Instruction manual

3. Class B applies only to push-pull amplifiers in audio; it SOMETIMES applies to RF power amplifiers with one tube. It

is like Class AB, except that the tubes idle at or near zero current. This gives even greater efficiency than Class A or

AB. It also results in increased distortion, unless the amplifier is carefully designed and uses some negative feedback.

If careful design is not undertaken, the result may be crossover distortion, which appears at the midpoint of the output

waveform and has very bad-sounding effects in audio. Most solid-state audio amplifiers use class B, because the

transistors undergo less heat stress when idling.

4. Ultralinear operation was invented by David Hafler and Herbert Keroes in 1951. It uses only beam tetrodes or

pentodes, and special taps on the output transformer. The taps connect to the screen grids of the tubes, causing the

screens to be driven with part of the output signal. This lowers distortion considerably. It is usually seen only in hi-fi

amplifiers that use power tubes such as the 6L6, 6550, EL84 or EL34.

E. Why are different kinds of power supplies used in various tube amplifiers? Why do some use tube

amplifiers? Why do some use tube rectifiers, while others use solid-state rectifiers, while still others have

electronic regulation?

Tube rectifiers are still used in power supplies of some hi-end amps, because the current a tube rectifier can produce

varies somewhat with the load. It is quite different in response from a solid-state rectifier. Many audiophiles also prefer

this classic design for much the same reasons. Also, inexpensive solid-state rectifiers can put "hash" into a power

supply, because of their slow transient capability while charging and recharging a filter capacitor 50/60 times a second.

Special high-speed silicon rectifiers are available at high cost. They are rarely used in products other than a few high-

end amplifiers. Tube rectifiers have faster transient response than most solid-state rectifiers, also making them useful

in some high-end designs.

Regulated DC plate power can be very helpful in a push-pull Class AB amplifier. Because the amp draws greatly

different current when at idle and when delivering full power, a regulated supply "sags" less at full power, producing

better transient response in the amplifier. It is expensive to regulate the high voltages in a tube amplifier, so it is done

only in expensive top-line models. Class A amplifiers have less need for regulation since they draw nearly the same

DC power at all times. It is dependent on the circuit design. The only way to see if you need an amplifier with a

regulated supply is to listen to it and carefully compare it with similar amps with unregulated supplies.

F. There's all this talk about "parallel feed", "shunt feed", SRPP, "mu followers", and the like. Which should I

use? What's the difference?

Parallel feed and shunt feed are the same technique. Basically, a choke is used to load the power tube (usually one, in

SE mode), while the output transformer is coupled to the plate of the tube through a capacitor. So, the plate current of

the tube does not flow through the output transformer. This can be a very expensive technique to implement, since the

choke must be as carefully wound as the output transformer. It does offer a possible performance improvement.

SRPP circuits and mu-follower circuits are special designs which use a lower tube (for gain), and an upper tube which

serves as the plate load for the lower tube. The upper tube also acts as both a cathode follower and as a constant-

current source for the lower tube. If properly designed, either circuit can offer improved performance over an ordinary

resistor-loaded tube stage. These circuits are used only in preamp stages and in the driver stages of power amps,

usually SE types, in high-end audio.

If you want to learn more of the technical details behind vacuum-tube electronic design, we recommend the following

books.

We recommend two recently-published books on circuit design, which the novice can derive much information from:

THE BEGINNER'S GUIDE TO TUBE AUDIO DESIGN, by Bruce Rozenblit (ISBN 1-882580-13-3);

and PRINCIPLES OF POWER, by Kevin O'Connor (ISBN 0-9698-6081-1).