt rating.

But that's nowhere close to the peak current emission of the cathode. I've measured 250 mA from a generic 300B, and the exotic European 300B's can slam out nearly 500 mA (transient). The only time I've ever seen a 300B current-limit around 80 mA were some particularly weak Chinese tubes from the mid-Eighties ... they sounded and measured pretty bad, and were near-defective. Other vacuum tubes are similar; the recommended quiescent currents are set by plate dissipation limits, not cathode emission maximums (which are left unspecified). Transistors will melt the internal copper links, but damaging the cathode in a vacuum tube is really hard to do unless the tube is operated with no B+ present.

It's transistors that have Safe Operating Area (SOA) curves that are log-log in both current and voltage (with an additional time dimension), not tubes. The current saturation mechanisms are totally different and have nothing in common.

Unlike transistors, vacuum tubes have very large areas of peak current emission that are left untapped by most circuits. Of course, plate heating goes up when these areas are explored, but unlike transistors, tubes do not fail in milliseconds (this is shown in the SOA curves of transistors, and must be respected). It takes sustained abuse, over many seconds, before mechanical deformation dooms the plate.

I think Ralph will agree that Class AB operation is not "false". In Class AB, one device cuts off (goes to infinite impedance and conducts no current) while the opposing device goes to a large multiple of the quiescent current. In conventional Class AB transistor amps, the idling current is a tiny fraction of the peak current, and in Class AB tube amps, it's still a small fraction.

Let's look at what happens in pure differential circuit, either tube or transistor, with a current source setting the quiescent current. This circuit must always operate in Class A. Unless something fails, the current source will always deliver the programmed current ... that is a hard limit that cannot be exceeded under any condition.

The late Allen Wright actually built a PP 300B power amplifier that had a current source under the pair of VV52B's (massive Czech power tubes). He stayed at my house during one of the VSAC shows, and we compared his amp to my early version of the Karna (which has bypassed cathodes and can operate in Class A, Class A2, or Class AB, or even Class AB2, depending on current demand). Allen's output stage was true differential, and true Class A, with a powerful solid-state current source running around 160 mA (if memory serves ... this was in 2003 or so).

The two amps sounded completely different. That's when Allen, and I, realized that differential, and balanced, are not in fact the same. This is a common illusion, a hangover from the Fifties. The question is what happens when one device cuts off.

When this happens in a current-sourced differential circuit, the "ON" device can never pass more than the total current programmed in the current source (by definition). That's a hard limit. It is a brick wall. The circuit, as a whole, will always pass whatever the current source is programmed to do ... no more, no less, always the same. This is why this circuit is seen in the Mullard topology as a low-power, medium-voltage phase splitter. Allen, as a big fan of differential circuits in Tek scopes, took it all the way and used it in a power stage.

This is quite different than a Class AB, or conventional Class A, power stage. Whether cathode or fixed-bias, current flow through the output pair is dynamic. IF (a very big if here) the output tubes were distortionless, perfectly matched, AND never voltage-clipped or driven into Class AB, yes, it would behave the same as a current-sourced pure differential stage. Only then are they the same.

But we don't live in a world of Platonic ideals. Tubes are not actually the same as the tube models, they are not perfectly assembled in perfect factories by robots, loudspeakers have odd ideas when they want lots of current, and bass drivers in particular are notorious for nonlinearity and very long energy storage .., all of which affects output stages.

So a power amplifier must deal with speakers as they are, not as we want them to be. So peak current excursions can be accommodated when necessary, without the amplifier grossly departing from basic design assumptions. The loudspeaker conforms to Theile/Small equations most of the time, but both Neville Theile and Richard Small warn us that these are only small-signal approximations. They are not valid once the voice coils start to move significantly. Speakers are only linear on average, not all the time.

My goal with Class A output is to synthesize a fixed output impedance that remains constant with real-world loudspeakers, which I have been designing since 1975. I know how awful speakers are. Most power amps use 20 to 50 dB of feedback to synthesize a perfect voltage source, and they do a pretty decent job of it. With zero feedback, the best I can hope for is a fixed, moderate-value equivalent resistor, about 2 ohms or so, which a low-Q vented or closed box speaker can deal with. And an output stage that does not have a hard current limit, but soft-clips in both voltage and current, without requiring protection circuits.