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Old 3rd April 2009, 08:01 PM   #21
Jeb-D. is offline Jeb-D.  United States
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although KT88's and 6550's have noticibly more bass than EL34's or 6L6GC's.
Have you tried the EH 6CA7? I've heard that they have more bass than EL34's, but haven't tried them in anything yet.

I've seen 2 types of EH 6550's floating around, some are strait bottles and some aren't. Anyone know the scoop on that?
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Old 3rd April 2009, 08:45 PM   #22
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Some 10 yrs ago I picked up several hundred 6550WA/B clear glass, zirconium pellet getter and find'em reliable despite very slow warm up...I complained to my vendor about this and got another 50 free so I became suspicious that they were manuf with incorrect heaters. Borrowing an infrared sensor and comparing with other 6.3V heaters of other 6550 series, I find the red spectrum exact at 6.5V not at 6.3V. So I run'em at 6.5V and all is happy. Without visible ident markings on tubes I cannot say this holds for all batches. In past, I believe both Sovtek and Svetlana manufactured the WA/B series but I could be wrong..The Svetlana B version was superceeded to the so claimed superior C version with gold plated grids. I have my doubts that gold plating does anything we ought to take notice of..
The 6550 WA/B straight sided transparent tubes in an amp chassis look rather boring with virtually no blueness in the dark. Historically, I haven't had one that failed outright on the bench nor sounded foul. Sonically sim as the Svet C version at a 1/3 of the price the WA/B is still available in surplus numbers at reasonable prices.
The distortion spectrum is low, sim to a good KT88 plugged in with adjustments, i.e end result no real differences..-However the proof test lies with FFT harmonic analysis which I intend to set up for.
The most beautifully engineered and ultimate performing KT88 was a Gold Lion replica made by the old salts of Tesla with a white base and highly sought after. .I never found a KT88 (even NOS) that bettered the results of this tube. Regretfully no longer produced. pic enclosed.

richy
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Old 3rd April 2009, 10:59 PM   #23
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I have my doubts that gold plating does anything we ought to take notice of..
I'm told it makes the tube much more fragile in AB2. This is consistent with my experiences, however I don't know how much power the grid was dissipating when I destroyed my tubes. Still, I'd pay well for quality KT88s without gold plated grids. Did KT88s and 6550s of the past have gold plated grids?
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Old 4th April 2009, 01:05 AM   #24
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The advantages of gold plated grids are discussed in this 1950s article describing the 5881 as compared to the 6L6:
http://www.qsl.net/dl7avf/roehren/e5881.html
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Old 4th April 2009, 02:28 AM   #25
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I found this statement a while back while searching for ham radio related material. These statements were found on this web site:

http://www.w8ji.com/

"In actuality, tungsten grids in tubes like 3-500Z's can be operated at temperatures where they show color without permanent damage. While tube operation at such temperatures is compromised, the grid and tube often suffers no permanent damage.

The same is not true for gold-plated grids, like those in metal-oxide cathode tubes. Grids plated with soft poorly-bound materials like gold suffer gradual long-term deterioration even with moderately low levels of grid current. Heat the grid of a metal-oxide cathode tube to temperatures even well below incandescence, and the result is instant irreversible failure."
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Old 4th April 2009, 04:03 AM   #26
PB2 is offline PB2  United States
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Ah, interesting, thanks for that link. It says with moderate grid current which does not apply to the more common amp designs. I noticed that you're into more exotic amps, but the average amp should not have G1 grid current unless there is a problem as I see it.

Not sure if they're plating the screen grid also, that could obviously be a problem.

Pete B.
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Old 4th April 2009, 06:32 AM   #27
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I took a look, and what I have are Sovtek 6550WE, straight side, 2 pocket pill-type getter on top, 3 round holes in plate, date code 99 08. Ok tubes or time bombs with octal pins?
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Old 4th April 2009, 02:30 PM   #28
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Differences between Svetlana 6550B and Sovtek 6550WB ? Electrically the same and for the physicals ? Same holes in anodes; the other external is the Svetlana vs. has bar over top of anode.

richy
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Old 4th April 2009, 02:43 PM   #29
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I've seen those before and always wondered why there is no visible getter flashing inside the glass.
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Old 4th April 2009, 08:40 PM   #30
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These are the beasts I have. The getter pills are much smaller, on top, and with the usual getter flash.
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