Almost Eighteen Years

I do not know what the record is for the longest tube life, however, this particular tube lasted 17 years, 11 months and 23 days.  That’s 157,596 hours.

I had written about this almost five years ago: http://www.engineeringradio.us/blog/2014/12/longest-tube-life/

The last one was last fall: http://www.engineeringradio.us/blog/2018/09/i-almost-hate-to-say-anything-but/

Eimac 4CX12000A power tube

This was installed new in a Broadcast Electronics FM20T transmitter which was placed on line on June 6, 2001.  It lasted until May 28th, 2019 with almost no down time.  Towards the end, the emissions started dropping off and we increased the filament voltage up to 10 volts.  When you have to increase the filament voltage, that really is the end for a tube.

The new tube was put in and I carefully marked out the date in the maintenance log.  The hour meter on the transmitter stopped working several years ago.

Prior to this, the longest tube life I’d experienced was an EEV 4CX35000C from an MW-50B transmitter RF section.  When that tube came out, it looked like it have been on fire.

The Broadcast Electronics STX-5 transmitter

Another install, this time a new BE product. I am familiar with the BE FM “C” series transmitter. Those are pretty solid units and we take care of many of them.

BE STX-5 LP transmitter
BE STX-5 LP transmitter

This new version of transmitter looks like it has a little bit of Elenos in its DNA.  Perhaps I am wrong about that.

The STXe exciter is an all purpose analog/digital unit that will do standard FM stereo, hybrid FM +HD radio, HD radio only, DRM+, or FM and DRM+.  I like that.  It gives the owner lots of options with regards to future planning.  Frankly, I would love to see some DRM+ testing done in the US.

We have actually installed a couple of “C” series transmitter with the STXe exciter as well.

BE FM2C with STXe exciter

The rest of the transmitter consists of four RF amps and an output combiner all in a short rack.  Frankly, if I were ordering one of these units, I’d order the taller rack.  Not that I am getting old or anything like that, but stooping over to program the date/time, frequency and power output introduced a slight discomfort in my lower back.

BE STX-5 LP controller/exciter
BE STX-5 LP controller/exciter

Running into the antenna.  At 4.1 KW, 18 watts reflected power is slightly high.  This antenna has always had a little bit of reflected power.

"The chicken coop, " WHUC and WZCR transmitter building
“The chicken coop,” WHUC and WZCR transmitter building

The building I installed this in is nick named “The Chicken Coop,” likely because it used to be an actual chicken coop.  I am not kidding.  The site was originally just the AM station (WHUC).  That station had a different transmitter building located some distance away which was fed with open transmission line.  This building was put in place sometime around 1969 when the FM station signed on as WHUC-FM (now WZCR).  It has seen better days, but we are working on fixing some of the issues with air conditioning and cleanliness.

Remains of open wire transmission line left over from orginal 1947 installation
Remains of open wire transmission line left over from original 1947 installation

The transmitter fired up without any issues and sounds much, much better than the QEI which it replaced.

Tired old QEI transmitter
Tired old QEI transmitter

The QEI transmitter had problems over the years, mostly burned out resistors in the RF combiner network.  It has since been scrapped.

The Sonotube subwoofer

Continuing with my speaker projects; I have finished building a Sonotube subwoofer for my Box One speakers. As I discovered, those speakers miss a significant part of the bottom end. I made up for this by using a Polk Audio PSW-10 subwoofer that was part of my home theater system.

Sonotube subwoofer, downward firing configuration
Sonotube subwoofer, downward firing configuration

I found the PSW-10 was certainly loud enough, but lacked definition which became noticeable when the new speakers where installed. Thus, I began thinking about a sealed box subwoofer. When designing the enclosure, there is a good bit of math involved in calculating the box volume.

Sonotube subwoofer, upward firing configuration
Sonotube subwoofer, upward firing configuration

I now have a bit of a dilemma; should I set the subwoofer up downward firing into the floor or should I set it up upward firing into a dispersion cone?  I have tried it both ways and in the downward firing position, it shakes the floor.  That is a great effect for watching movies like “The Hunt for Red October,” where there is a lot of bass rumbling.  It is also great for blasting some Led Zeppelin or Pink Floyd Dark Side of the Moon.   However, the idea of a sealed subwoofer was to add detail to the bass and that it does do.  The Polk Audio PSW-10 certainly had bass, but it was ill defined.  The sealed sono-sub has less low, low bass but the bass instruments now sound as good as the rest of the orchestra.  In the upward position, I get clean omni directional bass which sounds fantastic when listening to P.I. Tchaikovsky Concert #1 in B flat minor, opus 23.  That was the idea when I started making this.  Still, blasting Led Zeppelin is fun and I recommend it for everyone.  Decisions, decisions…

After a couple months of evaluation, I finally decided on the downward firing configuration.  It does add another dimension to watching movies.  Now, I kind of want to get a bass shaker for the floor underneath the sofa.

Piece of left over, 12 inch diameter sonotube
Piece of left over, 12 inch diameter sonotube

In any case, the technical details for this subwoofer are as follows:

  • Driver: Dayton Audio RSS265HF-4, 10″ subwoofer, 4 ohm
  • Plate Amp: Dayton Audio SA70 70 watt
  • 12″ (30.48 cm) diameter sonotube, 19 3/4″ (50.165 cm) long
  • Miscellaneous pieces of wood, paint, screws and foam that were laying around
Sonosub driver, Dayton Audio RSS265HF-4 4 ohm aluminum cone driver
Sonosub driver, Dayton Audio RSS265HF-4 4 ohm aluminum cone driver
Sonotube subwoofer foam lining inside of sonotube
Sonotube subwoofer foam lining inside of sonotube

I found a driver that will work well with a sealed enclosure.  This is important because some drivers work better with ported enclosures and some with sealed enclosure.  Another detail is whether or not the driver can be mounted horizontally.  Some driver cones will deform if placed in the upward or downward firing position.  The Dayton subwoofer selected will not suffer from that.

I used an online calculator called speakerbox lite to calculate the volume of my sealed subwoofer enclosure.  There are several choices for the type response the designer is looking for.  Initially, I thought about critically damped, but the box volume was 125 liters, which is more than a 12 x 48″ (30.48 x 122 cm) sonotube.  I settled on Bessel-2, which has a box volume of 56.68 liters.  That was easily obtainable with the sonotube I had on hand.

The plate amp is a run of the mill 70 watt Dayton unit.  Truth be told, it runs a little bit hot and I’d consider something else if I were making this design again.

For construction, I carefully cut the sonotube to the right length with a jig saw.  The outside of the sonotube was roughed up with some 220 grit sand paper before painting it flat black.

The individual pieces were glued together with gorilla glue.  The wood braces on the outside of the sonotube connect with wood braces in the inside of the sonotube with brass wood screws.  The plywood rounds were cut with a router to a close fit.  Being that this is a sealed design, I took some extra time with a non-silicone based sealant to make sure that the entire enclosure was tight.  All in all, the enclosure certainly feels tight.

It sounds great and I feel like my subwoofer now matches the quality of the other speakers I am using.