ARE TUBES RELIABLE?
Be a Beacon
Nothing is more frustrating and potential expensive than a piece of tube gear that is unreliable, and because this causes so much pain "Dr. Gizmo...is this piece of tube gear reliable?", is one of the most often asked email question, so let me address it.
TUBES ARE REILABLE: Which is why they are still used and have been used in all military services, radio stations and various industrial application. Tubes are in many ways much more rugged than transistors. Of course the key to achieving longevity is using them properly and using quality tubes. And it is absolutely essential to understand their duty cycle life. Some tubes are rated at 1500 hours and some tubes are rated at 40,000 hours, so KNOW YOUR TUBE.
One of the modern snafus of tube use is the use of solid state rectifiers which jolt tubes with the high plate voltage before they have warmed up. With a tube rectifier the filaments have a chance to turn on before the plate voltage slowly turns up. The sudden voltage surge to the plates of the tube shorten its life, and caring manufacturers either include a separate switch for the plate voltage or include a timing circuit which gives the filaments a chance to warm up before the plate voltage is turned on.
How a tube is operated also determines its life. Small signal triode are usually used in conservative ways, but you should be aware that in high gain tube circuits tubes can become noisey with life, especially if the equipment is turned on and off frequently.
As you know audio tubes are voltage amplifying devives and when power tubes are used improperly they will not last very long, and by that I mean in a manner which requires them to pass lots of current because they are being used with ineffecient low impedance speakers. There is nothing wrong with this if you dont mind replacing tubes often, and are prepared for random tube meltdown.
There is also a question of what is the best operating point for a tube, and I use 300Bs as an example. Western Electric 300Bs shouldnt be operated at more than about 400 volts at 60 ma, and I use mine at 350 volts at 60 ma. Which yields about five watts in a single-ended amp. At that rate I am going to get about 20 years of useful life out of them because I sued them with high effeciency/high impedance speakers. There are other brands of 300Bs that can be operated at much higher voltage and current which will produce much greater power but much shorter tube life. If you need the power you makes your choice and pays your price.
Power pentodes like the EL 34, 6550, KT 88, or 6L6 operated at 550 volts at higher current will not last as long as being run at 450 volts at lower current, so check with your manufacturer to discover how the tubes in your amp are being used.
My personal "longevity" strategy works like this: all of my tube equipment has separate on/off switches for the filament supplies and plate voltage. I turn on the filament voltage and then wait about ten minutes for the tubes to warm up, and then I turn on the plate voltage.
SIMPLER IS BETTER BECAUSE MURPHY RULES:
The most reliable amplifiers I have ever used are single-ended circuits because they have so few parts...less parts, less tubes, less to fail or go down. My warnings to all about big complicated tube amplifiers is not necessary an indictment against any companys build quality, but rather...if you are going to walk in the desert, you are going to get hot...big complicated tube amplifiers are much more prone to failure, because there is much more than that can fail...and these big horsepower amplifiers tend to be driven hard, abusing the output tubes. I have friends who have these big horsepower amps that have given years of trouble free service because they used them with very effecient speakers which need stress the output tubes.
BEWARE OF YOUR WALL:
A ten volt voltage spike from your AC outlet is equal to a forty to a fifty volt spike in your tube amplifiers power supply which can destroy your power supplys capacitors. The turn on surge can also stress your power supply capacitors. It appears that power supply capacitors are one of the weakest links in every tube amplifier which is why they should be of the highest quality and be conservative rated with at least an extra 100 volt safety margin. It seems an oddity of progress than tube rectifiers are much kinder to tube circuits when it comes to these spikes.
HEAT, CRUD AND CAT VOMIT:
At New York Audio Labs we once received a Futterman amplifier for repair because it blew up. When we took the cover off the circuit board was covered with dried vomit, which we later discovered was the from the owners cats. Tube amplifiers need ventilation if you are not able to let them ventilate at least use a whisper fan to keep air moving around the tubes...cooler is better. Tubes can also get covered with a flimy crud in most home environment, which means that periodically you should clean them with alcohol, because this crude is a form of insulation. Wait at least a few hours before turn your amps on after you clean them with any kind of cleaning fluid. The same crude can get on your circuit boards and cause a short, so it is a good idea, with your amplifier unplugged, that you use a good stiff paint frush to brush off all the hair, drandruff, cigarette butts and belly button fluff that accumulates on your circuit board. The high voltage charge of your amplifier is a real world dust attractor, so do your monthly maintenance.
e-mail Dr. Harvey "Gizmo" Rosenberg: firstname.lastname@example.org
Copyright© 2006 Meta-Gizmo.com and Dr.
Harvey "Gizmo" Rosenberg All