Jtm45 Questions

Discussion in 'Building the Classics' started by black knight, Oct 24, 2018.

  1. black knight

    black knight Member

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    Yes the bias was cold. 468 plate volts, 25.7 mA. Shooting for 37 mA, The bias pot maxed out at 30.7. Also, I could hear a sort of helicopter sound through the speakers when the volume is turned up. I've never noticed this when biasing. And I did not notice it when playing the amp. Is this DC ripple?
     
  2. neikeel

    neikeel Well-Known Member

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    First point is that the RI has two 50+50 cans but it uses one half for mains, the other half for the screens. The other can is 50uF for the PI and 50uF for pre-amp. The original uses 32uF mains from a single topside cap, the screens fed from half of the 32/16 internal chassis mounted cap and the PI the other half. Then the preamp has a 16uF on the board.
    Personally I would go for stock RI settings with the set up you have you can achieve this with linked 50+50 cans x2 by wiring them is series (so 100uF 500v cans become 50uF).
    Then check that the wiring on the topside blue can is wired for 32 screens PI and 32 screens.
    [​IMG]
    The easy way to do this is to disconnect the long bus wire from between the cans close to the left hand can and swing it to the +ve lug above it. Leave the ground wire going off to the left for now.
    Cut the ground wire coming from up front to the right hand can, again close to the lug and swing it to the black ground wire you can see.
    The helicopter noise is unlikely to be ripple, more likely to be reversed primaries.
    The way to check that is first do the minor wiring job above.
    Then to try the amp - see if it still has the noise with correct bias.
    If it does simply desolder and lift one end of the NFB resistor (the 27k resistor in the middle of the picture below with purple wires each end, does not matter which end of the resistor or which wire from it)
    [​IMG]
    If that eliminates the helicopter noise you need to switch the brown and blue wires going to pin 3 of the output tubes and resolder the NFB wire in place and you are good to go.
    Only bit of the amp I am not familiar with is the white cap on the end of the board but i presume that is some sort of snubber cap?
     
  3. ampmadscientist

    ampmadscientist Well-Known Member

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    You need to change 1 resistor to dial the bias in correctly for those tubes but,
    fix the oscillation first.

    Make sure the speaker is the correct impedance before you adjust any bias current.
    Use an ohm meter to test the speakers first.
    Make sure the amp is set to match the speaker first.

    1. try reversing the 2 wires that go to pin 3 of each output tube. (swap these wires around)
    Then re-test and see if the oscillation stops.

    jtm45.png
     
  4. ampmadscientist

    ampmadscientist Well-Known Member

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  5. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    Did everyone miss post #20?
     
  6. vivanchenko

    vivanchenko Active Member

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    Mixed up OT primaries cause much greater oscillations than what Black Knight describes. I know I've been there :) I thought my amp was going to explode :run: I don't think that OT primaries or a lack of shielding is the problem. Also, what's the maximum resistance of your bias pot? It should be at least 50 K and it shouldn't max out.

    I am not getting it, why bother modifying the original classic schematic without any tangible benefits? My JTM 45 wiring is 100% original design and it works like a charm. The tone is to die for and there are no any noise or buzz issues. I also didn't use any sort of wiring shielding. If I was the OP I would have modified the amp to the original JTM 45 schematic. This way I at least would be confident that design is not the issue. The way it is it very well might be.
     
  7. vivanchenko

    vivanchenko Active Member

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    Yes, what you said there very well might be the case. I just can't make out what's going on with that wiring there. As I mentioned previously I think that going back to the original JTM 45 filtering design is the best thing one can do about this amp. It would be very easy to do and wouldn't cost a dime.
     
  8. ampmadscientist

    ampmadscientist Well-Known Member

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    Oscillation will vary according to preamp tubes, caps used in power supply etc...
    Bigger filter cap can have less prone to oscillation.

    Reversing the 2 OT primary wires is an easy test, and often the cause of oscillation in a newly built amp. Try reversing those wires and test again.
    You have a 50% chance (at least) of resolving the problem.

    The worst that's going to happen is "even more worse" oscillation, and then reversing the wires again (a second time).
    The best that's going to happen is: the amp will work right, and you will be relieved.

    The reason it oscillates is : positive feedback in the feedback loop of PI instead of - correct negative feedback. Reversing the wires of the OT primary will fix this.
     
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  9. black knight

    black knight Member

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    Again, thanks to all you guys for taking the time. I VERY much appreciate it.

    Ok, so after adjusting the bias, I had to see if that got rid of the static in the decaying notes. It did indeed. Also, no pulsating sound. But here's where I admit I'm likely a moron. I had the chassis on the bench in my garage in close proximity (within 2 feet) of a wi-fi extender. I believe this was the cause of the pulsating sound. It is nonexistent with the chassis in the head box currently 10 feet away. I'm thinking it is a non-issue. Next time I have it out I will unplug the extender to see if if that was it.

    Speaking of the head box, another weird thing. the metal shield liner on the bottom has been cut out. There is about 1/2" remaining on both ends. I'm thinking why not leave it there?

    So next I plan on taking all your advice starting with Neikeel's suggestion for filtering. First, I need to prepare though. I need adequate lighting in my garage and since my eyes aren't what they used to be, I will get a bench magnifier. I also need to put together a bleed resistor for draining caps.

    I am not opposed to modifying this to the original schematic, like vivanchenko says, but I will need instruction. I assume the basic tone of the amp will remain close? I really like what I'm hearing now. The amp is very touch sensitive. Even with the crappy pots and electronics in the USA production les paul traditional, it is easy to back off for cleaner passages and dig in for some snottiness. The strat and R8 accentuate this characteristic further. It's a lot of fun to play and time gets away.

    Thanks ampmadscientist for indicating the bias resistor that needs changing. If after modification and rebiasing it requires it, this is exactly the information I need. I assume it will need changed.

    Now off to order a bench light magnifier. Sucks gettin old.
     
  10. neikeel

    neikeel Well-Known Member

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    No. I just told him how to implement what we both think was intended (ie series mains cans).
     
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  11. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    Get your lighting set up and a nice 3x or 4x at least for view scrutinizing.

    I do the same where needed. :thumb:

    Actually I also use a nice magnifier with light combo. That always comes in handy.


    [​IMG]
     
  12. ampmadscientist

    ampmadscientist Well-Known Member

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    No, the helicopter sound is oscillation, which is also called "motor-boating." It happens when the (wires of) primary of the output transformer are reversed.
    This is a really common bug in newly built amps. (easy to fix it)
    Ripple : is hum from the DC rectifier. You will hear a constant low frequency hum.
     
  13. ampmadscientist

    ampmadscientist Well-Known Member

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    I'll show you guys a way to stop the caps from surging.
    Then you can use bigger value caps and get less hum...

    Hook up the standby switch a different way, this is the Hiwatt method.

    And yes- stacking 2 caps in series is a good way to do it. This increases the voltage rating and makes the caps last a lot longer.

    JTM45POWER.png
     
  14. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    Whacking the negative bias supply to an extreme while having test equipment connected in some instances/situations can introduce enough to cause oscillation. So be warned, be aware and be careful.

    Feedback circuit carries on and exacerbates the issue.

    I have done it before, a long time ago and figured out what the F was causing oscillation. Yes it is weird but can happen. The real bitch is that my oscillations were not or barely audible. They were very low frequency and I saw it on the meter measuring power tube current.
     
    Last edited: Oct 31, 2018
  15. vivanchenko

    vivanchenko Active Member

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    As I mentioned previously hum is not an issue with JTM 45 amps. If done right they are very quiet even with 32 uF filter caps. Increasing capacity affects tone making it noticeably thinner and cleaner, in other words it won't be a JTM 45 any longer. Low filter capacitance adds some sort of desirable fuzziness to the sound. I experimented with filter caps a lot. If I wanted glass-clean tone I wouldn't use a JTM 45 for that in any case.
     

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