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EML / AVVT / KRON or KR / EAT output tubes

Posted by drlowmu (M) on May 7, 2007 at 15:58:21

Hi,

Here is what my friend Dennis tells me about EML, AVVT, etc. output tubes:

The best performing tube ever made for an output device was the RCA-designed Single-Plate 2A3-- a tube which is now rare, and very hard to find.

Experienced listeners will confirm this. The main reason for this tube's performance is the fact that it is symmetrical in design. The filament system is not a slanted "W" or a slanted "M". Instead, the RCA Single-Plate 2A3 filament system consisted of either 10 or 12 vertical filament strings, EACH welded at its top-- and bottom-- to a common horizontal bar at each end of each filament string. This produced a kind of "flat-plate" filament grid that conformed shape-wise to the grid structure, and to the shape of the directly-heated triodes plate. This tube was the best, most accurate output device that money could buy. RCA quit making them soon after their debut because they cost a lot to build. They went to the common type 2A3 and 45 tubes, which used a non-symmetrical and cheap-to-build "M" or "W" single-strand filament.

As you can observe, symmetrical DHT filament construction is not cheap to do, but this, or the rounded systems used in EIMAC transmitting triodes, for instance, are the only decent sounding DHT filament structures.

AVVT. EML, KRON, and perhaps EAT have rewarded us with modern tubes using the welded-bar, vertical filament system-- which is musically linear and does not produce spurious distortions because the filament geometry doesn't match that of the tubes plate, and grid structure. With AVVT, EML, KRON, and EAT, you are getting real tubes that reproduce real music perfectly, because this match-up is done properly.

An example of a tube which distorts music due to assymetry of structures would be the common NOS 45 tube, and the Western Electric 300B. Since this "W" or "M" filament structure-- which is cheap to build-- does not match the rest of the tube, the sound is euphonic and has its followers. Western Electric produced a tube somewhat similar to a 45-- this was the W.E. 275. Will any of these non-symmetrical tubes reproduce music accurately? They cannot, and will not.

Should one expect reliability problems with the newer, fabulously accurate tubes from AVVT, EML, KRON and EAT? Not if you run them conservatively, and protect them from mechanical shock-- the same thing you would do with any directly-heated triode.

If you wish to use tubes that are more rugged physically, you can use indirectly-heated triodes which have filament structures placed into protective metal rectangles, or cylindrical sleeves. While these types cannot reach the splendid musicality of the Prague-produced DHTs, they are certainly more physically rugged. A usable example of a physically rugged indirectly-heated tube would be the KT88.

Jeff Medwin

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