Selecting the Right Tube
(or checking out the monster in your junkbox)

Files are in pdf format and require Adobe Acrobat Reader

Available Free

The WB0NNI Law of Supply and Demand:

While selecting a tube around which to build a linear amplifier may seem the logical way to go about it, more often than not amplifiers are designed around a tube or tube pair that we already have, or have a hot lead on. For example, while a pair of 3-500Zs make a great pair of triodes around which to build an amp, a pair of 4-400Cs can be utilized to provide very close to the same amount of power at often less than a quarter of the cost of the triodes. The reason is simple - supply and demand. While the 3-500Z is a great tube for grounded grid service, they aren't in much demand by the commercial broadcast industry that generally uses tubes of that size in a grid driven configuration. Consequently, one of the major consumers of triodes such as the 3-500Z are amateurs.

Commercial broadcast stations across the country eat 4-400Cs and tubes of that ilk for breakfast. They are routinely changed out when their output level drops around 10 percent - these are called pulls and most often find their way to the amateur market at a small fraction of their original cost. In order to accomodate routine change out, boadcast stations often maintain a small stock. What do you suppose happens when they update equipment? Answer - the unused stock again finds its way onto the market, consumed by both other broadcast stations and amateurs alike, at a portion of their original cost. Finally, consider which would be more difficult to produce, a triode or a tetrode with its additional grid? Yet, go to any site that offers both and you will find that tubes such as the 3-500Z sell for more than heavily consumed tetrodes such as the 4-250 or 4-400C.

The great point, of course, is that you don't have to spend a fortune to obtain tubes that will perform remarkably well. Shop around a little - if you do, you will be able to put more money into other portions of your amplifier, rather that sinking it all into tubes that will provide you with absolutely no advantage.

And, just a thought - if you do have buckets of money and aren't sure what to do with it, drop me an email and I'll make a few suggestions!!!