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Maybe That's Why Using Bendix 6900's Made Such An Improvement?

Al,

I know tube microphonics play a role in the final "sound" of a tubed amp, although I'm not sure I'd say tube microphonics are the ultimate limiting factor. However your mentioning this started me thinking. When I replaced the stock Tung-Sol 5687s with Bendix 6900s which is considered by many to be a rugged version of the 5687 or a Super 5687 as some call it, a dramatic improvement for the better occcured.

Al I was so impressed with the 6900 I actually did quite a bit of research on the 6900 tube. Like I said previously the 6900 is considered by many to be a rugged version of the 5687 or a Super 5687 as some call it. The truth on that will probably never be known. What we do know is the 6900 tube was originally developed, circa 1954, for a US Air Force project by the Bendix Corporation of Red Bank, New Jersey. It was in fact used in Pulse applications in the USA's Intercontinental Ballistic Missles, as well as in aircraft and other military applications. Whether the 6900 was actually a 5687 made extremely durable & dependable or whether it was developed from scratch and just happened to come close to the 5687's specs is a subject of debate.

Due to it's intended purpose of use the 6900 needed to be extremely reliable and some of the prime prerequisites were freedom from early failures, long average service life and uniform operating characteristics. All 6900 tubes were subjected to a 45 hour break-in under the various overload, vibration & shock conditions, they were likely to be encounter in service. Thus each and every 6900 was tested for it's pulse capabilities under maximum grid-drive and duty-cycle conditions.

The 6900 was designed for use in equipment with high ambient tempatures (* this requirement permitted operation of the 6900 at bulb tempatures up to 300 degrees Centigrade! *) and where high levels of vibration & shock are encountered. This necessitated special materials & manufacturing technics to be employed. The 6900 is constructed from Nonex glass. This causes the 6900 to be quite heavy. It easily weighs as much as any other two 9-pin types together would! It also uses a Tungsten stem seal construction like many of the high-powered transmitting tubes do. The 3 spacers are made from ceramic. Each of these ceramic spacers has the equivalent thickness of about 4 of the typical spacers used in other 9-pin tubes, stacked together. Also the use of ceramic spacers eliminated one of the common sources of tube failure, which is the evolution of gas from other less costly materials such as mica which is commonly used for spacers. Moreover the ceramic spacers along with the use of of multi-pillar supports locked together via 12 welded eyelets, contribute to a sturdier sturcture. Special alloy snubbers, which maintain their postion, retain their spring properties at high temperatures, resulting in a structure resistant to shock accelerations as high as 500Gs! So it's most likely fairly immune to microphonics although I've never really tested them for such.

Even with all these demands on the Bendix 6900 tube it's life expectancy was 5,000 hrs when operated in Pulse applications as such:

Heater Voltage: 6.3v
Heater Current: 1.0a
Plate Volatge: 500V
Grid Pulse: +50v
Grid Voltage: -100V
Plate Current: 4.25a
Grid Current: .25a
Pulse Time: 10u sec.
Pulse Repetition Rate: 250pp sec

Perhaps it's due to the 6900's intended enviroment & requirements of use, that it's been claimed when a 6900 tube is placed in the gentle enviroment & requirements (gentle as opposed to it's intended use at 300 Centigrate while pulsing @ 5amps) of the typical tube amp it's been said to last virtually forever! Well not quite forever but some have claimed the 6900 will last 75,000 to 100,000hrs in a typical tube amp when replacing a 5687 tube. That's a minimum of 3125 days or 8.5 years of constant use! Now considering that no one leaves their tube amp on 24/7's you can see why some claim the tube lasts forever. I don't know if that's true or not, but this much I do know. About 2 years ago I came across 100prs or 100 pieces, I forget, of 6900 tubes. I hand selected 5 pairs for myself (I now wished I kept 10 pairs) of the 6900 that measured the very best and sold the rest. I had been told previously that a 6900 that measures almost dead would function perfectly in a tube amp when replacing a 5687. At the time I found that to be an absurd statement, but after looking at it's intended enviroment & requirements of use I took a pair of 6900 that measured almost dead and replaced the pair of 6900s that measured as NOS. Much to my amazement I could not hear a difference between the 2 different pairs. I was absolutely shocked, but when I contacted another seller of 6900's I found on ebay he told me, Why are you shocked? The 6900 is meant to work at almost 5amp pulses all day long. When it's in your tube amp as a driver it probably only passing milliamps. Under those conditions the 6900 doesn't even know it's being used.

Well as I start to end this tale of the 6900 tube I'll mention in the 1960s, the remaining stock of Bendix 6900 tubes were purchased and rebranded by MU Engineering, which continued to manufacture it in small runs at their Oceanside, California, plant. In the end only a few thousand of the 6900 tube were ever made. Some would tell you that partly because of its scarcity that the 6900 has acquired a mythical aura. But I'd say that it more due to those who have owned, used and then after having been bowled over by it's dramatic transformation of the sound of their system this tube created, they later allowed their friends to hear what the 6900 tube can do as well. I'm sure it was word of mouth from these people that actually created it's mythical aura. Much like how the mythical aura of the 300B tube was created, no?

Finally some people say the 6900 sounds more like a solid state device than a tube. Personally I only partially agree with that. Yes the 6900 has the added detail, bass control & extention of a transistor, but it has that while having the traits only the best of tubes have, i.e. a fuller & more realistic tone color, an incrediblly bigger, airier, 3-D image and a wider, deeper, layered soundstage that will cause your jaw to drop and goosebumps to raise. In the 37 years I've been involved in audio no single component change has equalled the difference I heard when changing the 5687 with a 6900! I know tube changes are usually subtle but perhaps it's due to it's incredible construction and structure resistant to shock accelerations as high as 500Gs that causes this tube to "sound" so damn good!

Thetubeguy1954


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