By Michael Miguest
The 6900 is considered by many to be a rugged version of the 5687 or a Super 5687 as some call it. 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.
Because 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 encounter in service. Thus each and every 6900 was tested for it's pulse capabilities under maximum grid-drive and duty-cycle conditions.
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 structure. Special alloy snubbers, which maintain their position, retain their spring properties at high temperatures, resulting in a structure resistant to shock accelerations as high as 500Gs.
A friend of mine recently purchased a pair to replace the Phillips 5687's in his AMR-77 phono preamp and invited me over to hear them. This initial listening session had me agreeing with Peter Qvortrup/Audio Note UK who said "The Bendix 6900, I know this is a popular tube with many, but I have to say I find it quite poor sounding, heavy and bright at the same time".
My friend agreed with my initial assessment and several days later invited me back over telling me he had discovered that he had crossed his left and right channel wires in his system. I brought 88 Basie Street by Count Basie and Soft Lights and Sweet Music by Scott Hamilton and Gerry Mulligan. We listened to Bluesville and Nobelesse. We also listened to a 45 RPM Reference Recording, Dance of the Macabre. I found this session sounded only a little better than before but was in no way jaw dropping. The hyperbole that the majority of so-called audiophiles bestow on this tube wasn't demonstrated to me in his system in no shape, form, or fashion.
I think its very important that the readers here understand that I have experienced this system with the stock Phillip's 5687 tubes many times before his purchase of the Bendix 6900 tubes and thoroughly enjoyed the musical presentation. My friend was most impressed with the added weight of the musical presentation that the 6900's brought to the table. However, I found it to be dark and heavy which mitigated a lively presentation, which I prefer, and subsequently a diminished magical quality which I experienced from his stock 5687's.
Too often in this hobby, hyperbole, specifications, and price rule the day as opposed to performance. In my opinion, the Bendix 6900's are "Overpriced", "Overrated", and with regard to the musical presentation, "Under Performed".
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