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6550 Tubes. Whats with the -A, -B or -C designations?

Discussion in 'Amps and Cabs [BG]' started by Rock&Roll, Jul 30, 2004.

  1. Rock&Roll


    Jul 21, 2002
    The title is pretty self explanitory. I couldn't find any good old threads by searching. So I needed to ask you guys what the deal is with these tubes. You'll see some called "6550B" or "6550C". Anyone know if there's any difference?
  2. In the very early days of tubes, the A would mean a revision, such as more power handling (example, the very first 7027 versus the 7027A). Now though, the suffixes have very little "real" significance, other than any particular feature the manufacturer wants to put on there. As far as specs, IMHO any 6550 out there is going to be pretty close.

    Groove Tubes has their own pecular marketing--er, I mean designation--scheme which has been debated in many previous threads.
  3. This is an often confusing issue derived from the fact that original EIA designations aren't used anymore and that's where the practice of suffixes came from. The original 6550 made by Tungsol came out in the late 1940's and had a plate disappation rating of 35 watts with a screen rating of 5 watts. GE introduced the 6550A in the late 1960's which had a plate disappation of 42 watts and a screen disappation of 8 watts. A version of this tube was also produced by Sylvania/Philips. Those are the LAST accurate EIA codes in regards to this tube type.

    The 6550B, is an early Svetlana variant made in the 80's which is not typically as robust as the GE 6550A with less voltage handling and plate disappation. They were imported by Magic Parts and Ruby in the early 90's when there was nothing other than Chinese 6550's being produced.

    The 6550C can be any number of tubes, and originally referred to a Chinese 6550 of VERY low quality that was known to fail spectacularly more often than not. There have never been any accurate ratings published for it, but most experienced techs treat it as a low-quality 6L6 if they have to use it. Groove Tubes sold these and introduced the "C" post designation to differentiate them from GE 6550A's they sold. Mesa also sold these for years, further confusing the issue by assigning them an "STR" designation number, even though no 6550 ever was produced to the STR specification, because US made 6550's already met it.

    When Svetlana began direct US export of audio tubes in the mid 1990's, they also used the 6550C designation as it was the next variant they produced after their 6550B, and aside from microphony, it is a MUCH superior tube to the junk Chinese 6550C. Sovtek also has sold two versions of the 6550 made in the Reflektor factory under the 6550C name. These are typically better tubes than the Svetlanas and comparable to the GE6550A. They now sell 6550's as 6550WD's, 6550WXT's, 6550WE's, and 6550EH, which is also sold as the KT88EH. Confused yet? It gets worse.

    Richardson Electronics bought the old GE/Ken Rad factory in Owensboro, KY in 1988 and made several lower quality versions of the 6550A, which were sold as Amperex KT88's or 6550A's, Richardson 6550a's or 6550C's, National 6550's, 6550A's, or KT88's, and SLM Hot Rod 6550's. They sold the tube jigs and machines as well as old stock plate material to the Chinese three years ago, and the Chinese now produce a true GE 6550A copy only slightly less in quality as the original sold as the Ruby and Shugang 6550A.
  4. Rock&Roll


    Jul 21, 2002
    Dang. Leave it to you two (Phyco Bass Guy and Nashvillebill) to be on the top of things. I swear that every time I read one of your posts here, I get a little bit smarter. Thanks for your expertice. I'm sure I'm not the only one who's grateful.

    As for the information given to me, I would gather two possible things. Either you are telling me that I should avoid Svetlana 6550C's and get Sovteks or something.........or I shouldn't worry about it and just get whatever I prefer becuase they're all gonna be of similar quality. (I'm more inclined to get Svets, Sovteks or J&J's tubes)
  5. Of all the current types, I like Sovtek 6550's and KT88's both the EH and Sovtek brands, the NEW Chinese 6550A's, KT88's, and KT100's, EI's KT90's, and JJ's KT88's, although lately, I've gotten some disturbing reports about JJ field failures with their power tubes. Svet 6550C's sound good, but they will go microphonic very easily unless you shock isolate your amp much more than normal.
  6. Psycho knows his tubes, I'm just an amateur...
  7. BassGreaser


    Aug 22, 2002
    Austin, TX
    Psycho I'm using Svetlana 6550s in my TE Hexa head...what do you think of these tubes as far as field failure?
  8. The microphony eventually makes them unusable. Power tube microphony is dangerous because the power tubes themselves are pulsing current that can burn out the windings in the output transformer. It's not a simple annoyance like preamp microphony.

    Tungsols are coke bottle shaped, and to my knowledge, China has never produced credible replicas, although they DID put out several blatant forgeries in an attempt to cash in on the Japanese hi-fi market. I wouldn't take the rating sticker seriously. As far as the tube being a copy of a Tungsol, I'd have to see it.
  9. All square plate holes and red plastic base are sure signs that its a Svet/Flying C 6550. It's probably from a few years back before China was producing decent 6550's and Ruby sold them as "their" tube for the SVT which is the most common amp using 6550's.