Alternate Individual Bias Adjustment Modification

Discussion in 'Fisher' started by dcgillespie, Jun 30, 2014.

  1. Tom Boley

    Tom Boley AK Subscriber Subscriber

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    Dave, thanks for clearing that up for me. I've read your article 3 times and think I understand the benefit of the adding that 10 ohm resistor between Pin 5 & Ground and adding the screen stability resistors. When you said that after adding the screen stability resistors you had no more instances of arc's that was enough for me. I'm waiting for the cans from Hayseed so I'll build but not install the IBBA. I'll wait until my initial issues are resolved before plowing forward. I still have 2 sections of the MetalBone update kit to complete, as well. Hopefully by next week I'll be ready for an inspection before the next power up. Tom
     
  2. Tom Boley

    Tom Boley AK Subscriber Subscriber

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    Dave, I've just about got my IBBA finished but have one more question. My board looks very similar to your drawing with the top control for the balance with the 2 (across) connections going to A, B, C or D. The second control (bias) is connected (at the across connections) to the empty tab on the upper control and to a 10k resistor on the bottom. What do I do with that orphan tab on the bottom control?
    Tom
     
  3. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    To make it simple, the "wiper" connection of the DC Balance Control connects to the wiper connection of the Bias Control. One of the outside connectors of the Bias Control connects to the 10K resistor. Which outside connector you chose to use will determine whether turning the Bias control clockwise increases current draw, or decreases it. I like to wire the control so that turning the control clockwise increases current draw. That will mean that the 10K resistor should connect to the outside terminal that decreases in resistance when the control is turned clockwise. Once that determination is made, the other (unused) outside terminal of the Bias Control can be either left with no connection, or (ideally), shorted to the wiper terminal (of the Bias Control).

    In reality, once the two terminals of the Bias Control are identified that you're going to use (wiper and particular outside terminal) for connection into the circuit, it matters not which goes to the wiper of the Balance Control, and which goes to the 10K resistor. In either case however, the unused outside terminal of the Bias Control should be shorted to its own wiper.

    I hope this helps!

    Dave
     
  4. Tom Boley

    Tom Boley AK Subscriber Subscriber

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    Thanks, Dave. I did have it wired incorrectly. Will digest above and reconnect. Tom
     
  5. Tom Boley

    Tom Boley AK Subscriber Subscriber

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    Dave, If I understood you correctly I removed the bias control, rotated in 90 degrees, connecting the two controls via the wiper terminals. I then shorted the unused terminal to the wiper terminal of the bias control. When I can see it operate the bias control should make more sense to me. I've come up with a pretty cool mounting for the board. I'll post photos when I get the cans installed. Thanks again, Tom.
     
  6. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Sounds good! I'll look forward to the photos!

    Dave
     
  7. Tom Boley

    Tom Boley AK Subscriber Subscriber

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    Hi Dave,
    I apologize for asking so many questions but here are 2 more. I'm ready to install the screen stability resistors. Looking at your 2nd photo:
    1) Are the 100ohm screen stability resistors connected to Pin #'s 1 & 4? I thought I saw another post where Larry said to connect Pins 3 & 8. Maybe that was for a different model.
    2) I think I see a second wire on Pin #1 of each tube. Do I need to run a wire connecting the output tubes? To anywhere else? I seem to recall a post where you discussed using Pin #1 for new connections since it was originally unused but I cannot find that post.
    Tom
     
  8. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    The 100 Ohm resistors are connected between pins 1 and 4, with the jumper remaining in place between pins 4 and 8. Then, all of the pin 1 terminals are daisy chained together and form the "screen buss", where anything that WAS connected to pins 4 or 8 (other than the resistor or jumper as mentioned) NOW connects to.

    I hope this helps!

    Dave
     
  9. larryderouin

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

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    Tom; It was Pins 1 & 7 on 7868 tubes (for the 400) on the FISHER COMMON PARTS Sticky.
     
  10. Tom Boley

    Tom Boley AK Subscriber Subscriber

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    Larry, I thought that was the case but I've learned to ask if there's any doubt.
    Dave, that answers my question perfectly. It looked like they were 'daisy-chained' but I wanted to make sure it wasn't 2 pairs of two sockets. Now all I need is those blasted caps and 1 tube. Both should arrive this week.
    Thank you both,
    Tom
     
  11. larryderouin

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

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    Tom; No problem.:bigok: We're all getting more and more of those..:confused:.....what?,,,:idea:, Huh, I can't find my (insert keys, glasses, wallet, etc here) moments. Go ahead and ask the questions, as it helps keep the rest of us on our.:oops:....??????? what was I trying to say.....:(..
     
  12. Tom Boley

    Tom Boley AK Subscriber Subscriber

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    Hi Dave, I'm not sure about the connection of the 2 pots on each side. Above I think you are saying the wiper connection of both pot on each side are connected together. That's how I think I built my IBBA but something is wrong. I know my wires running from the IBBA to the outputs is correct but the actual board wiring seems like it's wrong.
    One more time, can you tell me how to connect those pots? If my understanding about the pots is correct the outside pins of the balance pot go to the A/B & C/D leads to the 220k resistors. It looks like you have the center pin of the balance pot connected to one of the side pins of the bias pot not the wiper.
    The way I had it wired (center pins of both pots on each side wired together) I get a reading of 10.5 - 11 ohms when I touch any test point to ground. Turning the pot doesn't seem to make any difference. I get some over 20 ohms when I touch 2 test points together. Turning any of the pots doesn't seem to change the readings.
    I have removed the bias pots from the board. I've attached photos of the front and back of the board.
    Tom
     

    Attached Files:

    Last edited: Jun 19, 2016
  13. larryderouin

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

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    TOM; These pics should help you understand the connections and how to test for balance and bias once it's powered up.

    Bias pot setup.JPG

    Bias-balance test points.JPG
     
  14. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Thanks-you Larry!

    Dave
     
  15. Tom Boley

    Tom Boley AK Subscriber Subscriber

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    Larry,
    To be sure I'm wiring the board correctly:
    Am I correct in connecting the wiper (center pin) of the balance pot to Pin 1 of the bias pot? Do the center pin and Pin 3 connect to each other? Then, is Pin 3 also connected to one side of the 10k resistor?
    After reading your post I think I know what I supposed to connect but it doesn't seem like, when wired with no power, I don't get any change from turning either pot.
    this may against the rules but would it be possible for me to talk to you about my connections? I'm missing something.
    Tom
     
  16. larryderouin

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

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    Connect the BALANCE POT CENTER PIN (wiper) to either pin 1 or 3 of the BIAS POT, WHICH EVER IS CLOSEST to the BALANCE POT. The bias pot can go in either way (it's ambidexterous!!!) so it works the same way no matter which way you place it. Just as long as the pins are in a VERTICAL orientation for the bias pot, and a HORIZONTAL orientation for the BALANCE POT.

    Dave would be the one to talk to as he designed it. I've put it in 1 400, and that was over a year ago.
     
  17. Tom Boley

    Tom Boley AK Subscriber Subscriber

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    Larry,
    I've got the wiper of the balance pot connected to Pin 1 of the bias pot and Pin 3 of the bias pot connected to one end of the 10k resistor. Where do I connect the wiper of the bias pot? I've only installed the bias pot for V8 & 9 right now. The balance pot is horizontal and the bias pot is now vertical.
    I'm not getting a reading connecting Pin1 to Pin 3 of the bias pot. When I connect TP 8 to ground I get 10.8 ohms. When I touch TP 9 to ground I get 13.4.
    At this time the wiper of the bias pot is not connected to anything. Is that correct?
    When testing TP's 10 & 11 I get readings of 10.6 & 10.7. No bias pot installed yet for 10 & 11.
    Am I not getting any ohm reading when touching Pin 1 & Pin 3 of the bias pot because the wiper is not connected to anything? Turning the pot doesn't seem to change anything. When I touch Pin 1 and the loose Pin 2 I'm seeing a reading of 0.2. If I turn the dial all the way in the opposite direction I get nothing. But, if I then touch Pin 2 & Pin 3 I get 0.2 ohms.
    I'm sorry to be bothering you guys on Fathers' Day, by the way, Happy Father's Day to all you Dads. I can wait until sometime next week if that would be more convenient.
    Tom
     
    Last edited: Jun 19, 2016
  18. Tom Boley

    Tom Boley AK Subscriber Subscriber

    Messages:
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    Location:
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    Dave & Larry,

    I am very confused. Dave, in your post #84, above, you say the wipers of both pots are connected but in Larry's post #93, above, he says to connect the wiper of the balance pot to Pin 1 of the bias pot.

    I now have the wiper of the balance pot (for V8 & 9) connected to Pin 1 of the bias pot for V8 & 9 and the wiper of the balance pot (for V10 & 11) connected to Pin 3 of the bias pot for V10 & 11. The wipers of the bias pots are not connected to anything.

    I'm thinking the way I have them connected will result in having to turn one pot one direction to reduce the bias and the other direction to reduce the bias on the other pot.

    When I touch the wiper and one of the outside pins of the bias pot I get 0.2 ohms with the pot turned in one direction and 0 when turned in the other. When I turn the pot the all the way the other direction I get 0.2 on the opposite pin and 0 on the one I previously read 0.2 ohms. Does this help you to tell you what I've done?

    One last thing. In Dave's drawing he shows a chassis ground connection along the bottom line and a ground connection to the center test point. Should there be 2 ground connections?

    Would it be possible for me to talk to one of you about this? I promise I will not abuse the privilege. I can message you with my # or if you would rather I call you, please message me with your number. I'm thinking this will result with less of your time wasted.

    I really appreciate all of the help you guys have given me. To be honest, if I knew of a nearby repair facility I would throw in the towel and take it there. I'm about 2.5 hours east of Las Vegas and 4.5 hours south of Salt Lake City.

    Tom
     
  19. streetwise

    streetwise Active Member

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    Dave what are in your experience the difference in sound quality after balancing tubes? my outputs are currently at 19ma, 21ma, 21ma, 30ma. Also is there anything different about this installation with your efb, power amp, control section & SS Buffer installed? It would seem like the 100 ohm screen stability resistors arent used since the efb is still involved.
     
  20. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    1. Under-biased tubes will produce a restricted, thin sound that lacks any sense of responsiveness. Scientifically (in the lab that is), under-biased tubes will be shown to produce reduced power output, and copious amounts crossover distortion. While 30 Ma will produce reasonably good performance, there is no doubt that your other tubes are clearly not operating on the linear portion of their characteristic curve during the hand-off period between the tubes once Class AB1 operation commences with such a low bias setting.

    The control section and buffer modifications address completely different issues than that addressed by balancing output tube current flow and the installation of EFB(tm).

    The need for the installation of Screen Stability resistors is determined by the type of output tubes used and how they are operated. Recapping the power supply with modern low ESR caps and the installation of EFB are certainly influencing factors relative to the need for SS resistors, but the main criteria starts with the design of the circuit to begin with.

    Dave
     

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