1959hw ppimv with variable nfb

Discussion in 'The Workbench' started by Waldemar, Feb 25, 2021.

  1. Waldemar

    Waldemar Member

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    Hello, i want to install rich mod ii type ppimv in my 1959hw but i heard it would be better with some type of variable nfb and i have few questions. If i want 3 way switch than what values should resistors for nfb have or if i use potentiometer what value should it have? And if it would be pot then from which way signal goes in pot and which way out from pot signal goes back to circuit? And where should be pot grounded? I know which lugs are for in, out and ground in pots i don't know where it goes in the amp. IMG_20210225_114157.jpg IMG_20210225_114157.jpg
     
  2. neikeel

    neikeel Well-Known Member

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    I suggest a 15k resistor on the board, use a 250k log pot wiring it between the purple wire on your diagram and the 16ohm tap on your selector.
    That will give you a range from JTM45 tightness and late metal panel looseness and down to almost no NFB.
    If you are putting the pot on the back of the amp that would be best. Just use two lugs (input and wiper, you do not ground the pot in this circumstance.
    I can give you a simulation of the pot values and master interactions if you want but all the info is above;)
     
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  3. Waldemar

    Waldemar Member

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    As usual, best advice from neikeel, thank you again. So 15k resistor and 250k pot should be in parallel, if i understand it correctly?
     
  4. Max Gahne

    Max Gahne Active Member

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    I've done the NFB on a pot before when using a post PI MV. As you turn the NFB resistance down you're turning the NFB up. My memory is bad but I think I used the 16 ohm tap and a 50K linear pot with a 5K series resistor. That would give me moderate 55K resistance if the post PIMV was full up and if the post PIMV was at 10% I could go to 5K resistance and in theory have about the same NFB. For me it was important that the min and max NFB were what I wanted at 100% MV and 10% MV. As you turn a post PIMV down you're turning NFB down but the pre amp is still going into the PI 100%. So with the post PIMV full up, taking it out of circuit, I wanted my NFB to be where I wanted with the NFB pot full up. Then to keep the ratio sort of the same as I turned the post PIMV down, which is turning the NFB down, I would turn the NFB pot down, decreasing NFB resistance and increasing NFB. Turning the NFB up to offset the MV turning it down. You're thinking opposite when thinking NFB but I remember liking neikeel's way of wiring the pot because then you're turning both the NFB and MV pots in the same direction to keep the NFB ratio about the same.
     
    Last edited: Feb 25, 2021
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  5. neikeel

    neikeel Well-Known Member

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    No, think about what you are aiming for - wire them in series, so your max NFB will be 15k (stop resistor) and 250k pot in series using the voltage from the 16ohm tap.
     
  6. Waldemar

    Waldemar Member

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    O
    Ok so wire from 16ohm tap to the input of 250k pot, then wire from wiper of this pot to the 15k resistor soldered to the purple wire on my diagram, correct?
     
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  7. johan.b

    johan.b Well-Known Member

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    I started a thread on this about a year ago, using dual pot to combine crossline/type3 ppimv with variable nfb resistor...I think i called the thread "a better ppimv?"... but then i got covid and life got in the way...I never took it all the way, but as I remember, I came to the conclusion it was a viable way forward but you must not short the nfb completely.. it seemed it shouldn't be reduced to less than 1/4 of stock value... it seemed a sweet spot where presence still worked and gain didn't increase to much at low settings...
    J
     
  8. Waldemar

    Waldemar Member

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    I just installed mv and nfb pots and wanted to set bias so first check plate voltage on pin 3 and chassis and it gone to 649 volts before i turned it off, now when it turned off it still shows 618 volts. Help.
     
  9. Waldemar

    Waldemar Member

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    I just installed mv and nfb pots and wanted to set bias so first check plate voltage on pin 3 and chassis and it gone to 649 volts before i turned it off, now when it turned off it now shows 826 volts and still rises, help. Without power tubes it shows 9 milivolts. I put power tubes back in and it is 9 milivolts still but i'm afraid to turn it on again. Those pots have this little piece touching chassis, maybe this is the problem.
    IMG_20210318_202125.jpg
     
    Last edited: Mar 18, 2021
  10. Waldemar

    Waldemar Member

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    Ok, i just found another picture on which is noted that pot should not be grounded, i'll unground it but before i turn it on again tell me should i be aware of anything before i turn it on again? Can something be damaged? Beside when i measured dual pot on max difference between two was about 40k is it too much or acceptable? Maybe it can be even out by 40k resistor from wiper of the one that is lower, am i genius or not?
    Aldv3CB.jpg
     
    Last edited: Mar 18, 2021
  11. Waldemar

    Waldemar Member

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    I noticed something else, when i plugged multimeter to nfb pot (from input to wiper) it measured from 0 to 18k when turned, when i measured from input of pot to behind of 15 resistor it showed from 3 to 4k, is this fine?
    I connected it this way.
    IMG_20210318_214356.jpg
     
    Last edited: Mar 18, 2021

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