Question about using a trim pot to control 2 power tubes

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

  1. 1rebmem

    1rebmem AK Subscriber Subscriber

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    What you gonna do for the other 47 hours?
    You'll be reading, thinking and coming up with some other mod's.
    I see a pattern............
     
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  2. thornev

    thornev Well-Known Member

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    Jef... I'll be PLAYING through the amp ! I've been doing that with all the mods so far and I've got it sounding so very sweet. So even if I did nothing more, the mods so far have been quite beneficial. This new one will be the cake topper...hopefully. But you're right... I'm always looking for some new electronic project to make listening to sound an even more enjoyable experience. It's all about eargasms ! =:) Thanks for all your help. Your new schem really simplifies it for me. Thorne
     
  3. thornev

    thornev Well-Known Member

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    Attached diagram of physical layout as I understand it.

    UPDATED: removed link between R51 and R52 because the existing circuit board already has them connected. See post 45.
     

    Attached Files:

    Last edited: Jan 10, 2019
  4. 1rebmem

    1rebmem AK Subscriber Subscriber

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    Yep.....I believe you've got it.

    The jumper I'm pointing to below in purple should already be a run on your printed circuit board.
    Check that, it will help confirm your sketch accuracy.
    Screen Shot 2019-01-10 at 10.26.21 AM.png
     
  5. thornev

    thornev Well-Known Member

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    868
    Location:
    Mid Hudson Valley, NY
    Here's the schematic section of interest. I believe it does show that R51 and R52 are connected at C-. See attachments.
     

    Attached Files:

  6. gadget73

    gadget73 junk junkie Subscriber

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    Looks right. I'd do an 18k with the 10k pot, but a 20k will do it fine too. Use what you have. Only reason I'd go with 18k is because centering the pot will put it at stock adjustment vs having it set to approx 3/4 (or 1/4, depending how you look at it) travel to get there. Also 18k is a more common value than 20k, so I have a better chance of having one.
     

     

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  7. thornev

    thornev Well-Known Member

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    gadget - The difference of centering the 10K pot with 20K and 18K resistors is only 2K. On a 10K pot, that would be something like a 2/5 difference in travel of the pot. OK, maybe that is significant.

    What concerns me more is using a 10K pot and getting enough variation in controlling the voltage going to the tubes. Wouldn't I get more variability with a 25K pot + 1 ohm resistor? I guess it doesn't matter because what I want is for the tubes to operate at their most efficient which is around 70%. So what's important is finding where 70% is in the pot turning.

    But then again maybe there's something to running tubes hotter or colder to get different sounds from the amp.
     
    Last edited: Jan 10, 2019
  8. 1rebmem

    1rebmem AK Subscriber Subscriber

    Messages:
    194
    Location:
    Wilmington, NC
    Do it!

    Take an as found reading of what your bias voltage is now. To give an idea of what range of negative bias voltage is needed after the pot is installed.
    Implement the modification.
    Power it up without the output tubes installed and take voltage readings while rotating the pot end to end. Ideally, your as found value falls in the middle
    of the potentiometer range. If not we can tweak it in with a resistor swap.

    Jef
     
  9. 1rebmem

    1rebmem AK Subscriber Subscriber

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    Location:
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    Don't get concerned.
    You may not have any reason to!
    Implement the mod and find out.
    Worst case, you put the amp back to stock, right?
     
  10. gadget73

    gadget73 junk junkie Subscriber

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    Some math, making assumptions that the voltages on the schematic are accurate

    Stock voltage -10.6v

    33k upper resistor, 10k pot, 18k lower resistor
    max bias voltage -12.1v
    min bias voltage -9.3v

    33k upper, 10k pot, 20k lower resistor
    max bias voltage -12.6
    min voltage -10v

    Making the lower resistor smaller will push the bias voltage closer to zero, which you do not want. Making it larger will give more negative, which reduces current through the tube. Of course a larger bottom value also reduces the adjustment range possible.

    Honestly a >2.5v adjustment range should be plenty sufficient for an EL84 tube.
     
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  11. thornev

    thornev Well-Known Member

    Messages:
    868
    Location:
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    I completed the project tonight. Of course the amp sounds the same, no red glow, lots of nice orange. I'm not at all happy with the mounting of the trim pot as I didn't do that great a job at planning how it would sit on the board. I hot-waxed it to keep it stable and I did measure at C- as I turned the pot and watched the negative voltage go up and down. Now I have to figure out how to measure the current/bias since I don't have a resistor connected to the plate like with the IBAM. At C- I set whatever it is at -11.5 VDC. Foggy brain tonight ! But man, that amp absolutely sings when I have the master volume on 4 or above. Love love love it.
     

     

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  12. gadget73

    gadget73 junk junkie Subscriber

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    inserting a 10 ohm resistor between ground and the output tube cathodes is the easiest way of doing that. You can use a single resistor to handle both tubes, or one per tube. The other way is to build yourself (or buy yourself) a current monitor dealie that sits between the tube and the socket. Its basically a socket saver with connections out to a meter to monitor current.
     
  13. 1rebmem

    1rebmem AK Subscriber Subscriber

    Messages:
    194
    Location:
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    Soldering in the 10 ohm resistors is the cheapest way to check check power tube current. But it may not be convenient to get your meter in there. How often will you need to? Unless you plan some tube rolling. If thats the case you may want to build a bias probe (bias checker) as gadget suggested. Google "bias probe" or "bias checker" and you'll find plans to build one or buy one fairly cheap.
     
  14. thornev

    thornev Well-Known Member

    Messages:
    868
    Location:
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    Thanks, Joe. I'm used to that setup on my 500-C so I think I'll take your advice. The amp schematic shows many test points and their "designed-in" values so at least I can do those tests, but I'm not sure where to make adjustments should a value be off. With the plate resistor I know I can measure the health of the tubes and use the trim pot to keep them in the "safety zone".One good thing about the amp design is that it's easy to access PCB components. Getting meter probes in there is very easy as all components on the top of the PCB are exposed. The bottom of the PCB is where the soldering is done so the PCB has to be removed for replacing components.

    I rather like this little amp and I've got it sounding great with my 1970 Gibson SG. Other work I've performed on this amp:

    - I upgraded all the filter caps, replaced other components with better quality parts,
    - replaced the reverb tank with one that has a decreased reverb "tail" (i.e. the effect is not as long in duration),
    - tried an assortment of tubes (2 of EL84 and 3 of 12AX7) to get a sound I like (I settled on 1966 EL84 Mullards and new Groove Tubes for the preamp, tone stack and phase inverter circuits),
    - made the Negative Feedback circuit optional by installing connectors that can easily be disconnected (might add an on/off slide switch later),
    - put in a better and "darker" 12" speaker (more of an even frequency response across all frequencies but especially reduces the overly bright highs)
    - "dressed" the power transformer wiring and other wires, keeping some away from lytics that could introduce noise
    - replaced the output transformer with a larger, higher quality model
    - adopted some suggestions about which I read in various guitar amplifier blogs and forums that improve the efficiency and sound of the amp (phase inverter oscillation is apparently a concern with this amp that can be eliminated with one capacitor across a resistor)

    Unfortunately this amp (and probably other new ones by Fender) is not designed and built with the ambitious "modder" in mind because some parts, especially the multi-wire, plastic-sheathed wiring harness from the 5 tubes to the PCB, are brittle and are at risk of breaking from too much movement. It is known that the heat from soldering on the PCB can cause the traces to become separated from the PCB. After installing the trim pot, although I was so very careful with everything, somehow I damaged a separate small board with the speaker and foot pedal jacks because I have to jiggle it to get any sound from the amp. Something came loose somehow (have to go figure that one out now). There are only 1 or 2 additional changes I want to make and that is going to have to be it, as they say.

    Thorne
     
    Last edited: Jan 11, 2019
  15. thornev

    thornev Well-Known Member

    Messages:
    868
    Location:
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    I found that the jiggled board was a broken solder joint. I resoldered and all is well. Additionally I replaced 2 caps with 10X the capacitance for bigger sound, replaced the master volume pot with one whose volume increase is more linear than logarithmic, installed a standby switch so the tubes stay warm but current is zero, and I did a much better job with installing a round trim pot that is out of the way of everything. I dialed in the bias so I get 8.8 watts out of 12. This amp is sounding absolutely wonderful now. Time for the Unity Gain take 4 in my 500-C !
     

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