A concise guide to Valve Power Amplifiers. Part 1.
If you’re considering acquiring a valve amplifier, this article by Geoff Kremer will help raise any veil of confusion. Geoff has been an engineer working with valve amplifiers since 1968 and is the proprietor of English Valve Amplifier company. In addition to servicing valve amplifiers he hand-builds superb units himself. You honestly couldn’t find a better guide. He has penned three articles to aid our understanding of the art behind the enticing sonics.
Valve amplifiers come in two distinct types. Single Ended (SE) and Push Pull (PP). Single ended is the very simplest of designs. One output valve and its companion driver, drives the speaker via a matching transformer. They are generally of low power and deliver a sweet detailed sound, but need very sensitive speakers.
Push Pull requires two or more identical output valves, each handling half the sinusoidal signal. These valves are driven by a phase splitter which separates the positive and negative parts of the signal, which are then recombined in the output transformer and presented to the speaker. Depending on the output valves used, the power supply and the output transformer, Push Pull amplifiers are capable of generating huge amounts of power, and with the right design, can outperform a single-ended amplifier in terms of sweetness, speed and dynamics.
There are pros and cons for each type of amplifier. Single-ended amplifiers are cheaper to make, and by using multiple output valves connected in parallel, are capable of generating decent amounts of power, although at a loss of speed and refinement.
Each type of amplifier will sound different because of their inherent distortion characteristics. SE amplifiers contain within their signal predominately second harmonic distortion, whilst PP amplifiers contain third harmonics. Second harmonics tend to be easier on the ear, so a poor SE amplifier will tend to sound better than a poor PP amplifier, but either way a poor amplifier will never sound good!
All amplifiers, regardless of their type and design, produce some distortion. This is just a fact of life, and depending on how this distortion is controlled determines how the amplifier sounds and performs.
Triode, Tetrode and Pentode.
All valves operate in exactly the same way, but there are variants that have different characteristics.
The Triode is the simplest of all amplifying valves, comprising a source of electrons called the Cathode, a controlling device called a Grid and finally the Anode (or Plate in the USA), which collects the modified beam of electrons. The Triode is the purest and nicest of the group. In order for the Triode to develop more power a second grid was added, called the Screen Grid. It does however introduce a nasty kink in the valve's operational curve. To overcome this, the British GEC company originated the KT series of valves, KT66 and KT88. Called the Kinkless Tetrode, they are widely use today. And finally a third grid, called the suppressor grid was added to improve the gain or amplification factor and improve the linearity. Called the Pentode, the most commonly used ones today are the EL34 and EL84.
Most valves currently manufactured originate from Russia and China. Production is often on machines bought from the likes of Mullard, Osram and Siemens in the 1960's. I have had no end of trouble with valves made in China breaking down, so I honestly can't recommend their use. By far and away the best value valves I have come across are those branded 'Electro Harmonix'. These are obtainable in the UK from Edicron Valves.
The finest quality valves made are the British made Mullard's and GEC's of the 50's and 60's, but they are now worth a fortune, and rightly so. An indication of how good these original valves are is the fact that it has taken valve manufacturers 40 years to produce a valve that will match the original British made GEC KT 88. This is now called the KT 120, and having gone through various other designs and designations to reach this point, it is a very good valve.
And now I'm going to get on my hobbyhorse and talk a little about the ubiquitous 300 B Triode valve. In my opinion, this valve has no place in a Hi-Fi amplifier. It was originally designed in the USA to be buried in the desert and work for 50 years amplifying telegraph signals. I think, like so much modern H-Fi, this device has become a matter of style over substance.
A valve made at about the same time and developed specifically for audio amplification use is the PX 4, made by Marconi in 1928. This valve really does deliver the goods and is perhaps one of the best sounding Triodes ever made.
Looking after your valve amplifier. Some plain simple do's and don'ts:
Do allow plenty of ventilation around your valve amplifier.
Do use your valve amplifier as much as you can. They like to be used!
Do use copper speaker leads. Valves like plain copper. They do not like expensive esoteric cables, so throw out (or sell on e bay) those two thousand pound/metre cables, and get yourself some plain copper stuff. You'll notice the difference.
And now some don'ts:
Do not ever run a valve, (or any other kind of amplifier for that matter), without a speaker connected. Although there is little chance of damage being done, in some circumstances running a valve amplifier without a load can result in very high voltages being developed in the output transformer(s), possibly reaching thousands of volts. This high voltage can result in a damaging flash-over.
Do not switch off and then on quickly a valve amplifier. The resulting fast surge can damage the valve heaters and power supply. Allow at the very least 10 seconds before switching on again.
Do not try swapping valves over whilst the amp is running. Not only is there the obvious possibility of burning yourself, you may damage the amplifier. This is called 'tube rolling', although I have no idea why.
Do not, unless you know precisely what you are doing, use a different valve than originally used.
Do not, unless you know what you are doing, play with any accessible (or otherwise) bias settings, and never play around inside a running valve amplifier. There are voltages as high as 600 volts, and 600 volts shows no mercy if you touch it.