Question for Dave G. regarding 330K resistors...

Discussion in 'Fisher' started by gkargreen, Aug 2, 2017.

  1. larryderouin

    larryderouin Do I get Food, Med's, or more gear this Month? Subscriber

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    The old standard was 220K with a .1uf. It still works and can be used with no problems. However he cautioned against going higher due to possible LF oscillations. The time constant is a little larger but you can't hear the difference unless you get LF Oscillations. Dave did some calculations a few years ago and found .72uf or .73uf (Those numbers stuck for some reason but I can't remember which one. However it's irrelevant to this purpose as no one makes a cap that size.) was the ideal. But due to lack of a cap that size he recommended a 0.68uf or .082uf as the next best value.(At the time both values were fairly new and not widely available and hard to find). As .068uf is closer than .082uf it's a better value to use, although the .082 is good too. No audible difference is discerned, but it will show up on a scope that is sensitive enough.

    I don't remember the exact calculation but the resistance multiplied by the cap value sounds like it might be the correct . Lets see if this works.
    330K x .047 = 15510 Original Factory Values.
    220K x .071= 15620 Ideal replacement but not feasible due to lack of cap value.
    220K x .068 = 14960 Closest replacement to the 330K/.047uf. Parts available.
    220K x .082 = 18040 Last two values are useable if .068uf aren't available.
    220K x .1uf = 22000
     
  2. gkargreen

    gkargreen Active Member

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    Thanks, Dave. Is there a formula you use to calculate the values for the time constant, maybe like 1/RC or something?
     
  3. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Within the power amplifier, there are a number of LF time constants -- the OPT itself being one of them -- so it is something that must be determined after a basic amplifier design is established, and the stability testing portion of R&D begins. Basic formulas will determine the response of a given individual TC, but when multiple TCs exists, then it begins with measurements for gain and phase shift in the frequencies of interest and altering the available TCs to suit. It's just not as cut and dried as I know you'd like it be. That's why it's best to maintain the original constants if any values within them are altered.

    Dave
     
  4. gkargreen

    gkargreen Active Member

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    Yes, that makes sense. The OPT would certainly have to be considered as there would be no sense in making the circuit have a much lower response that the transformer could not itself pass, and making other circuits also be beyond that capability as well. BTW Dave, I reworked my W4-AM amps using your suggestions, they are sounding real good in my system right now, thanks for all the good suggestions!
     
  5. gkargreen

    gkargreen Active Member

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    Using the values that Larry provided = 330,000 x 0.047 x10*(-6) = 0.01551, reciprocal is 64.5 hz
    the Scott 330,000 x 0.1 x10*(-6) = 0.033, reciprocal is = 30.0 hz
    so the proper value for c (using a value of 220,000 for R) is 0.15 ufd, so I should up the coupling caps from 0.1 to 0.15 ufd to keep the same time constant. Well, this whole thread has been an enlightening and good teaching moment for me, and I think for others as well. Thanks to all for helping me puzzle this out!
     
  6. heyraz

    heyraz Super Member

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    If available capacitor value choices are limited, and the design Time Constant wants to be maintained...why can't the resistor value be altered? Why not parallel two resistors together to get the closest value to work with 0.068uf?
    Modern resistors are smaller too.
     
  7. gadget73

    gadget73 junk junkie Subscriber

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    you can, but that gets back to considering the drive abilities of the phase inverter. If you make the resistor too low, the inverter might not be able to generate enough voltage to drive the output stage to full power.
     
  8. gkargreen

    gkargreen Active Member

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    Thanks, gadget, this keeps getting better and better, I'm learning here!
     
  9. tube-a-lou

    tube-a-lou Super Member

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    This is what I finally did to my Fisher KX200, I was having a bit of a problem with bloated LF in my amp but when I change
    to the .068uf caps it went away. Great thread guy's Thank You.
     
  10. Patrice B

    Patrice B AK Subscriber Subscriber

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    Sorry to chime in and hope my intervention is ok: I asked a similar question back in 2009 when restoring my LK-72. The answer was, at the time, that the change wasn't necessary because the resistance seen in the circuit wasn't really 330k.

    I think it was in part because of the interaction of the D.C. Balance circuit just ahead. Mr Gillespie, please correct me if I'm wrong.

    http://hhscott.com/pdf/340A.pdf
     
  11. gkargreen

    gkargreen Active Member

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    Thanks for chiming in, Patrice! In looking at the circuit, I see what you mean, the DC balance control is in parallel with the grid bias resistors in what appears to be a resistor divider network, nice catch!
     
  12. Patrice B

    Patrice B AK Subscriber Subscriber

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    Found the post by GordonW: http://audiokarma.org/forums/index....ring-after-a-while.221779/page-2#post-2648959

    "Actually, the LK72 does NOT need the 330K grid resistors changed.

    It's funny to think about at first... but through Thevenin's and Ohm's laws, you can determine that, at DC, the grid actually sees the PARALLEL combination of the voltage divider created by the 330K resistor to ground and the roughly 520K combination of half the bias pot (500K divided by two) and the 270K resistor from the bias supply, as its DC feed resistance. That combination (330K || 520K) gives roughly 202K ohms. Well within spec for the 7591.

    I've never seen a Scott with this setup have problems maintaining grid bias...

    Regards,
    Gordon."
     
    Last edited: Aug 7, 2017
  13. Patrice B

    Patrice B AK Subscriber Subscriber

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    However, the 340A has 100k as D.C. Balance and not 500k like the LK-72...
     
  14. Patrice B

    Patrice B AK Subscriber Subscriber

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    Ok, parallel resistance gives around 160k if I follow correctly what Gordon said?
     
  15. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Absolutely correct. The total DC grid resistance is about 163.5K in the 340A, so as previously stated, there is no reason to adjust any of the grid resistances in the Scott design.

    Dave
     
  16. gkargreen

    gkargreen Active Member

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    See, Patrice, you moved the needle in the discussion and we learned even more, thanks for your contribution as well as Dave's confirmation, that makes good electrical sense!
     
  17. ducati2

    ducati2 AK Subscriber Subscriber

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    I want to replace my grid resistors, but first I need someone to tell me where they are located on my late model fisher 400. Neither my service manual nor my schematic has mention of a grid or grid resistor.
    Also, from the above posts, perhaps I should use 270k ohm?
    Also, how many are there to replace? 4?
     
  18. gkargreen

    gkargreen Active Member

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    It appears from your post that you don't do much electronic repair or have much familiarity with these types of vintage amps, you may be better off handing this over to a qualified electronics tech to have this done correctly, you don't want to create more problems poking around in a potentially dangerous amp.
     
  19. ducati2

    ducati2 AK Subscriber Subscriber

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    gkargreen
    I admit I am a hobby electronic guy in the eyes of the pros, but this has been my hobby for the last 8 years. I have brought back to life probably about 20 amps or receivers, SS and tube in that time. Sometimes I do use kits, but I also learn from each project. My elderly friend who got me started in this hobby has long since passed away and my only source for info is internet (AK mostly), service manuals and schematics. Because I am mostly alone in my research and self study, I sometimes come across a term that I have heard many times, but now I need to know what the heck it means. 'Grid resistor' is one of them.
    Here is another.. 'B+ power'. I can't find it on any of my schematics, but I hear the term once in a while. From a google search, it seems to be a main source for higher voltage, probably from a main filter cap, but still unsure.

    Perhaps there should be a separate section of AK for those that are non EE with PE cert. But then I would just be trading folklore back and forth with others guessing at terms and procedures. Maybe this is not the place, but I still need to find answers to my questions above. I also understand not everyone has the patience or ability to explain things to guys like me. No offense taken.
     
  20. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Ducati -- Your assumption is correct about the term "B+". It is principally the raw "B" voltage developed in the power supply (+ simply indicating that the voltage is positive with respect to ground) that is used to power the plate and screen grid elements within a vacuum tube. The term "B" harkins back to the earliest days of vacuum tubes, when the only source of power was batteries: Batteries designated to power the heaters (or filaments) of a tube were called "A" batteries, and so any power that is designated to power these elements is technically A power -- although today, with batteries being replaced as a source of power by AC line voltage, the A has been dropped in favor of simply calling any A voltage source the "heater supply or filament winding", etc. Voltage designated to power the plates and screen power is B power, and further, before cathode bias was developed, the batteries used to provide a negative charge (or bias) on the control grid was considered the "C" battery. Today, the C battery has been replaced by a "bias supply" and the C designation has been dropped. Therefore, only the "B" designation remains today, so that to operate a vacuum tube circuit, the modern designations will typically include a "heater supply", a "B+ supply", and possibly a "bias supply".

    The grid resistors you are looking for reference to refer to R117, R118, R119, and R120. These will always be a relatively large value resistor, and serve to reference the grid to ground if a cathode bias resistor is used, or reference the grid to the bias voltage provided by the bias supply if the cathodes are grounded, as they are in the 400. Their general value can be rather broad, but typically, if they are too large, they can cause problems with the tube they serve, and if too small, then it becomes too hard for the previous stage to develop a signal across them -- which is effectively the signal applied to the grid of the tube they serve.

    I hope this helps!

    Dave
     

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