Anybody tried KT66's in a Plexi?

Discussion in 'Amps and Cabs' started by EXP, Jul 18, 2006.

  1. EXP

    EXP Member

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    i want to swap the EL-34's for KT66's in my metroamp '68 ptp Superlead. i was told by George at Metroamp that its no problem, just set the bias. anybody tried this?

    also i was going to try the Valve Arts KT66, is it true they only get about 50-60 hours life on them?
     
  2. bstranathan

    bstranathan Member

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    4 KT66's are in the new Super 100JH Marshall (Hendrix stack).
     
  3. EXP

    EXP Member

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    yeah, only $8k!:eek:
     
  4. bstranathan

    bstranathan Member

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    how is the '68 metro superlead? can you get real hendrix tones out of it?
     
  5. EXP

    EXP Member

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    it sounds great, it does Hendrix well. lots of power and girth. i just dont like EL-34's period. im trying to find out if i can bias it for KT66's without changing the EL-34 transformer.

    i should clarify i have a Metroamp '68 Superlead ptp board, not the '' 12xxx series '' amp.
     
  6. illinimax

    illinimax Gold Supporting Member

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    My Cornell DC45/50 (Plexi style amp) runs EL34s or KT66s, no problem. Just a twist of the bias knob gets you where you need to be. The Chinese KT66s sound best amongst all the new production tubes to me.
     
  7. slider313

    slider313 Silver Supporting Member

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    I haven't tried them in a Marshall,but installed two in my '64 Fender Vibrolux. The Chinese KT66's gave me a great light show as one tube was drawing too much current.Sent those back and got a set of Tung Sol KT66's and they biased up nice and sound great. NO SOUP FOR YOU,
    NO MORE CHINESE FOR ME.
     
  8. noctilux1

    noctilux1 Member

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    I was wondering about trying two, KT66's in my '67 Park Plexi 50. I seem to recall Greg Germino saying something about not putting KT66's (UNLESS YOU USE GEC KT66's) in his Classic 45 because there are no good, current production KT66's. Anyone else hear this???
     
  9. kindburro

    kindburro Member

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    Totally fine to run KT-66's (your choice of brand) in the majority of amps that use EL-34's. Bias point is quite different for these two tube types.
    If you are going from EL-34's to a 6L6 family tube or a Kinkless Tetrode-66 you will need more negative grid bias than what you have. Some bias supplies have enough swing at the potentiometer to achieve this. Others you may need to change the resistor feeding the bias supply network.

    Current production KT-66's are at times questionable. I personally had about 60% failure rate on the GT KT-66HP. This tube when pushed into hard clipping would drift excessivly. I would see anywhere from a 5ma spread to as much as 20ma in some examples. A well made tube should not drift and hold bias steady. There were other OEM's having the same problem with this tube. I feel the Chinese KT-66 is a much better choice. In my Classic 45, which runs a high primary impedance (6.6K) I felt that some examples of the Chinese KT-66 sounded a bit thin in the midrange. I opted for using the TAD 6L6WGC since it has a very full midrange and it works well into the high primary impedance. It also works well with downwards impedance matching in the Classic 45 (running amp at 16 ohms into an 8 ohm load, which cuts primary impedance in half to 3.3K) Anyone buying the Chinese KT-66 should look for a good source that is testing these tubes and weeding out the ones that do not meet sonic criteria along with testing and matching the tube properly. This goes for any tube. The Valve Art KT-66 is made at the Shuguang factory. Any Chinese made KT-66 comes from this factory regardless of what is stenciled on the tube. Get 'em from Lord Valve or someone who is really testing tubes and putting them under the gun before selling them.
    True, the old GEC KT-66's have a great sound to them. Their long life span in most cases, is cause enough to justify their expense.

    Tetrodes sound great in Marshall's. I think 6L6's or 5881's in a 50 watter sound fab! It's been done quite a bit and don't be surpised if you happen to dig 'em better than EL-34's. Remember it's all subjective! Experiment and have fun!

    Greg G.
     
  10. noctilux1

    noctilux1 Member

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    Thanks for the help Greg.
     
  11. chappy

    chappy Member

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    I have a quartet of Chinese Kt66's in my Wizard Classic and they sound great. They have thicker low mids than 6L6's, very Hendrix on cleaner settings.
     
  12. KLB

    KLB Member

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    When installing KT-66 in an amp with an EL-34 output transformer, try putting the impedance selector to one half the speaker load. You may like the tone better that way.
     
  13. Beam Tetrode

    Beam Tetrode Member

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    Can KT66s be used in a Marshall with stock 6550 without rewiring the sockets?
     
  14. 5150loveeddie

    5150loveeddie Member

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    Yep the Chinese's Golden Dragon are very nice BUT get the 11th generation ones, earlier ones will die of horrible death, not the 11th generation.....

    Your amp will work fine with kt66 but you won't get all the beatifull colors of those tubes unless you swap the ot with a real kt66 transformer!
     
  15. EXP

    EXP Member

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    is this JF at the Vintage Amps forum with the cool avatar? :D
     
  16. myles111

    myles111 Member

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    Bias at 30mA and you will get more than ten times that life out of KT-66s.
     
  17. coco

    coco Member

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    What's a real KT66 Transformer? Can you describe the difference between the EL34 transformer and the KT66?

    Thanks.
     
  18. HeeHaw

    HeeHaw Member

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    Bob Reinhardt set my Vintage 50 up so I could bias it up with KT66's. He sure didn't change any transformer. He changed a few resistors to extend the bias range of my amp to accomodate the kt66's.
     
  19. kindburro

    kindburro Member

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    Most early Marshall style amps and replica's or designs that fall into this catagory which use EL-34's can be re-biased for use with any 6L6GC or KT-66 tetrode. The bias supply network which supplies negative grid bias to pin 5 of the output tubes must be adjusted for correct current draw parameters for the tube used. So, as long as there is enough "swing" at the adjustment pot for going between 6L6/KT-66 and EL-34's it is simply a matter of setting correct idle current based on plate voltage.

    In early Marshall amps such as the JTM-45/100 (of which the JH Limited Edition is a replica), and all JTM-45's, the output transformer primary impedance (plate load) is very high. It is in fact about twice that of later transfomers using EL-34's. Example #1 A early JTM-45 with R/S OPT has taps for 9K, 8K and 6.6K. The one's I have seen personally used the 6.6K primary. This is exactly double what the primary impedance of a
    JTM-50/JMP-50 OPT, which is 3.3K.
    Later JTM-45's with the Drake 784-103 OPT used a 8K primary impedance.

    A 6L6 will run optimumly into a 5K plate load (Class A/B) based on info given in a RCA receiving tube manual. Actually Fender used 4K as the plate load in all two output tube Fender amps to my knowledge.
    What happens when the primary impedance is high is the tube simply operates less efficiently putting out less power. The amp will respond as slightly less dynamic and more compressed. This is a classic "upwards mismatch" as the tube operates into this higher primary impedance.
    So a true KT-66 output tranny when we speak in reference to old Marshall designs is a OPT with a high primary impedance. Old KT-66's work well into this high primary impedance but I don't think the modern KT-66 tubes work as well into a high primary. That is a personal opinion.

    When you take a JTM-45 with a high primary impedance and leave the amp set at 16 ohms and run an 8 ohm cab, this halves the primary impedance of the output tranny. You are going from the 6.6K plate load (in a R/S output) down to 3.3K. A lot of JTM-45 combo's were set this way. The amp is punchier, puts out a bit more power and feels tighter, more dynamic. Simply put, more efficent. On later JTM-45's which used a 8K primary impedance, cutting this in half you get 4K.

    Figures are different when going to four valves, basically 1.7K for EL-34's in tradition Marshall designs, 2K in tradition Fender four output tube designs and in the early JTM-45/100's the OPT primary is 4K. Easily twice that of what Fender used and later EL-34 Marshalls.

    You can take a EL-34 50 watt Marshall style amp, set the impedance selector to 8 ohms and run into a 16 ohm cabinet which will double the primary impedance from 3.3K to 6.6K. However: Disclaimer!!!! You will run the risk of damaging EL-34's as they are not meant to run into such a high primary. A stout EL-34 will take this, some will not. I can only suggest this if trying 6L6GC's with a 50 watt Marshall design amp.

    This info is to help understand the difference between what is an OPT with a primary impedance specifically for EL-34's and what is an OPT with a primary impedance specifically for KT-66's. Hopefully this will not confuse and only enlighten. For those who have a RCA or Sylvania receiving tube manual take a look at the plate load parameters that are listed for the more popular tubes such as 6L6GC, 6V6, EL-34, 6550, KT-66, etc.
    You should be able to get a better idea of the relationship between primary impedance (plate load) and a given output tube design.

    Greg Germino
     
  20. Norjef

    Norjef Member

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    Mebbe try KT77's--tried sets in 2 EL34 amps--Egnater 50 w. and Ampeg VL503.

    These newer JJs sound like a fuller EL34 AND a crankier 6L6 at the same time--got 'em from Lord Valve
     

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