How to Build a Vacuum Tube Audio Amp

 Interest in vacuum tube amplifiers has enjoyed a tremendous resurgence in popularity. The components are not hard to come by as they once were. They are costly, however. Moreover, finding just the right schematic to build from is not easy either. I offer the  Blueberry schematic for $100. This circuit was two years in developing, and perfecting. I wanted to employ very old style vacuum tubes, the 6BX7,6BL7, & 6SL7 with phenolic resin bases, twin triodes for their reputation for "musicality", and rugged good looks. Incidentally folks think that vacuum tubes are "delicate", or burn out like light bulbs. No this was the World War era,when things were manufactured to last. Vacuum tube devices were in rough riding Army vehicles, and prop driven aircraft with enormous vibrations and hard landings. You can imagine how disastrous it would be for communication and navigation equipment operating on vacuum tubes to fail on any given mission. Trust me...these old tubes are tough and rugged. it isn't well known but there are some 22 rare elements and alloys in each vacuum tube. The precision required to manufacture them is staggering....miniature welding, assembly under magnification, insertion into a red hot glass envelope!...and an exotic chemical ignited by spark to remove the last remaining few molecules in the vacuum tube itself...a perfect vacuum being the result. Otherwise, clouds of electrons moving through empty space would be colliding with air molecules. It is all so exotic and impressive, all this retro technology. It is one of the reasons tube amps are so revered today. The main reason vacuum tubes have made a strong come back, in spite of the staggering costs to build one of these amps...is the enormously pleasant sound, and the "immediacy of presence" of musician / vocalist that tube amps provide....in part , owing to the free moving electrons in a powerful electromagnetic field generated by 450 Volts DC!  Whatever the sound pressure dynamic, whatever the instrument, or how many instruments...these light speed fast electrons EASILY keep up ! Music is a piece of cake for vacuum tubes.

For Accuracy, you will find the old HP bench test equipment a valuable tool for your shop.

Notice in the photo below here in the left column, there is an analog meter, which indicates 4.1 volts AC. The meter just below it is another HP meter known by its nickname "Nixie Tube". These are actual little vacuum tubes with a number of elements inside which glow red. The meter reads in such a way that you don't have to squint your eye to see that it also reads 4.1VAC. This comparison also demonstrates how spot on accurate these meters are - if you can find one in good working order.

Audio Signal Generator: Theimage below here is a home built signal generator, designed by R.W. Anderson. There are plenty of audio signal generators from decades past for sale on.e.g. ebay, et. al. The HP signal generator is a good one; but this one is small and handy. This piece of test equipment is not absolutely necessary, but it does help when diagnosing a problem in the wiring, or perhaps a faulty component. Absolutely most of the time, the problem is NOT a component..it is an error in your wiring. More on that later.

Just below here, is an HP Frequency Counter, "Nixie Tube" type , connected directly to the Audio Signal Generator, at very close to 1K Cycles Per Second, or"Hertz" for short, or " 1K Hz"

. Normally you apply the audio signal to the input jack of your amp, where the CD/DVD/TV audioRCA jack would plug in...and trace your signal through the amp with the Oscillloscope from stage to stage. You would also put the Signal Generator through its entire range, and watching the osilloscope, to see that the output wave maintains its amplitude.

Where you start to see the wave height diminish, this would be the "Roll Off" point of your amplifier-either at the low end or at the high end. You want you amp to maintain amplitude height from, say 100 Hz to 15K Hz or more. If the amplitude maintains its height all the way down to, say 40 Hz..you have a good amp with the ability to produce a good bass note. The only thing that might hinder reaching the lowest note would likely be the output transformer not big enough (heavy with iron, and dedicated to your 5 K in/8 ohms out ). With a smaller output transformer you have what is called "Core Saturation". Meaning the low note of 40Hz begins to behave like a DC current, 'magnetizing' the transformer core, and thus dissipating your sound signal before it can reach the speaker.

Some good used scopes can also be found on an auction site. For audio, onewould not need a scope capable of reading extremely high frequencies, such as over 10MHz. Thus these scopes are often far less costly, especially used. It happens to be a really useful device for tracing the sound wave from point to point. If the wave looks smooth like the above at one stage, but at the next stage it's all haywire, it is immediate indication that you have forgotten the ground. I mention this for many of the problems I encountered in the amp not seeming to work...was a forgotten ground connection. Once I forgot one of the 1 megohm grid resistors -( three for each channel )- and watched as the cathode resistor turned cherry red, and smoked itself to oblivion. This would also fry your vacuum tube, and output transformer- which underscores the importance of making certain every solder connection isabsolutely correct. It's one thing to see the conections on a schematic, and quite another down in the cramped space of the amp, and wires, seemingly a mess. I mention this for the most likely problem is not a failed component, but a neglect of careful, fastidious attention to wiring connections. It is imperative that once  having finished soldering.... Wait to the next day with fresh mind...and go over and over each and every connection...with a magnifying glass-a good idea. Before applying full 115VACpower to your amp, it is also critical to have a "Variac", which allows you to gradually crank up to full power - watching carefully for anything to glow hot, or smoke. Photo of that  Variac just below here.