[R-390] maintenance

[Flowertime01 at wmconnect.com]

Scott,

The military did semi PM on the receivers. Simi being twice a year. Six 
months of 24 7 was the half life of most tubes. Some would go 18 or 24 months when 
fired 24 7. But a year was typical.

Some Fellows reported the life expectancy of tubes being turned off and on
as tubes do die on filament turn on. Just like light bulbs blowing when 
turned on.

For most of us we get more life turning the receivers on and off with each 
use.
For heavy users its a wash, do leave one or two on all the time but let the 
others rest.

Age of New Old Stock does not appear to effect the life of tubes.
Stuff from the 50, 60, 70, still gives about 10,000 hours. Over a year of 24 
x 7
use.

Most tubes will yield 5,000 power off on cycles.
So if you leave the receiver on more than two hours at a time you are likely
to get the full emission out of a tube before you open the filaments.

I am not taken back that after six months of 24 7 your receiver needed a 
whole new set of tubes. It is a good thing you did a PM when you did and did not 
drive the receiver until you charred a plate resistor.


Every 4000 to 5000 operating hours the tubes need to be checked.
You expect a tube to go 5000 hours as very good.
Then from 5000 to 10,000 hours as good.
Then from 10,000 to 15,000 hours as OK it works.
Then from 15,000 to 20,000 hours as IT HAS NOT SMOKED SOMETHING YET.

Any thing after that you say thanks to the engineer that designed that tube 
stage to
operate conservatively and thus yielding longer than expected tube life.

A lot of tubes in a lot of circuits will give a longer life than expected.

There are reports of receivers going years and years on tube sets
24 7 for ever and ever. 

Do not just toss a tube because it has hours on it or even if it is noisy.

There are places the noisy tube can be used until it glows blue or its 
filaments go open or its emission drops to bad on the tube tester.

Use your tubes until you get all the use it will yield. Then save some to 
populate the receiver as a test set while you evaluate newer tubes for noise.

This is why every six months the tubes were tested and bad one replaced.

This is why there are rotation patterns where used tubes go to the back 
burners and new tubes go into the critical path.

Then there are in circuit comparison test to find the least noisy tubes on 
hand to place in the critical path. There are way to compare tube noise to 
select the least noisy tubes. There are places to best utilize the least noisy 
tubes.

Strange, new tubes can have more noise than a tube that has run 5,000 hours.

While you have the receiver on the signal generator and are doing an 
alignment and checking the signal to noise ratio as you go, swap a couple 5749 or 6BA6 
around in the IF deck and compare the difference in output level and noise 
level.

You trade a quite tube and a noisy tube into the first IF socket and see the 
difference and you become a believer. These tubes may both check the same on 
the tube tester for emission level. The quite tube may even have a lower 
emission level on the tube tester. Not always. There is no correlation of tube 
tester meter reading and signal to noise performance of a tube.

Tube testers sort out the shorts, gas, and low emissions. Check every tube 
about 5,000 operating hours or less in a tester to weed out the tubes that have 
reached the end of their rated life.

Then grade the tubes that pass the tube tester to determine their place in 
the receiver. You grade tubes in the receiver with a signal generator and power 
meter. (AC volt meter and 600 Ohm resistor [two, 1,200 ohm 1/2 watt 
resistors]) across the local audio output.

Compare all your 6DC6's 
Compare all the 6C4 in the 2nd mixer above 8Mhz.
Compare all the 5749 or 6BA6 in the first IF
Compare all the 6AK6 in the last IF stage.
Compare all the 5814's in the first audio stage.
Compare all the 6AK5 in the crystal osc deck because you can get that tube in 
and out.

If you have lots of low noise 5749 or 6BA6 then use a couple in the PTO and 
BFO. If not then trade off noise in the BFO for better tubes in the IF deck 
signal path. Then PTO noise. 

Compare the signal to noise and swap tube pairs around, Use the tubes for 
better signal to noise ratio. You can adjust the over all gain at the RF, Audio 
or IF deck gain controls. 

Roger AI4NI
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