WhitakerAudio
 
Audio Project Updates

After the book went to press, the author continued to tweak the circuits and experiment with different implementation ideas. Those efforts are documented in here and in linked pages from the menu above.

This page includes supporting information for the projects described in The TAB Guide to Vacuum Tube Audio, along with commentary from the author.

For readers that have a specific question about the projects described in the book, feel free to contact the author.

The individual project pages can be selected from the drop-down menu bar above.

Vacuum tubes

Some Comments from the Author

The classic RCA Receiving Tube Manual (1975) served as a starting point for the circuits used in the projects described in the book. In most cases, tweaks were found to make the circuits work better. In all cases, complete parts lists were developed (which, for decades-old circuits, was not always easy). Why start with the 1975 edition, rather that some other edition? Well, it seemed like a good place to begin the effort since by 1975 tube technology had arguably advanced about as far as it was going to, faced with very stiff competition from the transistor. Improvements would certainly continue to be made (and are even still being made today), but it is evident that by 1975 tube technology had reached a very mature state. One could argue about the best "era" for tube development, but I'll leave that for another forum.

The original idea when I began the book was to utilize vacuum tubes for the audio chain and current-technology devices and circuits for supporting systems. For example, solid-state (in some cases switching) power supplies, OP-AMP-based preamplifiers, and LED metering. After going down that road for a short period, I realized that a better approach would be to simply focus on the basics and attempt to build 'em like they used to. Readers can, of course, integrate new technologies into the basic designs described in the book if they wish. I didn't go that route, however.

Another basic principle that I developed early on was that hiding the tubes within the chassis made no sense whatsoever. Let's face it, part of the attraction of a vacuum tube amplifier is that you can see the tubes. Appearances are important in audio gear.

From the outset, I wanted to do in print what experimenters and hobbyists do in practice—namely, start with a basic circuit, built it, and then refine it. That seemed more interesting than just saying, “built it like this.” The other point here is that different things are important to different people. In online discussion groups you will find folks who claim that a tube amplifier should only be built a certain way, or only with certain tubes. I don’t subscribe to that. If you are happy with what you have built, who can possibly say you should have done it differently?

I want to stress this book is intended for the do-it-yourself hobbyist. It is not a book for audiophiles looking to build the ultimate audio system. A number of books have been published for the audiophile; this book is not one of them. Likewise, it is important to note that this book does not discuss guitar (and other types of music) amplifiers. Here again, there are a number of excellent books on that subject and interested readers are directed to those. The intent of the TAB Guide to Vacuum Tube Audio is to help hobbyists who want to build a tube-based amplifier do so. Pretty simple premise...


Hand-Wired or Circuit Board?

That's a good question, and I suggest the answer is "it depends." In Chapter 5 of the book (in particular Table 5.1) some of the trade-offs with regard to hand-wired construction using terminal strips and printed wiring boards (PWB) are discussed.

Hand-wired chassis

As described in the book, all of the projects began as hand-wired terminal strip-based implementations. This approach is ideal for one-off products and prototypes as it allows for rapid construction and facilitates change without too much difficulty.

For best performance, careful planning of component layout for a hand-wired unit is important. It is, for example, far easier to optimize a layout on paper than with a soldering iron. For that reason an accurate layout of the circuit components is advisable. This requires some amount of time, even if only done freehand as opposed to using a computer a drawing program.

As described in the book, PWB designs also require time to develop, and often require additional time to optimize. The real benefit of a PWB comes when multiple copies of a particular product or circuit are needed. Audio equipment usually has a built-in minium multiple of 2 (stereo).

Printed wiring board

When working on the first versions of the circuits described in the book, my expectation was that I would build only one of each unit. After building one, however, I decided to build another to put in my office, and then my brother wanted one for his home, and then a friend dropped broad hints about how he would like one too. So, at a certain point, designing a PWB seemed like a good idea. It also provided an interesting challenge. (The challenge is, after all, a big part of the fun of building something.)

A middle ground between hand-wired terminal strips and a PWB can be found in the turret-type circuit board (also described in the book). Here again, advance planning is advised. The turret-board approach has found good applications in countless projects—large and small.

One could argue that for a hobbyist, there really is no right or wrong way to go about building something. If it works correctly, meets the intended application, operates safely, is reliable, etc., then by definition it was built correctly. Personal preference comes into play at all levels of an audio project. So, coming back to the original question "hand-wired or circuit board", the answer is "it depends on how you want to build it."


PWB or PCB?

Well, I don't really care, but I needed to pick one. Printed circuit board (PCB) is more commonly used to describe a method of component interconnection, but "PCB" also is an acronym for Polychlorinated Biphenyls, a very nasty substance widely used as an insulator in high-voltage electrical equipment, and subsequently banned because it causes cancer. A Google search of "PCB" brings up about an even number of results for circuit boards and outlawed substances. Printed Wiring Board (PWB) is accurate and descriptive, and does not risk confusion, which I admit is only a remote risk.

If you are more comfortable with PCB, please feel free to substitute it wherever you see PWB on this site or in the book. In defense of my choice, I believe that PWB stands the test of an acronym as it: 1) is defined, 2) is consistently used, and 3) minimizes the possibility of confusion within a given domain.


Feeedback–Inverse or Negative?

Same as above. I needed to pick one. The terms "inverse feedback" and "negative feedback" describe the same process of returning an inverted (negative) sample of the output of a stage back to the input as a method of reducing distortion. The RCA Receiving Tube Manual (in the 1975 edition at least) describes this as "inverse feedback." Oher texts, including the classic volume Radio Engineering by F. E. Terman (McGraw-Hill, 1947, Third Edition) describe the same process as "negative feedback."

So, as with "PWB", please feel free to substitute "negative feedback" for "inverse feedback" where you see it in the book if that term is more familiar.

Not to dwell on this, but another term used in some circles is "degenerative feedback." I really don't care for that term, however.


Ultra-Linear Operation

The use of a screen tap at 40 to 50 percent of the full primary winding has been used for many decades in all types of audio gear. Commonly known as "Ultra-Linear" operation, the technique was patented in 1937 (Blumlein). This technique may be used in either a single-ended or push-pull output circuit. The primary benefit is a reduction, sometimes significant, in distortion. There is a power penalty for this mode of opertion, however. For example, a pair of 7868 power pentodes operated with identical parameters, can produce in the neighborhood of 34 W power output with fixed screen voltage, whereas the same circuit may produce about 24 W power output in the screen-tap configuration.

There are other considerations, of course, that determine the best circuit architecture. But as a first approximation, ultra-linear operation will reduce distortion but also reduce maximum power output. As documented in Chapter 10 of the book, the tradeoff is often worth it.


Caution: High Voltage

For the last 20 years or so, experimenters and hobbyists have been used to projects with power supply voltages typically in the 5 to 15 volt range for solid state circuits. That’s a far cry from the 400 V (or higher) plate supply of a vacuum tube amplifier. As such, extra caution is needed. We all know that, but it is worth reminding ourselves of the potential dangers every time we apply power to a tube circuit on the bench.

Danger High Voltage sign

Proper labeling of equipment is important for safety reasons. A number of "warning" signs are available from multiple vendors. One online vendor is safetysign.com. Among the items builders may want to consider are the following:

• "Warning Hazardous Voltages Inside", item #J5335-AF
• "Danger Risk of Electric Shock", item #J5305-AF
• "Danger High Voltage", item # E3360

These warning signs have an adhesive backing so they can be directly applied to the equipment chassis.

There are other on-line vendors in this space. You may want to also check out emedco.