This section is about valves - aka vacuum tubes. Although valves are a topic I've avoided since starting The Audio Pages in 1998, it's quite obvious that they are not going away. Don't expect much in the way of projects though. There are already countless websites that cover nothing else, and adding more is not in anyone's interests. That said, the odd project will come up from time to time, but not for anything that might be considered 'significant'.
Much of the material looks at the basics - the fundamental info on valves. There is also introductory info about the correct biasing of preamp valves and what gain you can expect from traditional valve stages. We also look at an analysis of an existing valve guitar amplifier, both to familiarise the reader with the basics of analysis and to point out that 'guitar amp' and 'careful engineering' generally do not belong in the same sentence. I've also looked at some of the myths that surround valves - while they may have considerable nostalgic value, that doesn't make them better than audio gear we can build today with more modern, lower voltage and more reliable components.
The voltages used in valve (tube) amplifiers are lethal, and must be treated with the utmost respect at all times. Contact with power supply
voltages may cause death or serious injury, including but not limited to burns caused by the arc when contact is broken. Never work on a valve amplifier unless
you are experienced with high voltage supplies, understand the risks involved and take proper care to avoid contact.
Capacitors may store a lethal charge for a long time after the amplifier is turned off or unplugged from the mains outlet.
Do not wear rings or other jewellery that may become caught on part of the chassis, thus preventing you from withdrawing your hand if accidental contact is made.
Ensure that all test equipment, probes and leads are rated for the voltages you will measure. Ensure that any operating valve equipment is secured from contact
(electrical or physical) by unsuspecting visitors, children, etc. Burns are not uncommon with hot valves. Please remember that your pets may also be at risk, and
as de-facto members of your family it is your duty to ensure their safety too.
It will become obvious as you read through the various sections here that I am not a fan of the Single Ended Triode (SET) amplifier. There is much information in the following pages as to my reasons, but it is very important to make one point very clear. At the height of the 'valve/ tube era' (just before transistor amps took over), not one high-end manufacturer built a SET design ... not one! Without exception, the best of the best (McIntosh, Quad, Audio Research, Leak, etc., etc.) were push-pull. In addition, these manufacturers built very high quality transformers to allow the maximum feedback practicable. They did not do this to make the sound worse !
Recording studio monitors and disc cutting lathes used these very amplifiers (or others of similar design), and push-pull amps were (and still are) also used by virtually every musician with a valve amplified instrument. Single-ended amplifiers were (and are) the sole domain of cheap low-end, low powered equipment. Practice amps, mantel radios, 'record players' and the like used single-ended output stages because they were cheap and the sound quality was considered 'adequate' for casual listening. With few exceptions, these low-end applications used Class-A pentode output stages.
Nothing has changed. The sound quality of a SET amp has zero magic qualities, but it remains adequate for casual listening or as a 'statement' (although I'm unsure what the statement might be). Claims that these amps are 'hi-fi' are false - high fidelity implies that the integrity of the input signal is not affected, but SET amplifiers often make profound changes. Such amplifiers are effects units, not hi-fi amplifiers. For those who enjoy the effects created, I say 'happy listening', and simply ask that you don't claim that your system is high fidelity and/or has magic qualities.
Introductory articles about valves and valve amps. Newcomers should read this first
Introduction to Valves
General info about valve types, terminology and performance
Do they really sound different? Includes a review of one of my valve amps
Valves vs. Transistors (Part I)
What are the differences? Surprisingly, not as great as you might think.
Valves vs. Transistors (Part II)
There are some interesting differences that are not commonly examined.
Bias and Gain
Understanding transfer curves, biasing and gain calculations
Valve Stage Analysis
Investigating an existing circuit to understand how everything works (or doesn't)
Classes of operation for valve amplifiers
Design Considerations - 1
A look at the design process, and rationalising wishful thinking into reality (Part 1)
Design Considerations - 2
Output and power transformers, DC filters, negative bias, etc. (Part 2)
There's a lot more to guitar amp clipping behaviour than meets the eye (or ear).
The important relationship between 'total harmonic' and 'intermodulation' distortion.
Obtaining very low distortion from a valve preamp isn't as simple as it seems
There are many valve myths, and some of the more common ones are exposed.
Valve Project Description
High Voltage DC Supply
If you want to experiment with valve ('tube') circuits, you need a power supply for the B+ and DC for the heaters.
High Voltage Time Delay
Delay the application of a valve amp's high voltage (B+) supply until the cathodes are up to temperature.
Output Valve Tester
A simple test set that allows you to measure output valves at the power levels where they are normally used. Ideal for service techs. (Project 165)
MOSFET Follower + Protection
Compared to a cathode follower, a MOSFET gives better results. However, it's essential to protect following equipment from high voltages. (Project 167)
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