Paired 2 x 6N8S 6H8C 6SN7 METZ from 1956. "Metal base". NOS. Foil getter.

A perfectly matched pair of 6N8S (6H8C) tubes, directly matched to the Western 6SN7 tubes.

These are the famous "metal base" tubes from METZ (Moscow). Both are from 1956. They are the "oval mica" version, a foil getter with rectangular holes in the anode and a dark anode.

Both tubes are military-grade "OTK" tubes (both have stamps, the last one from the inspection from 1975).

But be warned! This is a rare find: one of these tubes is an ultra-grade tube for ballistic missiles or radars in MiG aircraft (the stamp with the Soviet star).

This is a collector's item. Tubes with all possible inspection stamps at multiple levels (from factory OTK to periodic "warehouse" inspections) are very rare. They must have been removed directly from the device or, more likely (due to their measurements), from a so-called ЗИП (ZIP), "Запасные части, инструменты и принадлежности", meaning "Spares, Instruments, and Accessories," a repair kit for all types of military equipment. After the collapse of the Warsaw Pact, many such parts remained in Poland. I purchased this particular tubes from a company that traded in military supplies from the Polish People's Republic.

This particular tube has a defect characteristic of most "metal base" Soviet models: a cracked metal base. However, the glass is firmly attached. It sits in the base and doesn't move. This doesn't impede use in any way. In fact, many consider this an advantage, as they know the glass won't crack spontaneously, as sometimes happens with new metal bases when the lamp is first heated up.

Therefore, before selling, I gradually burn in each tube for at least a dozen or so hours to avoid this problem. This is also necessary for reliable measurements.

The tubes are measured and paired after burn-in and stabilization using a calibrated Elpo P-508 meter powered by 220V (from the Elpo era).

The "Ia" current measurements for both halves + transconductance curve measurements for nominal Ia (from the catalog values ​​of Ug = -8V and Ua = 250V). As an exception for such high-quality lamps, I measured the transconductance for the anode current Ia at the typical operating point, i.e., 9mA. This is important because in 99% of cases, this is the case. These tubes will operate at anode current, not at the measured currents Ia.

The transconductance parameter gm/S is, as you can see, ideal. A very rare occurrence for Russian tubes, but obviously – the military selection process by the OTK selected such perfectly even tubes from among thousands of "normal" ones intended for less critical applications than ballistic missiles.

The sound of these tubes:

How these tubes sound: a demon of speed, dynamics, and purity. There are no other tubes in the price range of up to thousand $ per pair that sound so clean, dynamic, and respond so quickly to transients. It may not be the king of "organic" filling with a thick, magical midrange like the Tung-Sol "round plate" or its "little brother" RCA "Grey Glass," but even here the Russian tube demonstrates its ability to reproduce this midrange so neutrally and musically that for some it may be a studio reference in the good sense of the word.

The only tube that has ever outperformed Russian metal base tubes in my listening sessions (every version, even the most expensive Melz "1578") is the GEC/OSRAM B65 "metal base." It has the same Dynamics, speed, and "purity," but it adds a bit more magic in the highs and midrange. The price for a pair of NOS B65s is around 3000$-$5,000!

The tubes have both the parameters of new ones, but I can't say (at least not about the one from Mig) that they're NOS. It shows that they may have been used for a while. However, their parameters are very stable above "new." The readings don't fluctuate, there are no shorts, and they have shiny getters without holes, etc. A very consistent tube of the highest military quality. A collector's item.

Both are structurally identical and even from the same year. The second one is 100% NOS from stock and, of course, also has all the measurements as new.

NOS tubes for these tubes are becoming rare. They're worth buying as an investment :) Their prices are constantly rising, and good deals are becoming scarcer.

PS: I know everyone dreams of a Melz "1578," not a "regular" Melz metal-base with rectangular holes. We're used to always looking for the best. I have Melz 1578s, and I've compared them many times with other tubes, including these ones. It was always a must-try when friends came over and wanted to hear various rare 6SN7s. Of course, the Meltz "round holes" 1578 sound a bit different. In many systems needing warmth, with their cosmic dynamics and speed, this touch of warmth can be charming. That's why the 1578s are so expensive and no longer found in nature... However, in most cases, if someone is looking for the speed, dynamics, transparency, and openness of the Melz, this tube will sound really good in a well-chosen analog setup. Especially if the system is on the darker, warmer (perhaps a bit muddy?) side, for example, with an EL-34 power amp or guitar amp.

This tube will add some bite, open up the system, and get it going. It's definitely a must-have for those who listen to rock (or playing), powerful electronica, and symphonic music. For jazz or chamber music I recommend other 6SN7s - e.g. Tung-Sol round plate, Sylvania metal base, Mullard Ecc32, or finally the king combining all the best aspects of 6SN7 in one tube, i.e. "metal base" from Marconi-Osram Valve Company (M-OV).

I'm selling these tubes because I converted my amplifier to 12.6V filament and now always use 12SN7s. Otherwise, I wouldn't be selling them. :)

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