Posted by Wayne Parham [ 65.69.120.160 ] on March 04, 2005 at 00:11:34:

I just bought four new Sovtek 6BM8 tubes for the Stoetkit. I am also hoping to find four Светлана 6BM8's too. I'd like to compare each of these with the stock RFT's that come with the kit. Of course, it's too early to make any real comparisons. I need to cook the cathodes a little while first.

There are a few observations I can make right off the bat though. First, physical construction of the Sovtek is nearly identical to the RFT. They look almost exactly the same, inside and out. The plate at the top is shaped a little differently, but the rest of the tube parts are formed the same.

Second, the cathodes are different, and act differently to current passing through them when at various temperatures. The RFT's seem to have very different resistance between tubes cold. When you power up the amp with RFT's, the filaments glow brightly and then settle down, and some tubes glow much brighter than the others on start up. Once hot, they all take on the same glow.

The Sovteks are all the same at power up as when at operating temperature. They glow a little brighter at first, but it is a uniform swell between tubes. They settle down in about a second, again all at the same time and to the same level. They also seem to run cooler than the RFT's, at least externally. I'll have to watch the filament colors closely and see if I can detect a difference in hue. Maybe I'll put a pair of RFT's and a pair of Sovteks in just to compare colors side-by-side. I can listen to the difference by comparing right and left channels that way while I'm at it too.

Third, one of the Sovtek tubes did the Geiger counter crackle and pop just like Valve Art 2A3's. Maybe the Sovtek tube will change with time. I am hopeful that the snap-crackle-pop tube will stop making noise after the cathode burns-off a little bit. It stopped after an hour of continuous playing time, so maybe it will stay gone. We'll see.

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