I almost hate to say anything, but

We have this certain transmitter which has been on the same tube since June 6, 2001.  Come to think of it, the transmitter itself has been on the air for the same amount of time without failure.  A testament to its designer…

Broadcast Electronics FM20T, WYJB, Albany, New York
Broadcast Electronics FM20T, WYJB, Albany, New York

According to my calculations, that is 151,691 hours or 17 years 3 months and 22 days.  The tube is the original EMIAC 4CX15000A that came with the transmitter.

I am a little nervous about turning it off to clean the cabinet.

EIMAC 4CX15000A tetrode
EIMAC 4CX15000A tetrode

We have a spare on the shelf for the eventual replacement of this tube, but I really want to see how long this thing will last.  This is also one of the last tube transmitters we have in main service.  There are several backup tube transmitters still around.

Longest tube life?

We may be going for a record here; this Broadcast Electronics FM20T was placed in service on June 6, 2001:

Broadcast Electronics FM20T, WYJB, Albany, New York
Broadcast Electronics FM20T, WYJB, Albany, New York

The original 4CX15000A tube is still in use.  I wrote about this a few years ago in this post: Longevity.

I thought by now, we would have changed out that tube.  A few quick calculations shows that the tube has been in use for 118,289 hours or 4,929 days or 13 years 6 months and 3 days.  Anyway you look at it, that is a long time for one tube in nearly continuous use.  I noticed the hour meter is lagging a bit:

Broadcast Electronics FM20T hour meter, WYJB, Albany, New York
Broadcast Electronics FM20T hour meter, WYJB, Albany, New York

Reads 113051.24, which is 5,238 hours different than what I calculated from the maintenance log.  I noticed a slight discrepancy in hours two years ago and attributed it to various off air periods.  However, between then and now, this transmitter has not been off at all.  Thus, the hour meter is wearing out before the tube.  I would say that this is because of excellent filament voltage management,  but I think we simply have a really good tube.

Has anyone else had a tube that lasted this long or longer?

The Raytheon RL10 Limiting Amplifier

Update: Apparently this is quite interesting to a number of people.  I have rescanned the manual, properly compressed it and which you may find it here.

Found this manual at one of the older transmitter sites:

Raytheon RL10 limiting amplifier manual cover
Raytheon RL10 limiting amplifier manual cover

Entire manual is available for your reading pleasure here: Raytheon RL10 limiting amplifier

As this is an older design than either the Gates Sta level or the Collins 26U, it may not be as useful to tube audio enthusiasts.

Raytheon RL-10 Schematic diagram
Raytheon RL-10 Schematic diagram

The main issue with the Gates and Collins unit is the GE 6386 remote cutoff triode used, which were great tubes, but very difficult to come by these days.  This design calls for a 1612 or 6L7, which is a pentagrid amplifier.  Feedback is provided by the screen of the following stage, a 6SJ7GT.  Anyway, perhaps it will give somebody some idea on how to make a good tube compressor limiter.