KT66 vs. 6L6

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers T' started by RMosack, Aug 21, 2009.

  1. RMosack

    RMosack Member

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    I'm pretty familiar with the "big four" American and British style power tubes: 6L6, 6V6, EL34 and EL84.

    On and off over the years, I've seen mention of the KT66 tube. Is this just a Euro equivalent to the 6L6, or is it a different tube entirely?

    As I understand it, there are Euro and American designations for what amounts to the same tube (e.g. 12AX7 and ECC83). Is this the case with the KT66 and 6L6, or are they actually different?

    If they are different, can anybody explain how they're different, particularly in terms of guitar amp tone?


    Also, how the heck does teh 6550 fit into this mess?
     
  2. davemccarthy707

    davemccarthy707 Member

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  3. frankencat

    frankencat Silver Supporting Member Supporting Member

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    KT66's have more bottom end and balls than 6L6's. They also have a little of the EL34 midrange/ To me they are kind of in between a 6L6 and an EL34.
     
  4. eugenedunn

    eugenedunn Member

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    Hey RMosack!!! Fancy meeting you here. The KT (Kinkless Tetrode) series of tubes are similar to 6L6's..... in that they're octal pinout tubes....but they have way beefier internal construction and higher heater requirements. This higher current requirements makes them incompatible with some amps because the power transformers would be overtaxed. The KT66 type of octal power tubes were developed in Britain in response to the 6L6 type tube.

    6550 tubes were a beefy US power tube developed "in response" to the huge British KT88 tube

    As you saw in my other posts, I've recently replaced my RCA blackplate 6L6GC's with a pair of JJ KT66's. I think I really notice a significant change in the bass response of my Allen Old Flame (just like yours). I feel a lot of the glassy clean high end that you enjoy in 6L6GC's is still there, but I have to say that the fuller and tighter bass is the real advantage in these tubes. I think it's especially cool since I like my amp running VERY clean.

    Another upgraded 6L6 type is the 7581..... the US upgraded version of the standard 6L6GC. Hard to find nowadays.... I don't believe they're in current production. In general, I feel it's pretty hard to beat RCA 6L6GC's......so I think the KT66 is a great option to try, IF your power transformer can take it.

    You're gonna need over-the-top spring tube retainers to hold those big bottles in the sockets if they're hanging upside down (like in our Allen Old Flames). Bias around 40 mV.
     
  5. RussB

    RussB low rent hobbyist Silver Supporting Member

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    In a Fender BF circuit, I find the Shuguang/Valve Arts KT66 to have a fatter midrange than some 6L6GC's (TAD short bottles, JJ's, SED's and Jan7581A's, Ruby MSTR's)

    They most notably start to break-up quicker and somewhat smoother, and will put out less total power than any 6L6GC.
     
  6. RMosack

    RMosack Member

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    Thanks for the replies!

    The main reason I'm asking about these is with regard to British style amps, not American. I've built an Allen Old Flame. This circuit with 6L6 tubes has me covered with the whole Blackface, American tone. I'm thinking about building a Marshall style kit next, and I was deciding between a few options, two of which are the JTM45 style amp and the 50 watt Plexi style.

    As I understand it, the early Marshalls like the JTM45 weren't much more than British takes on the Fender Tweed Bassman. It's obviously a cool amp, but I never think of it and a Marshall in the same terms. As I understand it, the JTM45 uses KT66 tubes while the Tweed Bassman uses 6L6 tubes. Pryor to this thread, I had always assumed that the KT66 was some sort of "British-sounding" tube, sort of like how the EL84 and EL84 are obviously "British" as compared to the American 6V6 and 6L6. If the KT66 is really not much more than a slight variation on the 6L6, then why does the JTM45 not sound more like a Bassman?

    Or, am I off base and the JTM45 sounds more like a Bassman than I ever thought it did? Or, maybe the preamp is different enough to make it not sound/feel like a Tweed Fender.

    Am I barking up the wrong tree? In other words, to get the Marshall/British contrast to my Fender/Allen/American Old Flame, do I need to look more at EL34 style amps - or even little EL84 amps?
     
  7. RussB

    RussB low rent hobbyist Silver Supporting Member

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    Transformers

    tube compliment

    speakers

    cabinets


    you can't just dumb it down to 6L6 vs. KT66
     
  8. wyatt

    wyatt Member

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    Early JTM45's did sound a whole lot like a Tweed Bassman, grab a real offset '62 JTM-45 or (more practically) one of the expensive handwired RI's and you'll hear a more Bassman-like tone. The JTM-45's we all think of is after Jim Marshall had years to tweak the voicing of the amp (most clones and RI's are based around a '65 or '66 circuit). Other major differences are all the components, the KT66's are the only UK-specific part (the Tweed Bassman spec'ed 5881's, a variation on the 6L6GB), there are also the Radio Spares transformers, and...well...just about every part under the hood was a British part. The differences are throughout, not just preamp, but remember the JTM45 is really only a 30 watt amp compared to Leo's 45-watt Bassman.

    eugenedunn mentioned something that I was surprised hasn't been expanded on. The KT stands for Kinkless Tetrode. The "kink" in question was the inherent scooped midrange of the American power-beam pentodes; the British called this the "midrange kink" and developed tubes that corrected this..."kinkless" KT66, KT88, etc., they were developed originally for hi-fi use. Also, note the British tubes are tetrodes, the American tubes are power-beam pentodes; IIRC, power tetrodes were under patent, so the American firms had to circumvent the patent with a pentode design.

    Personally, if you're looking for a big Marshall flavor to counter a Fender flavor, I wouldn't start with anything less aggressive than a Plexi.
     
  9. eugenedunn

    eugenedunn Member

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    Nice discussions going on.......I love all those flavors of the sonic rainbow.....
     
  10. RMosack

    RMosack Member

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    Interesting. I guess the "British Bassman" label only applies to the earliest JTM45. I would gather that the later models (i.e. the ones Marshall reissued and the ones that companies like Metro have cloned) diverged significantly from that original Bassman idea. Are saying that they didn't diverge nearly as much as a later Plexi?

    Basically, will a JTM 45 kit get you halfway between Bassman and Plexi, or is it still not British enough?
     
  11. eugenedunn

    eugenedunn Member

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    Hey RM,

    Why doncha build something TOTALLY different like a Ceriatone Trainwreck Clone? (^_^)

    I mean, if you're skatin' on thin ice, ya might as well be dancin'!
     
  12. Tomo

    Tomo Member

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    I love 7581A.

    Tomo
     
  13. jcs

    jcs Supporting Member

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    7581a Sylvania are great tubes!

    The closest to those in clean headroom are the twin getter big bottle 6l6gc Sylvania.

    They look identical, but the 7581a spec out as higher power output.

    They last too, i have some with very thin getter flashings and they still sound about the same as my newer or brand new NOS Sylvanias which is nice as a lot of modern power tubes go flat in tone quicker imo.
     
  14. wyatt

    wyatt Member

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    The JTM45 offers lower headroom which means overdrive at more manageable volume, but they are as tight and the Plexi has that immediate attack that just says "rawk!" thanks to it's SS rectifier and is more aggressive than the JTM45, you want to work your way into '70's hard rock and early metal, than the '70's JMP or even a JCM800 is great for even more crunch, but their cleans are as sweet.

    It's just been my experience that when someone is looking for something Marshall to pair with something Fender, they are looking for that classic Marshall overdrive. Which may be presumptuous of them since so many artists seem to pair clean Marshalls with dirty Fenders when recording multi-amp setups.

    If I want one amp to do it all -- cleans, bluesy OD, and rock -- then the JTM45 would be a big pick. If I'm looking to get that definitive Marshall sound, I lean toward a Plexi or JMP and an attenuator.
     
  15. Ben S.

    Ben S. Silver Supporting Member

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    Often repeated on the internets, but not the full story. It actually has more to to do with the anode current to voltage graph for tetrodes.
    http://www.vtodorow.com/index.php?o...7&587da7ee0ea3a0e0b9303be2c7da9f6b=xkhvblrvju
    Great read, and there is a good history of the progression from the Tetrode to the Pentode to the Beam Tetrode.
    Here is a quicker read: http://www.r-type.org/static/grid14.htm

    I have had a Bluesbreaker for almost 20 years and I love it. There is a definite difference in midrange qualities between the KT66 and 6L6, which I have finally been able to discover my self. But also the distortion has different harmonic content and sounds, well... "different".
    When I don't need the extra power, I tend to stick with 50's Tung Sol 5881's for a more balanced sound compared to some of the RCA 6L6's I have used in the amp. For the jams and small gathering gigs, I haven't really needed the extra watts.

    In comparing the Bassman to the JTM45 circuit, or more accurately for me, comparing the sound, I always think about what Doyle wrote in his Marshall book when discussing the differences with Steve Grinrod and Ken Bran of Marshall.
    They said because of the higher impedance in the JTM and there is a higher voltage being fed into the negative feedback circuit of the power section, thus changing the distortion characteristics.

    I am still learning about all of this as I am starting to build HiFi amps from kits and schematics, so hopefully in time I will have a better electrical understanding of all these facts.

    I agree about the 7581 tube also. Great sounding tube. Just last weekend I listened to a great old McIntosh 240 with some 7581's. Very nice indeed. Although I don't know what a distorted 7581 sounds like.
    Nice thread, enjoy!
     
  16. sabbenante5150

    sabbenante5150 Member

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    I swapped out all of my tubes in my triple channel Dual Rectifier. I went through eurotubes.com. Their staff is amazing. I would suggest going to their site and reading their tube descriptions and also their amp info.

    I wont go into the front-end section, but for my power section I am running KT88's in the outside holes and KT66's in the inside holes. Without even thinking twice, I swapped out the stock 5U4's rectifier tubes with the GZ34's. I can't tell you the improvement and full-bodied nature of my tone compared to the 6L6's. Talk about cutting through the mix without having to bring your volume up.

    The KT88's are very thunderous with a very non-muddy bottom end. The KT66's are a perfect combo to the 88's (for my application --> rich mid and smooth highs). I wish I tried the 66's by themselves to tell you more info --> eurotubes will give you all the info you need

    I play modern rock and was running my "cold" DR in hi=power mood and could never really, really enjoy my tone. KT88 + KT66 = the ballz!

    The guys at eurotubes know what they are doing. I would suggest going to their site for some extremely accurate information on tubes.

    You may want to also consider KT77 + 6V6 integrated quad in the power section.
     
  17. eugenedunn

    eugenedunn Member

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    WOW..... now that's what I call blenderific........ thanks for that account!

    Geno
     
  18. RMosack

    RMosack Member

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    Wow, found this thread in a search for KT type tubes. Forgot I start it 4 years ago!

    As it turned out, I did build a Marshall flavored amp to go along with my BF flavored Allen Old Flame. I settled on a Trinity 18 watt. I did the sIII v6 variety.
    - Channel 1 (volume,tone) is based on the normal Model 1974 circuit
    - Channel 2 (gain, volume, treble, mid, bass) is more or less in the realm of a JTM type preamp
    - Channel 2 has a boost switch that sort of heats things uip
    - The entire amp has a switch to go from a pair of EL84 to a pair of 6V6

    Absolutely love this thing. It's not really a Plexi, and certainly not a JCM, but it does do that Marshall thing when cranked - especially with a boost or OD out front.
     
  19. small axe

    small axe Supporting Member

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    ill take my 6L6 american made and my el34 british and my preamp to be dutch
     
  20. macula56

    macula56 Member

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    i just put a set of KT66s in my homebrew Vibrolux and it sounds terrific. a noticeable difference from the 6L6s that i had in it. i have also run the 7581s and they sound good to. the KT66s are killer in this amp. more bottom end and a fuller sound in general.
     

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