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 (vacuum 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.

There is absolutely no doubt that valves have enormous nostalgia value, and there will always be a fondness for an amplifying device that you can literally see into.  Knowing that substantial human effort has gone into the production of each valve also helps.  Valves were the first ever form of linear (electronic) amplification, and as such they have a permanent and important place in history.  Many articles (with vast research) exist, detailing some of the trials and tribulations that faced the early manufacturers and users.

I have read a great many accounts of early valve development, and (with great interest) the development of Radar by the British prior to WW II.  The historical aspect is fascinating and shows just how much effort went into the development of the valves and the equipment that used them.  Most of the things we take for granted today (including the computer) started with valves, and it's only comparatively recently that the largest valve most people ever saw (the cathode ray tube or CRT) has been replaced by flat screens in television receivers and computer monitors.

Having said that, there is equally no doubt that for the most part, the valve has had its day.  It will be some time before the last of them goes away (e.g. the magnetron used in microwave ovens and a (small) number of specialised high frequency transmitting valves), but for most other purposes they are part of history.  Undoubtedly an important part, and possibly one of the the most significant inventions of all time.  So, for people who love the historical aspect of the vacuum tube, there is some great joy to be had playing with valve circuits.

Much like the steam engine which created the industrial revolution, the valve started the electronic revolution.  Just like the steam engine has been replaced by electric motors (or internal combustion engines), the valve has been replaced by transistors and integrated circuits.  I doubt that anyone would claim that steam locomotives were somehow 'better' than their modern counterparts (apart from their nostalgia value), and there is equally no reason to imagine that valves are superior to transistors.

Valves are different from transistors (or opamps), but they are not 'better'.  There are still some things that can't be done economically with 'solid state' devices (the magnetron holds a special place here), but audio circuitry isn't one of them.  Over the years there have been a number of double-blind tests performed to discover if guitarists (in particular) could tell the difference between transistor and valve amplifiers.  Mostly, they could not!

Main Index Articles
Title	Description	Date
Lenard Audio	Introductory articles about valves and valve amps. Newcomers should read this first
Introduction to Valves	General info about valve types, terminology and performance	Oct 09
Valve Amplifiers	Do they really sound different? Includes a review of one of my valve amps	Nov 99
Valves vs. Transistors (Part I)	What are the differences? Surprisingly, not as great as you might think.	Dec 09
Valves vs. Transistors (Part II)	There are some interesting differences that are not commonly examined.	Sep 12
Bias and Gain	Understanding transfer curves, biasing and gain calculations	Oct 09
Valve Stage Analysis	Investigating an existing circuit to understand how everything works (or doesn't)	Oct 09
Classes	Classes of operation for valve amplifiers	Jan 10
Design Considerations - 1	A look at the design process, and rationalising wishful thinking into reality (Part 1)	Nov 09
Design Considerations - 2	Output and power transformers, DC filters, negative bias, etc. (Part 2)	Dec 09
Clipping	There's a lot more to guitar amp clipping behaviour than meets the eye (or ear).	Dec 09
Intermodulation Distortion	The important relationship between 'total harmonic' and 'intermodulation' distortion.	Jan 10
Preamps	Obtaining very low distortion from a valve preamp isn't as simple as it seems	Jan 19
Valve Myths	There are many valve myths, and some of the more common ones are exposed.	Nov 09
Title	Valve Project Description	Date
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.	Oct 14
High Voltage Time Delay	Delay the application of a valve amp's high voltage (B+) supply until the cathodes are up to temperature.	Apr 15
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)	Feb 16
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)	Sep 16

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