Notes About Output Tubes Back to Dusty Files Online

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The three major types of output tubes each have their own unique advantages.

Beam Tetrodes such as 6L6-GC, 5881 and 6550/KT88 are known for high efficiency at relatively low voltages, and in particular high peak current carrying capacity. The disadvantages are that (a) a relatively high amount of signal-grid current is drawn, which necessitates a low DC grid circuit resistance, and thus a fairly low-impedence driver circuit, (especially if used with fixed bias) and (b) a relatively high amount of intermodulation distortion, which may be defeated by using negative feedback and/or by connecting the tubes as "ultralinear", by hooking the screen grid to a tap on the output transformer. The ultralinear connection can give even lower IMD than triodes. The beam tetrode also can have a tendency to oscillate, especially big ones (like 6550's) or if run in multiple pairs. If the ultralinear or triode connections are not used then it is usually required to have a regulated voltage for the screen. Beam tetrodes make very efficient triodes by wiring the screen to the plate (anode), usually via a small 100 ohm resistor.

Pentodes, such as 6BQ5/EL84 and 6CA7/EL34 (beware the USA and the large Russian "EL34"'s are actually beam tetrodes!) are inherently more stable (read:less prone to oscillate) and have lower intermodulation distortion than beam tetrodes. They also draw less control grid current than tetrodes thus may have higher DC grid circuit resistance (usually up to 500K ohms versus 50 to 100K ohms operated fixed bias). This means that they may be driven even by relatively high impedance tubes such as 12AX7's. The pentode is by far the easiest tube for the beginner to design an amplifier around. Pentodes may be wired as triodes, but they are generally not as efficient in this arrangement as beam tetrodes.

Class A (low gain) Triodes, usually 300B, 2A3, or 6AS7-G, have the unique advantage of an inherently low plate resistance which gives an very high (for tube amplifiers) damping factor, even without negative feedback. Generally the distortion is fairly low and mostly even harmonic (although there is definitely third harmonic there!). The drawbacks from a designer's standpoint are numerous, low efficiency, expense per watt, low sensitivity requiring a high bias voltages and lots of AC voltage from the driver circuit, and in the case of fixed bias amplifiers in particular, a tendency to drawing signal-grid current. With fixed-bias triode amplifiers the driver circuit must be able to generate a large voltage swing with a very low output impedance.

Too much DC resistance in the signal grid circuit results is tube bias current drift and instability, if fixed bias is used. Using cathode bias resolves the bias drift and instability problem, but at the expense of lots of output power, for example a pair of 300B tubes can generate 40 watts fixed bias but only 20 watts cathode bias. There is another class of triodes designed to be used as Class B tubes (811A in particular) that are very efficient and have some interesting properties (see DF Part 1 for examples) but these usually require very high plate voltage and special driver circuits and are not covered here.

The real determining factor as to what types of tubes to use are the loudspeakers that the amplifier is attached to. There are a huge variety of different speaker designs available, the loads presented to amplifiers by them vary from one end of the galaxy to the other. The actual impedance seen by the amplifier tends to vary widely with the frequency of the signal, and the actual frequency response of speakers varies much more than the amplifiers that drive them. The interaction between the source (amplifier) and the load (speaker) is such that certain combinations of speaker, tubes, and amplifier can make distortion either much worse or much better than one might expect. The mind numbing number of variables, and the average person's inability to measure all of them, let alone knowing what to do with the numbers if they were available, means that the average user is best advised to find out what works best through careful experimentation and observation. The fact that what engineers predict will sound good and what listeners report sounds good often don't correlate tends to support this conclusion. (Actually, prior to World War II, before today's multiplicity of varieties of amplifiers and speakers, engineers could with some accuracy predict what people would and wouldn't like).

The general rules of thumb are:

(a) that if you have speakers that require lots of power for good fidelity, try beam tetrodes first.

(b) if you have very high efficiency speakers, try triodes or triode- connected tetrodes/pentodes.

(c) if you have electrostatics, try a medium power triode or ultralinear. (d) if you are somewhere in the middle, or aren't sure where to start, try pentodes or ultralinear connected pentodes, which tend to be the most stable.

Notes About Rectifier Tubes and Silicon Rectifiers

There is a whole group of 5 volt filament, octal base rectifier tubes that are pin compatible and sometimes interchangeable. These include 5V4-G/GA, 5Y3-GT, 5AR4, 5U4-G/GA/GB, 5V3, 5AU4, 5R4, 5AS4, and 274B. These all differ in various ways, from your standpoint, the main relevant charcateristics are the rated DC output current, maximum AC plate voltage, filament current, and voltage drop across the tube. Changing tubes is permissible if the replacement tube does not exceed its ratings, if the voltage drop is lower, that the voltage ratings of the filter capacitors are not exceeded, and the filament current rating of the transformer is not exceeded. There are three 4-pin types that are electrically identical to octal types, type 80 is equivalent to 5Y3-G/GT, types 5Z3 and 274A are electrically identical and interchangeable, and both are electrically identical to types 5U4-G and 274B.

There are a number of TV damper tubes which can and have been used as rectifiers, 6AU4-GT being the best known, but there are a lot of other tubes of this type that could certainly be used, including but not limited to 6AX4, 12AX4, EY500, 6CG3, etc.

One of the most overlooked specs regarding tube amplifiers is the one about the maximum size of capacitor that may be hooked directly to the tube's filament or cathode. This spec has been pushed a lot by OEM's in recent years, resulting in a higher than normal instance of tube arc-over failure in these amplifiers, when the capacitors charge up initially, the maximum surge current rating is exceeded. A good rule of thumb is that capacitors no larger than 100 uF should be attached directly to a filament-cathode (5U4,etc), no larger than 50 uF to a heater- cathode (5AR4, etc). Large filter capacitors up to 500 uF (maybe more) can be used with a tube recitifier, provided enough impedance (usually a choke at least 2 henries) is between the tube and the capacitor is used. Thus a pi-filter can be made with a 50 to 100 uF cap, a choke, then another capacitor of much higher value after that.

If silicon rectifiers are used, the ratings should be at least double what the diode is likely to see. Silicon diodes can not be paralleled to increase current rating, but they can be wired in series to increase voltage rating. If run in series each diode shold be paralleled by a 470K resistor (any wattage will do) and a 1000 pf capacitor (same voltage rating as the diode).

Back to Dusty Files Online

Small Signal Tube Notes

Capacitor Notes

Resistor & Transformer Notes

Altec, Acrosound & Dynaco

Eico, Grommes, HK & Heath

Pederson thru Stromberg