I want to build a tube amplifier that will be on for ~7 hours/day. I was wondering whether the life expectancy of the tube would be higher if I left the heater on all the time and just turned the plate supply on and off.
The tube will most likely be a 6N1P or a similar general purpose small tube.
What is your opinion/experience with this?
A heater's failure mode is typically a stress-related fracture of the tungsten wire or at a weld point and usually occurs after many thermal on/off cycles. One way to mitigate this of course is not to turn off the heaters at all (one of your options). Another is to employ a negative temperature coefficient (NTC) device such as a thermistor in the power supply servicing the heaters. to allow the heaters to reach operating temperature more gradually.
When the ENIAC computer was built in 1946 using over 17,000 vacuum tubes, the failure rate was initially several tubes a day. Of course they were already on all the time. They derated the voltage (and current) going to the heaters and reduced the failure rate to one tube every two days (longest time recorded without failure was five days).
Leaving tubes on all the time can accelerate failures which occur over long periods of time (thousands of hours of operation). Cathode depletion is the loss of emission after thousands of hours of normal use, as it is poisoned by atoms from other elements in the tube. However, according to page 34 of the 1960's era book getting the most out of Vacuum Tubes, this is fairly rare since by the time the cathode has lost its emission, the tube is pretty much dead for other reasons.
This same book, on page 14, makes another suggestion re keeping the heaters on; during standby, reduce their voltage to half instead of either leaving them on full voltage or turning them off.
I would personally probably provide a VERY soft start heater powering circuit and just possible an (almost unheard of) soft stop circuit (ie ramp up and down heater current over some seconds to minimise thermal shock and inrush current. You could even use a "maximum current circuit" to ensure Imax was hardly greater than I_operating_warm at any time.
Note that while general advice is that lowering Cathode temperature and reducing operating time increases lifetimes. one reference below, which very loudly claims to be authoritative, makes some radically unconventional claims. I'd be wary of taking his claim at face value unresearched - but also wary of not doing so.
TCrosley notes that
"Cathode depletion is the loss of emission after thousands of hours of normal use"
However, 7 hours a day = 2555 hours per year and 24 hours a day = 8765 hours per year, so you have "thousands of hours" per year in both cases but 24/7 = 3.4+ times as many thousands if operated for 24 hours a day rather than 7.
Extending vacuum tube life targeted at thoriated tungsten directly heated transmitter tubes but has some good general advice.
Tungsten Filament Life under Constant‐Current Heating -1969
Abstract only of for $ article BUT notes -
Wikipedia - Vacuum tube says a number of useful related things, but of special note is
Tubes on standby for long periods, with heater voltage applied, may develop high cathode interface resistance and display poor emission characteristics. This effect occurred especially in pulse and digital circuits, where tubes had no plate current flowing for extended times. Tubes designed specifically for this mode of operation were made.
Cathode depletion is the loss of emission after thousands of hours of normal use. ...
BUT this authorative SOUNDING page contradicts advice from a number of other sources Vacuum tubes and vaccum tube failures
Claims include:
The big enemy of high gain power grid tubes (or valves) using metal oxide cathodes is grid current and excessive cathode current, or low cathode temperatures.
The ARRL, as careful as they sometimes try to be, has published more than a few incorrect articles about amplifier and tube life.
A thoriated tungsten filament tube can be run "hard", to the point of complete filament-cathode emission saturation, and the life will be no shorter or longer than when run easy,
Provided the elements being bombarded by electrons or the envelope does not overheat and suffer permanent thermal damage. We can lower filament voltage in a thoriated tungsten tube, and provided it doesn't get contaminated from prolonged operation at excessively low voltage, all that happens is peak clipping.
A metal oxide cathode tube can quickly suffer damage if operated that way. This is why they sometimes have to start on a timer that prevents current before the cathode is fully heated, which sometimes can be the warm-up time of a rectifier tube and other tubes in the system! Lower the filament voltage in a metal oxide cathode tube too low, and you can ruin it in seconds!
Within emission and thermal damage limits, tubes basically do not wear any faster or slower if just idled or operated. It isn't like a mechanical engine, where high RPM operation greatly increases wear by mechanically loading internal parts increasing friction. As a matter of fact, too cold is often much worse than hot.
The later MIT Whirlwind 1 had robust statistics on failures of the many valve models it used (very large valve population and many years of statistics with almost 24/7 operation). A 1958 paper by the main powering eng (Gano) associated with that project identified that heater filament failure was low (3.6%) compared to total failure population. The statistics also showed that the filament failure rate halved when a controlled power on supply was used, but the report didn't identify the frequency of power on events. I guess the message is that filament failure is a rare event for starters, and although measures can be taken to lower that failure rate, it is highly likely that other reasons for failure will occur first and foremost.
Paper on "Thermistors for the Gradual Application of Heater Voltage to Thermionic Tubes"
Cathodes must not be operated at lower temperatures than emission temperature or you'll damage the cathodes by ion emission. Of course, "operated" means current passing through the cathode. If there is no emission field (namely anode or at least gate at cathode potential), this does not apply.
So if you have some standby filament current that is lower than the operating current, be sure to have the rest of the tube powered off entirely.
A slow start by thermistor from standby is likely still going to prolong overall lifetime even when the anode comes live earlier than full filament temperature.
The replies have touched on many of the issues, yet no seemingly firm, reliable, technique out there for indirect heated tubes. So, here's my plan...
1a. If you use DC and turn the device consider switching the polarity on occasion, before a cold start is best.
1b. Assuming the tube ages gracefully, you might extend its life by slowly raising the heater's operating voltage as the tube nears end-of-life.
Run the tubes, w/o B+, at around 80% voltage if you plan on using the device again within, say, 3 days or so. Turn off completely if not.
When turning up the device, wait at least 2 minutes for full heat before applying B+. Adjust the voltage up to, or down from, operational the value in a measured way.
3a. Turn up power most slowly when powering up for a cold shutdown.
Make sure the tube are kept as cool as possible. Insure the best possible air movement, always. Well placed fans, if quiet enough are not a bad idea.
Set operating parameters for the tube to the lowest acceptable power parameters possible. For example, use a KT120 tube biased like it was a KT90.
Always use the lowest volume/gain setting possible when listening.
If possible, apply B+ in gradual fashion as well. Perhaps by using a big, nasty, variac. And then, only after heaters have been brought the tube to full heat.
Do not place the device in an area with vibration. Be careful of where you put that fan, btw. Never on top of a speaker, for example. Or even the same shelf that may also have speakers on it. Don't let the cats, dogs, children, jump, walk, or play around it.
For a specific tube, like yours, verify - then verify again - it does not operate over its rated parameters, ever. Check ALL volts and currents. Check again, periodically, over the life of the tube. If you do nothing else, you must do this.
Keep the tubes free of dust. Do not touch them with your bare hands. Don't use "Pledge", or such. The glass must remain spotlessly clean at all times.
Don't use tube cans, dampers, or anything that will block infrared radiation from leaving the tube, ever.
Prevent oxidation on the pins and socket. I use an ever, ever, so small quantity of "Mobil One 0w40 motor oil". Don't get any, at all, even the smallest bit, on the glass envelope. Mind your fingers!
In the end, your tube is GOING to die. It just is. So stock up on some spares now, they simply ain't going to get any cheaper.
The heater runs at 6.3 volts, 0.175 amperes, or 1.16 watts. That's a kilowatt hour roughly every 860 hours, or about ten kilowatt hours a year. At 15 cents per kilowatt hour, running the tube 24/7 would cost $1.50 in electricity annually. It would provide a modicum of background heat in winter!
Balance that against the cost of a tube. The 6AK5 is relatively inexpensive but might cost the same as three or four years of electricity consumption.
