Total posts : 45366
Tubes are very nice devices when they work. The trouble is that the tubes available today have very poor quality. That there are any tubes at all available today is probably because of the alleged superiority of tubes in audio amplifiers. There are audiphiles who will pay a premium price for an audio amplifier made with tubes. Actually, tube amplifiers have a lot more distortion than a decent solid-state design because they have a lot less surplus gain that can be used for negative feedback to correct for distortion. However, since tubes are approximately square-law devices, they produce largely even-order distortion products, which are pleasing to the ear.
I think that tubes can be used to design more efficient output stages for Part 15 AM transmitters than is possible with solid-state devices. MOSFETs will not make highly efficient RF output stages because of an internal diode that absorbs power when it is forward-biased during a portion of the cycle. JFETs are much better, but all of them have very high interelectrode capacitances. This makes them difficult to use for efficiently driving high-Q, low radiation resistance, antennas of the kind used for Part 15 AM. Tubes, on the other hand, have very low interelectrode capacitances.
The characteristic curves in tube specs actually mean something. They describe how a tube will perform over a wide temperature range. Early transistor specs also had charecteristic curves, but they were unlikely to accurately represent the actual device. So, eventually, transistor manufacturers stopped supplying characteristic curves. Characteristic curves that accurately describe the device can be used for designing for exacting applications.
Unfortunately, the tubes that are available today do not perform as well as tubes performed years ago. I have had fairly recent experience with tubes. A company I worked for used a marginal HF oscillator using a 6C4 or a 12AU7 as a detector for differentiating between different species of white blood cells. This was a critical application in the company’s flagship product. Everything was fine as long as good tubes were available. A small company in the U.S. manufactured good-quality tubes using equipment purchased from large companies that had stopped making tubes. Eventually, this small company stopped making tubes, and nothing but trouble followed. Tubes purchased from Russia and China were terrible. Serbian tubes were a little better, but then there was a trade embargo against Serbia at the time. The tubes that worked well were stocks of old unused tubes manufactured in the U.S., England, Germany, and Holland. There was no choice but to embark on a project to develop a solid-state replacement for the tube oscillator. The tube oscillator had characteristics, some of which were not well-understood, which were difficult to duplicate in a solid-state version. It took six years before an adequate solid-state design was ready for production. The company would have ordinarily shut down a project that took so long, but this was something that had to be done. The JFET that was used was expensive because it had to be a selected device. The selection criteria had to be as wide as possible so that there would be any yield at all in the JFET manufacturer’s process. Temperature changes were a problem and had to be compensated for. In the meantime, there were supply problems with the tubes that increased as time went on.
My opinion is that tubes should not be used unless there is a compelling technical reason for using them. Using tubes simply because they are quaint is not a good enough reason. Using a tube to get superior performance presents the difficulty that good tubes are now hard to obtain.
