JTM 45 to 50 watts, can it be done?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by jiml, Jul 10, 2008.

  1. jiml

    jiml Supporting Member

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    I want to stick with my Rockytop JTM45, just want to get some more out of it. Can it easily be converted to 50 watts? Can I retain the lower gain voiced JTM45 front end?
     
  2. jay42

    jay42 Member

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    I know squat about Rockytop amps...an accurate JTM45 is good for roughly 30 watts clean. The high internal voltage might be around 400V. The 50 watt amp would be more like 465V, so there's a power transformer (PT) issue. Additionally, the PT might not be capable of delivering the ~250mA required for the 50 watts.

    Next, the older amp should be set up with KT-66's at either 6.6K or 8K primary impedance on the output transformer (OT). The 50W might be about 3.4K to 3.8K primary impedance, so you'd want either need to change the OT or change the impedance selector for a better match. The OT may or may not have been designed to hold up at the higher power level.

    All this said, I think there's no reason not to try it with EL-34's. Bias will have to be adjusted, which could require a resistor change in the bias circuit. Hope all this helps, though I doubt you're liking my answers.
     
  3. jiml

    jiml Supporting Member

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    It's the answer I was looking for, though. I am going to convert it to EL34's, it has adjustable bias. Just looking for more umph out of it, while retaining the lower gain front end...
     
  4. jiml

    jiml Supporting Member

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    My cat says I need more headroom.
     
  5. gldtp99

    gldtp99 Member

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    I don't agree that the difference in vol between a 30 watt JTM45 w/KT66 and a 50 watter version w/EL34 would be imperceptible to the human ear at all.
    Since a healthy Marshall 50 watter usually puts out closer to 70 watts, the vol difference should be noticable.
    This is considering the proper PT with higher B+ and current rating (@250mA) is used along with a @3.4k primary OT.
    Just converting a JTM45 to EL34's without changing the transformers isn't going to make a lot of difference................gldtp99
     
  6. jay42

    jay42 Member

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    So my 2204 is sick? I measure 49W clean with =C='s. B+ is 465V. The OT was a Drake, swapped it with an OEI...same power.

    I do agree with the notion that you probably will notice. The drummer sets a volume threshold below which absolute power is meaningless...and a 50 watt will throw farther than a 30 watt. Still less than 3dB.
     

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