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Tube swap- JTM45

Discussion in 'The Workbench' started by Terry Cooper, Jul 17, 2021.

  1. Terry Cooper

    Terry Cooper Member Silver Supporting Member

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    Ok......I'm not even in the same library with y'all, little on the same page LOL. THAT BEING SAID DOES THE KT 66 GIVE IT MORE HEADROOM SAY I WANTED TO GET A LITTLE MORE GAIN OUT OF IT NOT A TREMENDOUS AMOUNT I'M NOT BUT IF I WENT BACK TO ELs WOULD THAT GIVE ME A LITTLE MORE GAIN? And I apologize about the Caps there I didn't know that it was on till after I had finished talking into this but I've always liked the sound of an el-34 amp and to me my jtm doesn't have as much gain as a couple of the others that I've seen that were factory also, and sometimes that clean tone would be nice if it was something else but come on it's a marshall it's a Plex it's a jtm 45 I'm not looking for clean tones lol
     
  2. Terry Cooper

    Terry Cooper Member Silver Supporting Member

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    Even through my power brake dimed out it doesn't seem to have as much gain as a couple others that I played and I am no expert that is why I am on here talking listing with y'all, but that does coincide with my original question going back to El 34s is that something that I could do or no like I said I'm no amp tech but I've done electrical work my whole life did some low voltage control wiring ladder logic that kind of stuff also so I'm familiar
     
  3. mickeydg5

    mickeydg5 Well-Known Member

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    Why would you want to put 6CA7/KT77/EL34 in a JTM45 model?
    It has a lower B+ and a 6k-8k output transformer.
    The KT66 are excellent tubes being kin to KT77 style.
    The EL34 are going to be cleaner when used with the same 6k-8k output transformer.
    You will not get more grind/overdrive out of EL34 but may get more mid focus.

    It seems what you want is a 1987 model and not a JTM45.

    You can try the EL34 if you can get a decent bias level with the JTM45 design but I think the bias will be cold due to the fixed circuit. The bias circuit may have to be modified in order to get it hot enough. Pretty sure the JTM45 has no adjustment with a trimmer/potentiometer.

    Still with a boost or overdrive with some equalizing up front the same can be acquired.
     
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  4. Matthews Guitars

    Matthews Guitars Well-Known Member

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    The KT66 sounds better in my opinion. But the fixed resistor in the bias circuit will have to be changed to get the bias into the proper range for EL34s.

    I predict you'll go back to the KT66s.

    What to change: If I remember correctly, and I won't be sure without opening up the amp, for a 1959 at least the fixed bias set resistor is 47K for a EL34 amp and I think I went to a 27K that gave me the right bias range. In fact, I think that gave me enough range to bias up either KT66s or EL34s. But that's for a 1959.

    In any event, the bias range resistors should be decreased in an EL34 amp if installing KT66s, and increased if installing EL34s in a KT66 amp.
     
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  5. mickeydg5

    mickeydg5 Well-Known Member

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    My 1960 Bassman sounds great with 5881, vintage tubes you know. :)
    One day I may try and compare some nice KT66 even though they require more filament current, just to listen.
    Plenty grind and overdrive for that type amplifier but of course it has the original 4k output transformer and operates at over 475V B+.
     
  6. Terry Cooper

    Terry Cooper Member Silver Supporting Member

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    Ok, well that is why I'm here to find out those things. Your absolutely right though if the Kt66s are that friendly with a jtm then I may just keep it the way it is.Thanks mikeydg5, actually all of you. A 1987 was what I was leaning to, actually I was looking for a Germino lead 35 when I bought my Marshall and couldn't find what I wanted. I'm thinking that should have been my choice. Unless anyone has one of the forementioned amps they wanna trade just saying
     
    Last edited: Jul 24, 2021
  7. Stephen H

    Stephen H Member

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    I don't know how they sound, but you could try the Tungsol KT66 with no worries, it only requires 0.9A, same as a 6L6 type tube.
     
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  8. Terry Cooper

    Terry Cooper Member Silver Supporting Member

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    So technically I could run 6L6s in it without having to do anything also huh. These are the things I didn't know. It's got Gold lion KT66 in it now
     
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  9. mickeydg5

    mickeydg5 Well-Known Member

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    That I may.
     
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  10. mickeydg5

    mickeydg5 Well-Known Member

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    All of these are equivalents worth trying: 6L6WGB, 6L6GC, 5881, 7581 and KT66
     
  11. Pete Farrington

    Pete Farrington Well-Known Member

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    The 80mA cathode current limit on Tung Sol’s ‘KT66’ would seem to make it unsuitable for most applications https://tungsol.com/specs/kt66-tung-sol.pdf
     
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  12. neikeel

    neikeel Well-Known Member

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    That is a really interesting thing to pick up on!
    The GECs I like have Ik limit200mA (Class AB, fixed bias) and the ones I have taken a liking to recently are the Gold Lions 175mA - so quite a difference compared with Tung Sols. I know that quite a few people use the Tungsols but when you consider at idle in a JTM45 you will have 40mA on the plates plus 6 or 7mA on the screens it does not give you much room to manoeuvre.
     
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  13. Stephen H

    Stephen H Member

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    I find it "interesting" how once I've seen the info I'm looking for my mind seems to automatically block out some other sometimes critical contrary detail from view...
    Luckily you guys are there to watch out for us less technically gifted enthusiasts.

    And for that, a heartfelt thanks, sometimes saves a lot of grief.
     
  14. pedecamp

    pedecamp Well-Known Member

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    Tube swap wont make a big difference in your gain that much unless they're just really dead tubes.

    Look up something called a "one wire mod", it might give you that bit more gain youre looking for, I hear its real easy to do and is reversible if you don't like it.

    Theres also a little gadget called the evo mod v2 some of the guys here are messing with and getting good gain results, look up "evo mod v2".
     
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  15. _Steve

    _Steve Well-Known Member

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    Are you suggesting it's something else (eg 6L6) in a KT66 envelope? I've heard that sort of thing happens.
     
  16. Pete Farrington

    Pete Farrington Well-Known Member

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    It doesn’t seem conform to KT66 ratings or characteristics, so I dunno why the shysters feel empowered to market them as such. I suppose because they know they’ll get away with it.

    Whatever, it can’t be anywhere near a 6L6, because though there’s no actual limiting value that I can find, even the earliest type can accommodate well over 200mA cathode current, see p3 of https://tubedata.altanatubes.com.br/sheets/127/6/6L6G.pdf
    Even a little EL84 can manage a 65mA cathode current limit, see p6 of https://tubedata.altanatubes.com.br/sheets/010/e/EL84.pdf
     
  17. myersbw

    myersbw Well-Known Member

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    I have a Dr. Z Antidote prototype that has a switch to change the bias circuit between EL34's and KT 66's. The Antidote is "loosely based on a JTM45"...the differences are...a footswitcheable tone stack lift...the two channels are jumpered together at V1 grids...and a "vintage/modern" switch that varies the pass-thru frequency response slightly for an upper frequency boost for "modern"...plus a 100pF bright cap and separate V1 cathode circuits. As I can easily insert and A/B with the flick of a switch, the KT66's give a larger bell-like expansive tone on the cleans vs. the EL34. A friend preferred the EL34's when pushing them hard. I MUCH preferred the versatility of the KT66's when using mostly clean and pushing pedals through it.

    I'll add to that comment by saying I've since modded the "JTM45" mode into a '78 2203 circuit as it took very little on the mod-side and completely reversible. I'm ok with either tube selection, but...for clean tones? The KT66 wins my vote any day. (lol, but, change the speaker and it'll be a whole 'nother ballgame!)
     
  18. mickeydg5

    mickeydg5 Well-Known Member

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    Yes, I am not liking that 80mA cathode current limit.
     
  19. Matthews Guitars

    Matthews Guitars Well-Known Member

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    A little creative engineering can be applied. Install a second 6.3vac (OK, 12.6v centertapped) transformer of adequate current capacity and use that to run the filaments. You could separate the filament string where power and preamp tube circuits meet, and run the power tube filaments with the new transformer and the preamps with the original.
     
  20. mickeydg5

    mickeydg5 Well-Known Member

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    It seems the JTM45 2245 30W came with 5881 tubes which run .9A on the filaments.
     

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