How damaging is long idle time for a tetrode ?

[AB1KC]

I would never intentionally leave an amp in standby and just walk away but I find that my Acom 2000a is SO quiet that on a couple of occasions I have done exactly that. Yesterday I walked into the shack after a long day and found that the amplifier was sitting in the operate mode quietly waiting for me exactly as I had  left it  the evening before  !
What exactly is the damage I am doing to the poor tubes when I do this ?
73,
Bruce

[WX7G]

The 4CX800A tube used in your Acom 2000A amp uses an oxide cathode and these can exhibit reduced cathode emission if left hot but not drawing cathode current for long periods of time. But realistically for ham use most of the time the tubes are not drawing cathode current anyway so why worry?

[N6AJR]

I don't know about that particular tube, but a lot of tubes have a Getter inside to scavange the ocassional air molecue out of the tube. ( all tubes leak at the atomic level) so the getter needs you to turn on the set and let it run for a while, like hours or even a day.  that actually helps the tube.  Most tubes do this in stand by mode.  just be sure the air flow on the amp ios not restricted. 

[WX7G]

I don't know about that particular tube, but a lot of tubes have a Getter inside to scavange the ocassional air molecue out of the tube. ( all tubes leak at the atomic level) so the getter needs you to turn on the set and let it run for a while, like hours or even a day.  that actually helps the tube.  Most tubes do this in stand by mode.  just be sure the air flow on the amp ios not restricted. 

The 4CX800 does not have a getter. The oxide cathode makes a fine getter and therein lies the problem. Without drawing cathode current yet left hot the cathode will slowly "poison."

[K6AER]

Ceramic Triodes and Tetodes can be left in standby forever. The power dissipation from just filament heat is miniscue as compaired to actual operation. My alpha 86 would be on for weeks at a time. It still is on the original tubes that are 35 years old.

Some tubes with high filiment wattage such as 3CX3000 will run hot but that is what proper cooling is for.

[WX7G]

It's not heat, it is cathode poisoning that can get an OXIDE cathode.

[K6AER]

I went through all my Eimac engineering stuff on ceramics. No mention about Cathode poising from long periods of standby idle. Do you have an article you can forward? All the3 amps I have encountered the tubes are in cut off when not in use.

[WX7G]

Here is a quote from the Wikipedia article "Vacuum Tube."

"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".

Here is a quote from SM0AOM at amps@contesting.com, June 2003.

"On the other hand, the filament hours obtained on 4CX250B's in the Standard Radio ST1610 ship's transmitters usually were less than 8000 when the transmitters were used for automatic radiotelex service. A lot of effort was spent to find the causes, and poor filament voltage regulation together with long intervals of stand-by without plate current was deemed to be the culprit."

Here is a quote from the UK Vintage Radio Repair and Restoration website August 9, 2010.
 
"The Brimar Valve and Cathode Ray Manual No 10 says they made two sorts of special quality valves, ruggedised valves designed to cope with shock and vibe and Other Special Quality Valves ruggedised but also with a special cathode tube alloy designed to prevent the growth of a resistive layer between the metal cathode tube and its emissive surface. It says this layer was especially likely to form when valves were run with very low cathode current or in cut-off condition for long periods.

Some of these Other SQ valves were recommended for use in computers. Examples include the 7032 heptode, and E88CC, 7489 and 7492 double triodes."


My comments again...

For the vacuum tube computers this was a problem and special tubes were designed. Some of these are listed in the RCA Tube Manual. When I worked in the TWT industry this was a problem for customers who would keep their TWT amps in standby most of the time. A bit of time drawing cathode current would fix the problem. As far as I know this problem occurs with oxide cathodes and not thoriated tungsten cathodes. The 4CX250B's mentioned by SM0AOM have oxide cathodes.

In any event cathode depletion will occur as least as fast with the tube in standby as in transmit because cathode depletion is due to the electron emitting cathode material evaporating.

So, the original question is how damaging is long idle (standby time)? Since it is not a well known phenomenom to amateur radio operators I would say it is not very damagine.

[K0TF]

The 4CX800 does not have a getter. The oxide cathode makes a fine getter and therein lies the problem. Without drawing cathode current yet left hot the cathode will slowly "poison."

And what is this? http://www.cqham.ru/forum/attachment.php?attachmentid=13646&d=1190554941