by GP49 Tue Feb 09, 2016 11:17 am
The "one tube" pentode voltage amp/triode phase splitter was a widely used design, not only by Dynaco. Most manufacturers may not have used the 7199, but a 6AN8 or 6U8 instead. But the circuitry was otherwise essentially the same and its operating principles identical. What is true is that few of the higher-end amplifiers used the one-tube driver (and that is a misnomer, actually two tubes...two independent sections in one envelope).
Distortion in an output stage will behave exactly as Gillespie states. His hypothesis applies whether the push-pull output stage is driven by a single-tube 7199/6AN8 or a multi-tube setup using triodes as voltage amps. In fact the THEORY is the same if one assumes an absolutely perfect driver stage, which of course does not exist. The mechanism of distortion reduction from ideal biasing into Class A is independent of driver circuit type. Overall performance may be better with the more complex circuit...or worse if the more complex circuit is badly designed. But the principle remains the same. It even remains the same if the output tubes are driven by a SOLID STATE driver, which has been seen in commercial Stereo 70 modifications.
When I was doing audio work professionally, I was privileged enough to be using the best test bench distortion analyzer on the market, and did the same thing Gillespie writes about, on different amp drivers, driving output stages where idle current is altered by adjustment of that "fixed" bias voltage, the classic design used by Dynaco. His findings are correct. They are correct even in cathode-bias amplifiers, where the unit under test can have its bias, and therefore idle current, "adjusted" by altering its cathode resistor. For a given tube type, the results were always the same and dependent upon the type and design of the output tube. Reducing the idle current lower in the Class AB range invariably increased measured distortion.
Why, then, run the EL34 at a lower idle current? Because of tube life. The current-production EL34 is not really an EL34 at all, NONE of them. There is a specific set of specifications that define the EL34 and amplifier designers worked with those definitions when designing output stages. Variations in the current production tubes that are called "EL34" result in lower durability and tube life. For instance, the maximum plate voltage in the EL34 specification is a whopping 800 volts. Put that to today's Russian, Chinese or other "EL34" and they will arc over and be destroyed. Similarly the margin of safety in plate dissipation is lower. Tube life will be increased, and the likelihood of catastrophic failure reduced, if idle current is reduced. It is a TRADEOFF, not an optimization. Gillespie's figures and conclusion are not wrong, but his numbers for the distortion increase at lower idle current may sound worse than they are. If distortion in the output stage is 0.08% under ideal idle current, a not unreasonable figure, what would seem a whopping increase of 300% would be 0.24%, still not bad...and that increase could be swamped by the distortion in a non-ideal driver stage, so you wouldn't hear it. It could also be virtually inaudible if it is largely second-harmonic...and likely it would be.
I used the EL34 as an example because that is what is in my modified Mark II amplifiers. But the same applies no matter what the output tube. The behavior of the push-pull output circuitry remains the same. Unfortunately the difference between genuine old tubes by their original makers, and the copies, often reverse-engineered, from current makers remains the same, too.
