Adding a negative feedback trim pot on 5f1

Discussion in 'Amp Tech Center' started by markeyd123, Jan 22, 2020.

  1. markeyd123

    markeyd123 Tele-Meister

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    I recently built the following 5F1 amp based on Doug Hoffman's schematic. This replaced my previous iteration of the amp which utilized a PCB from a tube depot kit. One thing I liked about the tube depot version was the 100k negative feedback trim pot in line with a 22K resistor.

    Hoffman circuit (attached as PDF):
    Screenshot 2020-01-22 at 7.13.46 PM - Edited.png
    Screenshot 2020-01-22 at 7.14.38 PM - Edited.png

    Picture of trim pot on PCB (old version of my build):
    Screenshot 2020-01-22 at 6.57.28 PM - Edited.png
    PCB Schematic is here: https://s3.amazonaws.com/tubedepot-com-production/spree/attached_files/tweed_5f1_schematic.pdf

    It looks like the traces on the PCB (nothing visible on back of PCB, trace-wise) connect the R8 22k resistor to the top lead on the trim pot, then the bottom right lead is connected to the right yellow wire which is the negative feedback wire to the speaker output jack. It appears as if the right side lead on the pot has nothing connected to it. Is that right?

    If that is the case, and I want to add this pot to the Hoffman layout above could I do so by simply placing it between the 22K resistor already on the turret board and the "negative feedback" wire on the diagram. I would connect the top lead of the pot to the 22k resistor, the bottom right to the negative feedback wire, and the bottom left lead connected nothing?

    Thanks.
     

    Attached Files:

  2. Jon Snell

    Jon Snell Tele-Meister

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    "One thing I liked about the tube depot version was the 100k negative feedback trim pot in line with a 22K resistor".
    Yes, wire the pot in series with the 22k to give you standard nfb to 122k minimum nfb.
    Lift one leg of the 22k. Fit the centre or wiper pin of the pot to the original 22k fixing that has been unsoldered with a little fresh solder. Solder the left or anticlockwise leg of the pot to the 22k.
    That will give you your adjustment that you require, increasing the noise level and overall gain of the amplifier.
     
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  3. jimgchord

    jimgchord Tele-Afflicted

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    Seems inconvienent to have it on the board like that
     
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  4. markeyd123

    markeyd123 Tele-Meister

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    I found that I basically set it and forget it. It wasn't something I fiddle with, but I have seen some add a pot to the outside. I have no room for that. I already replaced the fuse position with a switch and moved the fuse down by the tubes.

    [EDIT]: But now you have me thinking . . .
     
    Last edited: Jan 23, 2020
  5. markeyd123

    markeyd123 Tele-Meister

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    when you say "22k" above I think you intend to refer to the NFB wire going to the output jack instead because you already described soldering the center or wiper leg to the 22k. That would have taken care of the 22k, yes? So your quote should probably read: ". . . Solder the left or anticlockwise leg of the pot to the NFB wire going to the output jack."

    Just to make sure: 1 leg remains disconnected from everything, the leg on top to the 22k, and the other to the wire going to the input jack. You say connect the left leg, but on the PCB it is the right leg of the pot that is soldered to the NFB wire. Does it matter, or is it a case of me confusing my right from left (depends on which way you are looking at i.e. stage right, etc.)?

    Thank you for your help!
     
    Last edited: Jan 23, 2020
  6. markeyd123

    markeyd123 Tele-Meister

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    I also suppose I can splice the pot (or any 100k pot) inline with that wire from the 22k to the input jack, and that would give me the freedom to mount the pot onto the bottom of the chassis or elsewhere . . .
     
  7. Jon Snell

    Jon Snell Tele-Meister

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    Apologies but being dyslexic has its downfalls.
    I am not good at explaining things but yes you have it correct. Placing the pot on the front panel after splicing the nfb lead is what a lot of players have done, gives the added freedom of an adjustment that can be made at will without taking the back off etc.
    Being dyslexic has advantages; I am good at my trade and can repair things other cannot. It allows me powers of observation most haven't got.
     
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  8. markeyd123

    markeyd123 Tele-Meister

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    Thanks, and remember: with great power comes great responsibility. Use your powers for good, and not evil.
     
  9. Jon Snell

    Jon Snell Tele-Meister

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    Of course.
     
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