PT With No Dedicated Bias Supply

Discussion in 'The Workbench' started by mAx___, Sep 14, 2019.

  1. mAx___

    mAx___ Active Member

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    I got a Merren 560V PT that came with no dedicated bias tap. He suggested feeding from the lowest voltage secondary, which is 360VAC. I tried drop resistors up to 1M and I still get very high negative voltage on one end of the trimpot (around -260v). The bias circuit is the 1959 type. Like I said, I changed the original 27K drop resistor before the diode up to 1M to no avail.
    What can I try next? Thanks guys.
     
  2. ampmadscientist

    ampmadscientist Well-Known Member

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    Start the reactor... Free Mars!
    The rest of the bias circuit needs to be connected.
    If voltage goes through a resistor there will be no voltage drop until the output of the resistor is loaded to ground.
    The CT of the winding needs to be grounded, the rest of the circuit needs to all be in place.

    2. Put a capacitor in series with the resistor to drop the AC voltage.

    It sounds to me like some of the circuit is missing / not built or connected yet.

    upload_2019-9-14_10-25-37.jpeg See how the .1uF cap is used to couple the bias to the HT winding?
     
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  3. mAx___

    mAx___ Active Member

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    Thanks AMS! Can I use a regular .1uF 500V Sozo coupling cap for that?
    Also, the trimpots I have are all 25K, do I need to adjust for that?
    Edit: Center tap of the secondaries is grounded.
     
    Last edited: Sep 14, 2019
  4. ampmadscientist

    ampmadscientist Well-Known Member

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    Start the reactor... Free Mars!
    Different value caps will give different voltage drops.
    I still don't understand why the voltage is high, it's like the ground of the bias circuit is disconnected.
     
  5. mAx___

    mAx___ Active Member

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    So far in other amps I built (which had a different PT) I had around 105VAC before the diode. With this one I have 360VAC before the diode. The circuit is grounded as per the 1959 schematic. I'll try your 0.1uF cap in series with a resistor to ground tomorrow (Sunday) if you still think is the way to go. I tried everything else to no avail. Thanks again.
     
  6. ampmadscientist

    ampmadscientist Well-Known Member

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    Start the reactor... Free Mars!
    Wait
    Do you know how to use the continuity tester?
    Can you check that all the ground points in the bias circuit are grounded using the meter?
    Take the tubes out.
    What are you reading with the ohm meter if you measure across the bias adjust pot? Maybe the pot is open?
     
  7. South Park

    South Park Well-Known Member

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    Check for negative dc set the bias way down and check with tube in place the amp draw will reduce the voltage . then set bias you are on the right track .check voltage on the whole circuit to see what it is doing to properly bias the amp you will need a amp meter but this will get you going
     
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  8. john hammond

    john hammond Well-Known Member

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    that power transformer, if its a copy of a 1202-43, has a resistor ( 150k), attached to one of the h.t taps ( not the lowest, rather the highest on that copy by merren which has taps for lower b+), then that resistor is attached to the bias diode ( make sure at the end with the silver band on it ), then to a couple of 10uf axial caps in parallel ( with a 15k resistor joining the two negative leads of those filtering caps )....the positive side of those two filtering caps ( 10uf..100v?) are joined together and go to ground. the 15k resistor that is at the negative end of those ripple filtering bias caps..the right hand leg of that resistor goes to your 220k PI feed resistors that feed neg. bias into your circuit.
    The bias pot, is actually a resistor (56k) on the metropoulos layout i have in front of me ( and that I cant paste here because its a pdf and i can't seem to work out how to do that)
    - there is a notation that a bias slider pot ( 100k should be fine) is substituted instead of that 56k resistor to have adjustable bias current. This pot is placed from the right hand side of that 15k resistor at the neg ends of those bias ripple filter caps, across to the positive side of that same filter cap ( ground) .
    the ground reference for this standard 45/100 bias setup is the centre tap itself for the two H>T taps, one of which feeds that first 150k resistor.
    I'll try and paste some layout schematics later for you, its a shame i cant send this one to you.

    this bias pot and all the rest of it is easy to determine the layout thereof, it will be the same as other marshall amps. the value of this post is merely to show you the values that work with that model transformer in the 45/100 amp, if it is that model. Its the only 560v one that i could see on C. Merrens page
     
    Last edited: Sep 15, 2019
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  9. john hammond

    john hammond Well-Known Member

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    same thing.

    you can see the cluster of six diodes sort of in the middle and to the left, well your bias diode is that single diode sort of in the middle and to the right.



    JTM100.jpg
     
    Last edited: Sep 15, 2019
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  10. mAx___

    mAx___ Active Member

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    Thanks so much guys. The pot was new. I checked it and it was open. Before installing a new one, I changed the bias circuit like that of the JTM100 layout posted above. Now everything works fine. So far voltages are steady and they check correctly in the preamp too. Thanks again!!
     
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  11. mAx___

    mAx___ Active Member

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    Santiago posted this on another thread and now I'm wondering about the reliability of what I did. Thoughts?

     
  12. neikeel

    neikeel Well-Known Member

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    Yours is a jtm45/100.
    His is a multistage hi gain amp.
    No comparison
    Concentrate on good lead dress, Larry grounding good tight twists on your heaters and it will be quiet as a church mouse at idle.
     
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  13. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    Is there a specification/data sheet and diagram for that Merren transformer?

    Most amplifier manufacturers did go the separate LT tap(s) for the negative power supply.
    Marshall did use the HT tap for negative power supply in a lot of 50W and lower amplifiers.

    It makes me wonder. All you said was a 1959 type bias circuit. What kind of amplifier is this and which Merren PT is that?
     
  14. mAx___

    mAx___ Active Member

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    It's a 100W 2xKT88 Major circuit. I asked Merren for a more powerful PT and he sent me his 560V Gold Series. He told me to use the LT secondary tap for the negative power supply, and a 1959 bias circuit.
     

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  15. santiall

    santiall Well-Known Member

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    try to use a good 1000V polypropilene cap. I've seen the circuit failing in several 30 anniversary amps (including mine) and some 900s as well. The problem is always the capacitor breaking, not that the circuit itself is wrong but probably the parts being used and how they age.
     
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  16. john hammond

    john hammond Well-Known Member

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    +1 jcm900
     
  17. john hammond

    john hammond Well-Known Member

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    hes told you to use the lower power tap and its good advice, taking bias from higher V tap means more heat/higher resistor values, more stress on components because the bias requirement is so low.
    the thing is though, when youre troubleshooting, a known factory setup is so handy because you know it works.
    you can re-configure your bias pathway to use that lower tap , just use different value resistors, neikeel reckons change that 47k one for effect( on another thread), and i'd say its good advice.

    maybe use a 120k from the lowest H.T tap instead of 150k..its like a juggling act between that first drop down resistor and the bias range resistor and what neikeel called a ' stop' resistor?

    edit, if its working now, i'd leave it.
     
    Last edited: Sep 16, 2019
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  18. mAx___

    mAx___ Active Member

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  19. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    I believe I remember your KT88 100W build from another thread.

    There is no LT secondary supply except for the 6.3VAC filament/heater taps.
    What he has there is a multi-tap HT secondary with 3 options. He is telling you to use the lowest VAC tap for your negative voltage supply which I suppose you have done.
     
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