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OTL Operation: Class A or Class AB

Customers often ask if Transcendent amps operate Class A or Class AB.  The only practical way to operate an OTL and generate more than a few watts of power is Class AB.  (The Transcendent Single-Ended OTL operates Class A.)  Apparently, this is an area of confusion among audiophiles because other OTL manufacturers claim that their amps operate Class A.

 These claims are all entirely false.  All of these so called Class A amplifiers operate Class AB.  An analysis of Class A operation will prove that these claims are incorrect.   

 The definition of Class A operation is:
An amplifier in which the grid bias and alternating grid voltages are such that plate current in a specific tube flows at all times.  RCA Receiving Tube Manual, ed. RC-30, page 25.

An amplifier in which the grid bias and alternating grid voltages are such that the plate current of the output valve or valves flows at all times.  The Radio Designer’s Handbook, by Langford-Smith, page 572. 

The definitions describe three parameters.  They are the grid bias, alternating grid voltages (the audio drive signal), and the plate current in the output tube (or tubes for push-pull.)

 In all amplifiers, the grid bias is a fixed value.  It does not change or is modulated by the audio signal.

 The actual signal that drives the tube is the combination of the grid bias and the audio drive signal.  The two are added together.

 The operating plate current through a tube (or bias current) is set by the bias voltage which is always a negative value.  Let’s look at a real tube as an example.    


Take a 6AS7 tube (6080 is equivalent) with a plate current of 80 mA, a grid bias of -60 volts and a plate voltage of 150 volts.  To get the maximum current out of the tube, which is about 500 mA, the voltage peak of the drive signal has to reach 0 volt.  (With a transconductance of 7 mA per volt times 60 volts, the plate current will increase by 420 mA.  Adding 420 mA to the 80 mA we started with yields a net current of 500 mA. The calculation verifies measured results)  That means a drive signal of 120 volts peak to peak is required for maximum output.  A 120 volt peak to peak signal has a 60 volt positive peak portion and a 60 volt negative peak portion.  The positive 60 volt peak of the drive signal when added to -60 volts bias results in a net signal voltage of 0 volt, thereby achieving full power.

All audio signals are symmetrical.  The negative portion of the signal cannot be eliminated.  What happens to this tube when driven by a 120 volt audio signal during the negative half of the signal.

The drive signal combines with the bias voltage to yield a net input signal of -120 volts at the extreme of the negative peak.  At the positive peak, the tube is producing maximum current, and is being overloaded.  As the audio signal goes negative, the current through the tube starts to decrease.  At the zero crossing of the audio drive, the net signal is the bias voltage, in this case -60 volts.  As the audio drive signal continues to go more negative, the current through the tube continues to decrease.  At some point, around -90 volts, the signal becomes so negative that all current flow in the tube stops and we say the tube is “in cutoff “.  The above chart demonstrates the cutoff region, the region of zero plate current.  The drive signal continues to reach -120 volts, fully 30 volts beyond cutoff.  This is class AB operation.

Class AB operation is defined as:
An amplifier in which the grid bias and alternating grid voltages are such that the plate current in a specific tube flows for appreciably more than half but less than the entire electrical cycle.  RCA receiving tube manual RC-30, page 25.

An amplifier in which the grid bias and alternating grid voltages are such that the plate current in any specific valve flows for appreciably more than half, but less than the entire, input cycle.  The Radio Designer’s Handbook, Langford-Smith, page 572.

The tube stops conducting current during a large portion of the negative part of the input signal, therefore operates class AB.

As long as there is a plate voltage, some minute vestige of current will flow.  In other words, tubes never completely shut off and conduct no current whatsoever.  Only if the the negative grid voltage equals or exceeds the magnitude of the plate voltage can these almost immeasurable currents stop flowing by canceling out the positive electrical field generated by the plate.  All of the curves actually continue on and terminate at the lower left corner, at 0,0 on the chart.  The currents are so low that the width of the line exceeds their magnitude. 

To illustrate this point, actual measurement of three 6AS7 tubes were taken with the Transcendent Tube Analyzer.  Plate voltage was held at 150 volts.  This is the plate voltage used in a different manufacturer's commercial OTL amplifier.   The following data was obtained:

Tube 1) Bias -85V, current 350 micro amps.  Bias -100V, current 95 micro amps.

Tube 2) Bias -85V, current 185 micro amps.  Bias -100V, current 75 micro amps.

Tube 3) Bias -85V, current 160 micro amps.  Bias -100V, current 12 micro amps.

As can be plainly seen, these currents are minutely small.  In fact, they are so small, they are beneath the noise level of the tube and are mostly the result of leakage currents produced by the filament heater.  What we are actually measuring here is not signal but noise.

Some may claim that because these current flows are detectable, then the amp is operating class A.  This is nothing more than a play on words.  If this were true, then all Class AB amps could be called Class A. 

The above argument does not address the extreme nonlinearity that exists at these micro current levels.  Transconductance is about 500 times lower and plate resistance is about 500 times higher.  The result is distortion is several hundred times higher.  The whole purpose of utilizing Class A is to reduce distortion.  So why does distortion increase so much at these levels?  Because the tube is not operating Class A.  It is in fact operating in a range it can't operate in.  The tube is in cutoff and for all practical purposes was in cutoff when plate current dropped below two or three mA.           

So why not just reduce the bias voltage so a smaller drive signal is required to reach 0 volt?  Then the tube can be kept out of the cutoff region.

This is not possible.  The maximum plate dissipation for each triode in a 6AS7 is 13 watts.  With 150 volts on the plate, the maximum allowable plate current is 87 mA for 13 watts dissipation.  At best the bias voltage could be reduced by 2 volts to 58 volts to increase the plate current to 87 mA.  The tube still has to operate class AB.  There is far too little plate dissipation for Class A operation at the claimed power level of 60 watts.  The amplifier does operate Class A at low power.  In this example the Class A power can be calculated by:

87 mA x 8 x .707 = .492 amp. 
.492 amp x .492 amp x 8 ohms = 1.94 watts

This calculation shows that the amplifier operates class A up to about 2 watts and then switches to class AB.   This is exactly how Class AB amplifiers function.  

Well then, why not lower the plate voltage to reduce the dissipation?  That won’t work either because then the tube cannot produce enough current at lower plate voltages.  For true Class A operation, the plate voltage would have to be reduced to 50 volts which would only yield about 100 mA of maximum available current.  Power output would drop by nearly 90%.     

It is important to note that the configuration of the push-pull output stage has no bearing on these calculations.  It makes no difference if the output stage is balanced, or bridged, or series connected.  Tubes only respond to the electrical parameters they are subjected to.  The circuit configuration has nothing to do with class of operation.  Any push-pull configuration can operate class A or class AB depending upon how it is biased and what signal levels are passed through it.  All single-ended amplifiers must operate Class A or they cannot function. 

If an OTL amplifier were to operate push-pull with the 6AS7 tube, how should it be biased and what kind of heat output would result?  The linear range of plate current operation would be from 80 mA to 500 mA. 

For equal current swings for both positive and negative portions of the signal, the plate current should be set at the midpoint of the range or 250 mA.  At this current level, the plates would dissipate 250 mA x 150 volts = 37.5 watts which is 290% over the tubes rating which would destroy the tube.  It can’t be done.  Class A is not possible.

If 37.5 watts could be continuously dissipated by the 6AS7, how much heat would such a theoretical amplifier create? Let’s look at a commercial OTL that uses 8- 6AS7 tubes with a claimed a power output of 60 watts. 

There are 8 tubes with 2 triodes each so the total heat from the plates would be 16 x 37.5 = 600 watts.  There are 8 filaments at 15.75 watts for a total of 126 watts.  Add 50 watts for driver circuitry and the total is up to 776 watts.  Adding in 5% for power supply losses and the final total is 815 watts.  Of course, this number must be multiplied times 2 for stereo giving a total heat load of 1630 watts.  A portable electric heater puts out 1500 watts of heat.  Measuring the current draw of one of these amplifiers is a good indication of Class of operation.  If the current is not there, then the amp must be Class AB.  Watts are calculated as current draw multiplied by line voltage.        

It is important to note that the above paragraph describes a theoretical OTL that would burn out all the output tubes in a matter of minutes.  A real world OTL that did not overload the output tubes would require 24 tubes for each amp and produce about 2800 watts of heat for two 60 watt class A OTL amplifiers.  No such OTL exists.

OTLs that use the 6C33Cb tube suffer from the same problem of insufficient plate dissipation.  This tube can dissipate 60 watts per plate.  Using the same methodology, an OTL using 6 of these tubes that outputs 100 watts of Class A power would have to dissipate 120 watts on the plates, fully double the maximum allowable rating.  To reach 100 watts Class A and not overload the plates would require 12 tubes per channel.  Total heat output would approach 3000 watts for a stereo pair!  The thermal efficiency is slightly higher with this tube than the 6AS7 because the current levels are greater for each tube but the heat output is still enormous.       

The calculations clearly show that all push-pull OTLs must operate Class AB.  There is insufficient plate dissipation for Class A operation. 

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