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

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

  1. gkargreen

    gkargreen Active Member

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    Hi Dave (and any one else wanting to contribute), I have heard that one should replace the 330K grid resistors on the 7591 output tubes with a lower value, say 220K as 330K is too high for today's modern tubes in these old integrated amps/receivers. Is this something that is important for using modern 7591 tubes? Thanks!
     
  2. Thumper

    Thumper Active Member

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    I'd wait for the guru himself to chime in, but having read and more or less understood his explanation on the subject - replace them with 220K. I don't think it's a new/old tube thing rather than a less than ideal original design of the output stage.
     
  3. larryderouin

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

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    YES! In fixed bias 330K is higher than the manufacturer recommended 300K maximum even for old tubes. Let me see if I can find Dave's good explanation of why they should be run down to 200-220K. The newer Russian tubes can't handle the 330K period. The old tube could handle it but they ended up wearing out in a year or 2 because all the individual component ratings were too high compared to the maximums. This was mainly due to the wattage wars even back in the 50's. The output coupling caps should be increased to between .068uf and .1uf max to keep the R/C network timing as close as possible to the original. .068uf is closest to the ideal .072uf. .1uf was used at the beginning of the great changeover back in the early 2000's as .068 and .082 were not readily available.

    I can't find it on this computer (it's probably on my shed desktop, but it's after midnight right now). If Dave hasn't answered by morning, I'll try and find it and post the explanation up. You should do this upgrade on all fixed bias 7591's. Also the 100ohm screen stability resistor upgrade should be done to lessen
     
  4. larryderouin

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

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    This is from the 1964 RCA Tube Manual. This is for all 7591's built old or new. Note Maximum circuit values.
    7591 specs.jpg
     
  5. Thumper

    Thumper Active Member

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    Yep, 300K max.
     
  6. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    The maximum value of Grid #1 resistance applies to both original production as well as modern production tubes of any given type. Since modern tubes seem to be more trigger happy relative to their own self destruction, most feel that it applies even more to modern production tubes.

    The issue involves an effect known as "reverse grid current" which can occur from a number of reasons -- with all of them basically a product of heat, or too much of it. If the heater is operated at too high of a temperature, or out-gassing occurs due to exceeding dissipation specifications, it can all lead to the #1 grid effectively becoming an emitter of current within the tube. When that happens, there is a slight current drop across the gird #1 resistance (more commonly known as the grid return resistor, or the 300K resistance in this case), that works to produce a positive voltage acting against any negative bias voltage applied to the resistor. This can produce a run away condition, since reducing the negative bias voltage causes the tube to draw more current producing more heat, causing more reverse grid current to flow, causing more tube current, and so on. To limit this run away condition from occurring, a maximum value of Grid #1 resistance is specified for most tubes, limiting the amount of reverse bias voltage that can be developed. Since cathode bias is a self regulating form of bias, the maximum value of grid return resistance is greater for cathode bias designs versus fixed bias designs.

    It is important to understand that the maximum value of resistance specified is based on the tube operating at maximum rated dissipation levels; as a tube is operated at lower dissipation levels, then the value can be safely exceeded -- which is how Fisher (and many others) got away with exceeding the value. Theoretically, if the tube is operated at only 70-80% of its maximum dissipation ratings, then the maximum resistance can be exceeded somewhat. However, with today's higher line voltages and therefore higher heater voltages, it can cause cathode material to boil off and deposit itself on the grid. If that happens, hello negative grid current. Or if the tubes are not matched closely enough in the original circuit, allowing one tube to be a current hog and run very hot -- hello negative grid current. Or if the tubes are not properly ventilated, hello........ It all goes back to heat and what happens when there is too much of it.

    Reducing the 330K grid return resistance to about 220K is a very good move in the stock circuit and operating conditions to add some protection for the tubes. Let's face it, the suckers pretty much cooked due to a variety of reasons. However, as IBAM type circuits are installed (equalizing the current for all tubes), the tubes are adjusted to operate comfortably below rated dissipation levels, care is taken to ensure that the heater voltage is within reasonable limits (5%), and truly proper ventilation is provided, then the need to reduce the value of these resistors nearly vanishes. When new back in the day, Fisher could control the match on the tubes initially installed in the set, and AC line voltages were an established, lower value to design for. The original manufacture tubes were also much cooler in terms of current draw relative to a given bias voltage, so exceeding dissipation ratings were generally not a problem. The main problem Fisher had to be concerned with then was temps when the unit was installed in a case. It is a testament to the quality of the original manufacture tubes that so many lasted as long as they did in that environment without catastrophic failure

    Today however, with much hotter running modern manufacture tubes (relative to bias voltage), notably higher AC line voltages, and a potential mish-mash of tubes installed (i.e., unmatched), then operation with the stock design plugged straight into today's AC power with the unit installed in a case can spell certain doom. Lowering the value of these resistors is critical in this case, and likely still won't be enough to save the unit and its tubes from disaster. If all the common sense guidelines are followed however, lowering the value of these resistors won't hurt, as long as the coupling caps are adjusted accordingly as well.

    I hope this helps!

    Dave
     
    Last edited: Aug 3, 2017
  7. Dave451

    Dave451 AK Subscriber Subscriber

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    Dave,
    Outstanding explanation, as always, and clearly stated. One question I've had is why the manufacturers installed such high value resistors in the first place given these downsides? What was the benefit of using high value grid return resistors?
    Thanks
    Dave
     
  8. gkargreen

    gkargreen Active Member

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    Thanks, everyone, for chiming in on this question, as always, a great and knowledgeable group here provides superior help! And great explanation, Dave!
     
  9. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Dave -- The reason for using the highest possible grid return resistance is two fold in this case:

    1. Back in the day, a given component's physical size -- compared to that same component today -- is much larger. Therefore, for a given desired R/C time constant to appear at the output tube coupling point, making the grid return resistance as large as possible allows the physical size of the coupling cap to be comparatively reduced in physical size, were the grid return resistance otherwise smaller. Back then as now, reducing the size of the audio components was a big deal, and it starts with keeping the physical size of the components used as small as can be. When/if these resistors are in fact reduced in value today, the resulting increased coupling cap value required still likely results in a physically smaller component than that of the smaller value coupling cap originally installed -- unless boutique caps are chosen as the replacements.

    2. Keeping the grid return resistance value as high as possible also minimizes the AC load placed on the phase inverter stage, which has the job of developing enough distortion free drive voltage across these resistors to be able to drive the output stage to full power output if need be. As the value of these resistors are reduced, it becomes harder for the phase inverter stage to do this. Reducing the value to 220K should not present any real problems for the inverter stage, but I would not go any lower than this.

    Dave
     
  10. Dave451

    Dave451 AK Subscriber Subscriber

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    Thanks very much, Dave. I've been wondering about this.
     
  11. audiodon

    audiodon AK Subscriber Subscriber

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    Reading that puts the puzzle pieces in place.
     
  12. larryderouin

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

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    Don: So you finally got that 1,000 piece "BLACK Hole" puzzle:naughty: finished!?! BRAVO!!:banana:
     
  13. gkargreen

    gkargreen Active Member

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    Dave, love the explanation! The coupling caps in this set are 0.1 mfd, I am looking to go to a 0.15 mfd, or do you have a better recommendation? Thanks!
     
  14. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    What is the value of grid return resistance currently installed, and why do you want to increase the coupling cap value further?
     
  15. gkargreen

    gkargreen Active Member

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    It's 330K, it was my understanding that is I replace the 330K with a smaller value (220K) I would need to change the coupling cap value which is currently 0.1 mfd. Thanks.
     
  16. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Using a .1uF cap with a 220K grid return resistors is already providing a slightly longer time constant than the original components do (.047uF and 330K), so there would be no need to go to any larger value cap than your unit already has installed in this position.

    Dave
     
  17. larryderouin

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

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    gkargreen; Exactly what unit are you talking about???? Receiver or integrated amp??
     
  18. gkargreen

    gkargreen Active Member

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    Its a Scott 340A receiver. Like a lot of the Fishers of the day, it too uses 330K resistors for the output tube grids, so I am looking to determine the best approach for restoring this. Is there a formula for determining the combination of the coupling caps and grid return resistors, i.e., the time constant? Thanks!
     
  19. Thumper

    Thumper Active Member

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    From what I've read from Dave's posts, seems like 220K/0.068uF is the gold standard. Works in my 400 quite well.
     
  20. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Besides controlling LF response, it plays into the much bigger issue of LF stability. It would be best to keep the same time constant as originally designed into the unit at that point. If you elect to lower the grid return resistance, then increase the coupling capacitance by the same relative amount.

    Dave
     

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