[R-390] Tube longevity (summary)

[Flowertime01 at wmconnect.com]

G, Kwitka asked,

To prolong tube longevity  is it better to keep the set on with the volume 
and RF gain turned down or use the standby position on the function switch?

Charles replied that,

Military BAs generally don't beat their tubes (unlike a lot of home hi-fi 
equipment), so leaving them on is generally OK. 

Charles replied further,

Some oxide-cathode tubes don't like having no B+ for long periods. 

Mark Huss replied,

With mil-spec tubes, heater failure from startup current is rare, though the 
old manuals said it better to keep the set on with the volume and RF gain 
turned down.

Mark Huss further replied,

If you have done a solid-state mod to the Power supply rectifiers do not use 
the Standby position. If you do not have the 26Z5 tubes in the power supply, 
turning off the plate voltage by switching to standby will cause the plate 
voltage to raise excessively, and can cause some caps to blow. Never use standby.
 
Mark Huss recommends,

Probably the best thing to do is to add a CL-80 surge suppressor in series 
with the 2 Amp fuse in back.
 
Mark Huss point to the counter issue

The exception is the Ballast Tube. That is rated in cycles (about 2000-5000). 
On-Off cycles will definitely affect the life of that tube. 

Roger adds,

However if the ballast tube has been sand stated or replaced with a tube 
filament this limitation becomes a non-issue.

Mark Huss offers,

Personally, I don't think tube life is reduced significantly by turning the 
receiver on and off. I suppose you will get a few hundred hours more from a 
tube that is about to loose it's heater if you leave it on continuously. 

Tim Shoppa offers,

In some industries part of regular maintenance is to "margin" the power 
supplies up and down just to find (and in some cases blow up) the weak parts. Thus, 
turning off the plate voltage by switching to standby will cause the plate 
voltage to raise excessively, and can cause some caps to blow. Blowing the weak 
parts up is an added bonus - it makes sure that nobody will try to reuse them! 
I'm thinking mainly of the original electrolytics and black beauties, 
although I'm sure some of the metal-can Westcaps/Vitamin Q's and yellow Aerovox-type 
caps will blow too. Some of us hate old crappy caps with a vengeance. (Or 
not-so-crappy-originally-but-now-50-years-later....) We're probably the same guys 
who stuck electrolytics across 120VAC to watch them blow and now hold titles 
like "Director of Destructive Testing"!

Roger L. Ruszkowski offers,

It was published R390 standard practice in the ASA to never park an R390 (or 
its newer version for clarity R390/A) in the standby mode.

So what's a body to do? How many hours are you going to listen to the 
receiver and how many hours is it going to just sit idle waiting for use? The 
military run them 24 x 7 for years. They got turned off twice a year for the semi 
annual maintenance. A technician would likely turn it on and off 4 or five times 
while doing the maintenance. Once the receiver went back in the rack, it was 
just left on until some maintenance guy come along again to do service.

If you only use your receiver once or twice a week, you may as well turn it 
off when not in use. If you are going to listen to it every day, even if only 
for an hour or so, you should leave it on. If you are going to do a week end 
event, turn the receiver on several hours early to let it warm up and stabilize. 
Then leave it on for the duration.

A tube has a working life. The military used them 24 x 7 until beyond 
acceptable noise performance. At six month intervals, the receiver was tested for 
signal to noise performance. Tubes were tested for shorts and put back into the 
receivers. The test set up was performed. Maintenance would try to get a 20:1 
signal to noise performance out of the receiver. Noisy tubes would be replaced 
until the ratio was achieved. The receiver then went back into service for the 
next six months at 24 hours a day. 4380 hours. Unless the receiver died, it 
received no additional tube changes for about six months. Over this time the 
new and old tubes would "age" and get more noisy. Hopefully the signal to noise 
ratio would stay above the required 10:1 noise level. Most receivers did make 
the cut of 10:1 after six months use.

>From this we can conclude that tubes will make at least 4380 hours of good 
quiet life. Hours on a tube with no ears to listen is wasted tube life.

One trade is receiver stability and warm up time. Most R390 receivers will 
drift some as they warm up. But are you using the receiver in a mode that exact 
frequency is critical? Even if you have to ride the knob to keep a SSB signal 
in the band pass, the drift of the receiver is not that much. 

Most of us have solid stated the rectifiers and the replaced the ballast 
tube, so the issue is cold filament surge failure against time on a tube until its 
emissions noise gets excessive. Life cycles on a ballast tube has a high end 
of 5000 cycles. A tube will likely do as many cycles.

So do not leave the receiver on more than twice as long as you are going to 
listen to it. Turn it on and off 5000 times and enjoy 5000 hours of low noise 
reception. If you use it for more than an hour at a time, you will then likely 
get more than 5000 hours of use with good noise performance, before you have a 
tube filament fail. If you leave the receiver on for 5000 hours the tubes 
will start to get noisy and performance will suffer. You age the tubes and get no 
return.

For the casual user, the analysis is to turn the receiver off if it will not 
be used in the next 6 hours. The exception would be, to leave the receiver on 
for the duration of an event to avoid a failure at power on and have a 
temperature stabilized receiver for use during the event.