The early Tube Drivers would work without the tubes installed. The tubes in the Tube Drivers aren't really in the amplification circuit...used for some sort of resonance feedback thing. They also put LEDs near the tubes to give them the tube "glow." They are not the only people who do this. Really, if a tube is glowing as much as some of these you see on these amps, you have a bad tube or are running a really high plate voltage- 1000+ volts on some sort of transmission tube.
The HSS is the only class A all tube amp in any sort of production. The difference between A and A/B is that the output devices are never "off", they are getting both sides of the waveform in one tube. Hence why they are so wastefull of energy.
The Milbert (I have one too) is more class B in operation than A/B.
From the Rane Technical Notes:
amplifier classes Audio power amplifiers were originally classified according to the relationship between the output voltage swing and the input voltage swing; thus it was primarily the design of the output stage that defined each class. Classification was based on the amount of time the output devices operate during one complete cycle of signal swing. Classes were also defined in terms of output bias current [the amount of current flowing in the output devices with no applied signal]. For discussion purposes (with the exception of class A), assume a simple output stage consisting of two complementary devices (one positive polarity and one negative polarity) using tubes (valves) or any type of transistor (bipolar, MOSFET, JFET, IGFET, IGBT, etc.).
[Historical Notes marked "GRS" provided by Gerald R. Stanley, Senior V.P. of Research, Crown International, Inc., designer of the famous Crown DC-300, inventor of the Crown K Series switchmode amplifier line and holder of over 30 U.S. Patents.]
[GRS on amplifiers: "At first there were no amplifiers as the very thought of amplification had yet to enter the vocabulary of electronics (another word which had yet to be birthed!). The invention of a three-terminaled device (DeForest Audion U.S. patent 841,386 or later triode) was the invention in 1906 of a more sensitive radio detector and not an element for an amplifier.
By 1912 the triode had become both a vacuum tube and an amplifier (multiple names can be attached to this collective achievement). The oscillator also dates to 1912 giving proof to the saying "When you set out to make an amplifier you get an oscillator and when you attempt to make an oscillator you get an amplifier."]
[GRS on amplifier classes: "Originally it was adequate to distinguish amplifier classes only by the conduction angles of the control elements (tubes or valves). More recently it has been necessary to add distinctions that relate to topology, degrees of conduction and control methods to be able to determine class."]
Class A operation is where both devices conduct continuously for the entire cycle of signal swing, or the bias current flows in the output devices at all times. The key ingredient of class A operation is that both devices are always on. There is no condition where one or the other is turned off. Because of this, class A amplifiers in reality are not complementary designs. They are single-ended designs with only one type polarity output devices. They may have "bottom side" transistors but these are operated as fixed current sources, not amplifying devices. Consequently class A is the most inefficient of all power amplifier designs, averaging only around 20% (meaning you draw about 5 times as much power from the source as you deliver to the load.) Thus class A amplifiers are large, heavy and run very hot. All this is due to the amplifier constantly operating at full power. The positive effect of all this is that class A designs are inherently the most linear, with the least amount of distortion. [Much mystique and confusion surrounds the term class A. Many mistakenly think it means circuitry comprised of discrete components (as opposed to integrated circuits). Such is not the case. A great many integrated circuits incorporate class A designs, while just as many discrete component circuits do not use class A designs.]
[GRS Historical Note: "Class A - The most basic of operating modes saw both single-ended and push-pull embodiments by 1913. The first known use of push-pull appears in a patent of E.F.W. Alexanderson of GE U.S. 1,173,079 filed in 1913. While Alexanderson would have been aware of other levels of biasing his push-pull stage, such as classes B and C, he would have only been able to produce a useful result with a tuned stage such as a transmitter where resonant filtering would have managed the distortion problem. Negative feedback is not understood in 1913 to be able to cope with distortion problems."]
Class B operation is the opposite of class A. Both output devices are never allowed to be on at the same time, or the bias is set so that current flow in a specific output device is zero when not stimulated with an input signal, i.e., the current in a specific output flows for one half cycle. Thus each output device is on for exactly one half of a complete sinusoidal signal cycle. Due to this operation, class B designs show high efficiency but poor linearity around the crossover region. This is due to the time it takes to turn one device off and the other device on, which translates into extreme crossover distortion. Thus restricting class B designs to power consumption critical applications, e.g., battery operated equipment, such as 2-way radio and other communications audio.
[GRS Historical Note: "Class B - This class has no obvious inventor, but it does have its master and perfector. Loy Barton working for RCA developed tube designs and biasing methods to manage the open loop distortion of class B push-pull power stages. His IRE paper in 1931 titled "High Output Power from Relatively Small Tubes" is a landmark in the history of class B. Technically he only used class AB but the distinction was not in the language. Class AB is a later and probably unnecessary class fabrication."]
Class AB operation is the intermediate case. Here both devices are allowed to be on at the same time (like in class A), but just barely. The output bias is set so that current flows in a specific output device appreciably more than a half cycle but less than the entire cycle. That is, only a small amount of current is allowed to flow through both devices, unlike the complete load current of class A designs, but enough to keep each device operating so they respond instantly to input voltage demand s. Thus the inherent non-linearity of class B designs is eliminated, without the gross inefficiencies of the class A design. It is this combination of good efficiency (around 50%) with excellent linearity that makes class AB the most popular audio amplifier design.
Class AB1 & AB2 Subdivisions of Class AB developed for vacuum tube design. These subsets primarily describe grid current behavior: Class AB1 has no current flowing into the grid of the tube, and Class AB2 has some current flowing into the grid. Class AB1 operates closer to Class A, while Class AB2 operates closer to Class B. Most bipolar solid-state amplifiers would be classified as Class AB2, while power JFET designs mimic Class AB1.
Class AB plus B design involves two pairs of output devices: one pair operates class AB while the other (slave) pair operates class B.
[GRS Historical Note: "Class AB+B is a term that I'd coined and is intended to be very descriptive but is not truly worthy of its own class. The Crown DC-300 was the first to use this mode of operation in 1968."]
Class BD Invented by Robert B. Herbert in 1971 U.S. patent 3,585,517 and improved on by Neil Edward Walker as disclosed in his 1971 U.S. patent 3,629,616. Both patents are concerned with improving original class D design efficiencies by using various bridge connections and cancellation techniques. And most recently more improvements are claimed by inventors James C. Strickland & Carlos A. Castrejon in their U.S. patent 6,097,249 assigned to Rockford Corporation in 2000 for their Fosgate-brand automotive amplifier.
[GRS comments: "This is a class designation that would best be forgotten. It has been applied to multiple modulation schemes on a class D derived full-bridge. This is perhaps the most reinvented class design in recent history with "filter-less amplifiers" and other such things. An interleave of two class D full-bridge is what we actually have here, and it is a good improvement to an interleave of one class D full-bridge. However an interleave of four is actually possible on a full-bridge if one uses Class I design."]
Class C use is restricted to the broadcast industry for radio frequency (RF) transmission. Its operation is characterized by turning on one device at a time for less than one half cycle. In essence, each output device is pulsed-on for some percentage of the half cycle, instead of operating continuously for the entire half cycle. This makes for an extremely efficient design capable of enormous output power. It is the magic of RF tuned circuits (flywheel effect) that overcomes the distortion create d by class C pulsed operation.
Class D operation is switching, hence the term switching power amplifier. Here the output devices are rapidly switched on and off at least twice for each cycle (Sampling Theorem). Theoretically since the output devices are either completely on or completely off they do not dissipate any power. If a device is on there is a large amount of current flowing through it, but all the voltage is across the load, so the power dissipated by the device is zero (found by multiplying the voltage across the device [zero] times the current flowing through the device [big], so 0 x big = 0); and when the device is off, the voltage is large, but the current is zero so you get the same answer. Consequently class D operation is theoretically 100% efficient, but this requires zero on-impedance switches with infinitely fast switching times -- a product we're still waiting for; meanwhile designs do exist with true efficiencies approaching 90%.
[Historical note: the original use of the term "Class D" referred to switching amplifiers that employed a resonant circuit at the output to remove the harmonics of the switching frequency. Today's use is much closer to the original "Class S" designs.]
[GRS Historical Note: "Class D is a subset of all possible switch-mode amplifier topologies that is typified by use of the half-bridge (totem-pole) output stage that has two interconnected switches operating in time alternation. The paradigm is that of Loy Barton's class B, but uses the statistics of conduction angle to produce amplification (PWM). There are many subclasses within class D that describe the origins of the modulation. Class D is at least as old as 1954 when Bright patented a solid-state full-bridge servo amplifier U.S. 2,821,639."]
Class E operation involves amplifiers designed for rectangular input pulses, not sinusoidal audio waveforms. The output load is a tuned circuit, with the output voltage resembling a damped single pulse. Normally Class E employs a single transistor driven to act as a switch.
The following terms, while generally agreed upon, are not official classifications:
Class F Also known by such terms as "biharmonic," "polyharmonic," "Class DC," "single-ended Class D," "High-efficiency Class C," and "multiresonator." Another example of a tuned power amplifier, whereby the load is a tuned resonant circuit. One of the differences here is the circuit is tuned for one or more harmonic frequencies as well as the carrier frequency. See References Krauss, et al. for complete details.
[GRS Historical Note: "Classes E and F are distinguished by their resonant topology and not conduction angle else we would class them with C. A good reference to these is found in the many patents of Nathan Sokal. Also class S which is very old (1929-1930) has similar applications (resonant RF)."]
Class G operation involves changing the power supply voltage from a lower level to a higher level when larger output swings are required. There have been several ways to do this. The simplest involves a single class AB output stage that is connected to two power supply rails by a diode, or a transistor switch. The design is such that for most musical program material, the output stage is connected to the lower supply voltage, and automatically switches to the higher rails for large signal peaks [thus the nickname rail-switcher]. Another approach uses two class AB output stages, each connected to a different power supply voltage, with the magnitude of the input signal determining the signal path. Using two power supplies improves efficiency enough to allow significantly more power for a given size and weight. Class G is common for pro audio designs.
[Historical note: Hitachi is credited with popularizing class G designs with their 1977 Dynaharmony HMA 8300 power amplifier, however it is shown much older by GRS: "Class G - I have been searching for the proper inventor of this class, but have not been able to find a reference older than 1965 when I first encountered it in a college text "Handbook of Basic Transistor Circuits and Measurements" by Thornton et al., SEEC vol. 7. The method is introduced without references or fanfare. One is led to believe that it was common knowledge in 1965 and earlier. This is not the first known use of extended quasi-linear methods (beyond class B), as there is a dual found in Fisher U.S. 2,379,513 from 1942."]
Class H operation takes the class G design one step further and actually modulates the higher power supply voltage by the input signal. This allows the power supply to track the audio input and provide just enough voltage for optimum operation of the output devices [thus the nickname rail-tracker or tracking power amplifier]. The efficiency of class H is comparable to class G designs.
[Historical note: Soundcraftsmen is credited with popularizing class H designs with their 1977 Vari-proportional MA5002 power amplifier, however like class H above GRS finds precedence: "Class H - The apparent inventor of class-H in full-blown multi-level form was Manuel Kramer of NASA in 1964 U.S. patent 3,319,175. Class H optimally applied to a full-bridge was invented in 1987 (Stanley) U.S. 4,788,452. Classes G and H are all members of a class of amplifiers that has articulated rail voltages to improve the efficiency of class B power stages. Examples are available of tracking using binarily weighted segments, (Stanley) U.S. 5,045,990. Continuously variable tracking with switch-mode PWM appears to have been first done by Hamada in 1976 U.S. 4,054,843. The ultimate rail tracker using interleaved technology is found in (Stanley) U.S. 5,513,094. Only with interleave is the converter fast enough to meet the needs of full-bandwidth audio and yet have low switching losses."]
Class I operation invented and named by Gerald R. Stanley for amplifiers based on his patent U.S. 5,657,219 covering opposed current converters.
[GRS explains: The "I" of the class is short for "interleave" as this is the only four-quadrant converter known that uses two switches yet has an interleave number of 2 in the terminology of interleave. When used with fixed-frequency natural two-sided PWM it forms a theoretically optimum converter having the least unnecessary/undesirable PWM spectra. A good reference is found in the IEEE Transactions on Power Electronics Vol. 14, No. 2, March 1999, pages 372-380."]
Class J operation is the category/name suggested by Gerald R. Stanley for amplifiers that combine class B and class D where converters act in parallel to drive the load.
[GRS elaborates: "There are serious problems with the power efficiency of these products when processing fast signals into arbitrary loads. The class B stage is used to actively remove the ripple of the class D stage and other distortion problems that plague class D. No solution is offered for the MOSFET CSOA (current safe operating area) problem of class D. To solve that problem it would be necessary to parallel a class I and class B amplifier but this would be without merit as the class I amplifier generally does not need the class B amplifier to meet fidelity requirements."]
Class S First invented in 1932, this technique is used for both amplification and amplitude modulation. Similar to Class D except the rectangular PWM voltage waveform is applied to a low-pass filter that allows only the slowly varying dc or average voltage component to appear across the load. Essentially this is what is termed "Class D" today. See References Krauss for details.
[Final GRS Amplifier Historical Note: "All of our amplifier classes have thrived under a very important invention, without which most would have floundered. That invention is, of course, negative feedback. Harold Black in 1927 changed our world forever while riding to work on the Lackawanna Ferry. (See U.S. patent 2,102,671.) Harold Black did not stop there however, he also in 1953 wrote the text "Modulation Theory" which we today use to understand the fundamentals of PWM. In 1935, Terman, in his now famous "Fundamentals of Radio" handbook, wrote that it was good that class B was only used in places like radio stations as there needed to be an engineer on duty full time to keep the bias tweaked to where the distortion was acceptable. Thanks go to Harold Black for changing all that and leading us into the next century of amplification."]