Question about using a trim pot to control 2 power tubes

Discussion in 'Fisher' started by thornev, Jan 8, 2019.

  1. thornev

    thornev Well-Known Member

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    This question is not specifically about Fishers. I was studying the IBBA schematic to try to answer my question since it does what I'm trying to do.

    I have an amp that has 2 EL84 power tubes in push-pull configuration and they were fixed biased with 2 resistors, both 22K. I wanted to be able to adjust the bias so I replaced the 2 resistors with one 50K trim pot. I connected the pot wiper to the hole at the "bottom" of one of the resistors, and attached one of the side pot leads to the "top" hole of the same resistor. The remaining side pot lead I attached to the "top" hole for the other resistor. This arrangement leaves the "bottom" hole of one of the resistors unoccupied.

    I believe what I've done is make only 1 tube adjustable because the wiper is attached only to one of the 2 power tubes. Am I correct? If I want to control the bias on both tubes with the one trim pot, do I need to bridge the wiper to the same side of both resistors? Or do I need a second trim pot?

    Thorne

    UPDATE: I guess this is one way to go?
    https://robrobinette.com/Generic_Tube_Amp_Mods.htm#Balanced_Bias_With_Single_Balance_Pot
    That means I need a "balance pot" as well as the trimmer pot.
     
    Last edited: Jan 8, 2019

     

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  2. 1rebmem

    1rebmem AK Subscriber Subscriber

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    Thorne,

    EDIT: Thorne, this ckt may not help you if working working with a cathode biased amp, not fixed bias which is what the info below is for....sorry.

    The circuit you linked IS for fixed bias.

    I have built about a dozen guitar amps and always add dual bias pot circuitry.
    Here is a sketch of an example I use:

    Screen Shot 2019-01-08 at 3.48.13 PM.png


    Look at the following Marshall 50 watt head schematic. They tap off the PT secondary HV red wire, knock the voltage down with a 120K resistor then half wave rectify it for a negative voltage supply feeding the bias circuitry and 22K adjustment pot. My sketch is doing the same thing. You can vary the first resistor that is connected to the red HV lead to vary your overall voltage to the ckt. Then on my sketch I adjust the tail resistor (the 56K) to get the desired range of negative bias voltage needed for whatever power tubes being used.

    Screen Shot 2019-01-08 at 3.44.48 PM.png

    You can adjust resistor values to use other adjustment pots such as 25K.

    That Robrob website you linked is a wealth of knowledge. So is the AX84 website. Both geared toward guitar amps but great information.

    Jef
     
    Last edited: Jan 8, 2019
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  3. 1rebmem

    1rebmem AK Subscriber Subscriber

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    Thorne,
    We probably need to know more about your amp/circuit.
    Schematic or model number?

    Jef
     
  4. thornev

    thornev Well-Known Member

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    642
    Location:
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    Thanks so much, Jef. So many questions...

    Your ckt indicates that 2 x 50K trim pots are required, one for each tube, yes? So one pot cannot bias 2 tubes. Bummer.

    Where does the ckt attach to the 2 tubes?

    I would guess the wiper of each trim pot goes to each cathode grid?

    It says a 1K 5W resistor attaches between pins 4 & 6. What purpose do they serve? What tubes are they? I know they're not EL84.

    And which tube grids are the "RG" in your diagram?

    Thorne
     
  5. thornev

    thornev Well-Known Member

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    Attached Files:

    Last edited: Jan 8, 2019
  6. 1rebmem

    1rebmem AK Subscriber Subscriber

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    The "RG" resistors are the bias feed resistors on the Marshall 50 watt head. Marshall designated them RG.

    On that Marshall ckt, the jumper wire connecting the 2 "RG" resistors would be removed and the bias pot wipers connect to the individual "RG" resistors.
     

     

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  7. thornev

    thornev Well-Known Member

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    In my amp R51 and R52 are the RG resistors. Youll find them at "C-" in the schematic (bottom right quadrant). Somehow R31 and R32 figure into this because I have to measure voltage from them. They are at C- also but in the top right quadrant.
     
    Last edited: Jan 8, 2019
  8. 1rebmem

    1rebmem AK Subscriber Subscriber

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    So you want an IBBA, bias/balance circuit.
    NOT an IBAM, individual bias adjustment, like I was suggesting.
    Disregard my prior postings, it won't help with this Blues Jr amp.

    I think you will need two pots either way.

    I'm sure it can be done, somehow, like you were thinking.
    But we have to be sure to not affect the other circuitry on the secondary brown wiring, which is where the C- bias supply is from.
    I will ponder it some more. I'm sure others will chime in......

    Jef
     
  9. thornev

    thornev Well-Known Member

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    Location:
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    So if I simply install 2 trim pots, one at R51 and the other at R52, will that work? The wiper at one side, the side lead on the other side and the 2nd side lead does nothing. It's not ideal that I would have to be swapping the test leads back and forth between the 2 tubes, but it's not that much of an inconvenience.
     
  10. 1rebmem

    1rebmem AK Subscriber Subscriber

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    To go the IBAM route, I think you could replace R51 with two 50K pots in parallel. Then remove the jumper from between R31 and R32 (220K's)
    and connect the wipers from the new pots to the free ends of the 220k's. One wiper to one 220K resistor. After that, R52 (22K) may need to be tweaked up to get the power tube bias required.

    Thorne, we probably could come up with a bias and balance pot deal. I need to study that first.

    Someone please sanity/idiot check me!

    Jef
     
  11. gadget73

    gadget73 junk junkie Subscriber

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    If you only need a single adjustment, replace R52 with a 10K resistor and a 25k trim pot in series. Half travel on the pot will be essentially the stock setting. Adjusting either way will adjust bias, and if the pot fails it goes to full negative bias rather than melting the tube.
     
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  12. thornev

    thornev Well-Known Member

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    gadget - the 10K and trim pot will adjust the bias for both power tubes?
     
  13. 1rebmem

    1rebmem AK Subscriber Subscriber

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    Here is Gadget's solution:

    Screen Shot 2019-01-09 at 9.38.55 AM.png

    The 25K will adjust bias to both power tubes.
    You will still have to verify power output of both tubes.
    This is the solution using one pot.

    Is this more what you had in mind Thorne?
    Jef
     
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  14. thornev

    thornev Well-Known Member

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    If it means using 1 trim pot and no other pot, yes, that's exactly what I am wanting. THANK YOU, All ! I'll have to digest this.
     
  15. 1rebmem

    1rebmem AK Subscriber Subscriber

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    The robrob linked idea can work also.
    He soldered/glued a 50K trim pot to the metal balance pot, so it is a 2 pot solution. It will be like an IBBA. Bias/balance arrangement.

    Jef
     
  16. thornev

    thornev Well-Known Member

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    Yes, Jef, I was digesting that solution last night. My questions with that solution are "What are the 'grid leak resistors' and to which pins on the EL84 do they connect?" Also, from where do I get the 50 VAC to which the IN4007 diode is connected? I know it says Power Transformer, but that sucker has 5 or so leads. I don't know which one has 50 VAC. But it seems gadget's solution is much easier anyway so I'd rather take that route.
     

     

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  17. thornev

    thornev Well-Known Member

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    Jef, the 25K is a trim pot in the schematic in post 13? If so, where does the wiper get connected? It looks like the 2 side leads of the trim pot are connected to the 10K resistor and the 22K (R52) resistor, right?

    In effect R51 (33K) is replaced with the 25K trim pot and the 10K resistor, right?

    Can the trim pot be 50K instead of 25K? (It's what I have on hand) I guess that would mean more pot turns than are really needed?
     
    Last edited: Jan 9, 2019
  18. 1rebmem

    1rebmem AK Subscriber Subscriber

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    The grid leaks robrob was talking about would be R31 and R32 (220K) on your Blues Jr schematic.
    The 50vac in robrob's example is a Bassman 50vac bias winding. Your Jr doesn't have one.

    Gadget's solution is would be the most simple and it meets your original desire for one adjustment pot.
     
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  19. 1rebmem

    1rebmem AK Subscriber Subscriber

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    Correct.

    Yes, the 25K is a potentiometer of some type. The wiper of it connects to the C- point at the junction of R31 and R32.

    Yup
    So........
    If you had a 10K pot and a 24Kish resistor, that would work.
    Any combination of pot and resistor equal to ABOUT 33K will work.

    Jef
     
    Last edited: Jan 9, 2019
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  20. gadget73

    gadget73 junk junkie Subscriber

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    I actually meant the 10k on the bottom, not on the top.

    C- would come from the same point it does now, not from the wiper of the pot. That makes it fail-safe in case the pot fails.
     

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