Alternate Individual Bias Adjustment Modification

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

  1. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Now that I've restored proper operation in the AM and FM tuners, MPX decoder, and FM Automatic matrix switching circuits in my 800C (there were problems in all four of these sections), I've now turned my attention to the audio section.

    When received, it still had the "original" Fisher 7591 output tubes installed, which were advertised as having tested fine, but in reality were so bad that although "functional", they would only allow a small fraction of the unit's performance to be produced. But, what the hey, everything else was advertised as operating properly in this unit as well, so it fit right in with the Grand Canyon of difference that existed between truth and reality by the seller. At any rate, with the unit really starting to come together, a decent set of low mileage Westinghouse 7591's was purchased as the next order of business to attend to, in the quest to make this unit be all that Fisher intended it to be.

    As is so often the case today however, while the new tubes were a great improvement over the old ones (almost anything would have been!), one was out in left field with it's grid bias voltage requirements. This was aggravated by the fact that the tech who serviced the unit in readying it for sale put in a bias modification to cool down the output tubes. Boy did it ever cool them down! Good tubes were only drawing 9, 11 and 13 ma of total cathode current each, with the odd-ball pulling 22 ma, and collectively producing an abundance of crossover distortion at even modest power levels. Throttling back the bias voltage a bit brought the three tubes up into the 16-18 ma territory, and the odd-ball up to about 28 ma. Clearly, this would hardly do for the otherwise great tubes this receiver now had, and the condition it was rising to. The ability to adjust each output tube individually then became top of the list important.

    When it comes to installing individual output tube bias adjustments, the approach incorporating DC Bias and Balance controls in each channel offers a number of advantages over having four separate bias controls:

    1. The overall adjustment process is easier to execute, with only two bias controls requiring adjustment to match channel performance rather than four.

    2. It offers greater fail-safe protection when properly designed, where if the wiper of either the DC Bias or Balance control in a given channel lifts away from its carbon track, both output tubes in that channel immediate go to a maximum negative grid bias condition.

    3. It is the only configuration that allows for the complete output stage quiescent current draw to be easily adjusted up or down (both tubes in unison) to set the stage for the lowest distortion operating point.

    When Fisher included a FULL complement of output tube adjustment controls, the DC Bias and Balance approach is always the approach they used. Think SA-100, X-202/X-202B, X-1000 (EL34) and K(X)-200 integrated amplifiers as some examples of this type of design. If I was going to install adjustment controls to optimize output stage operation, this is the type I wanted to employ.

    Also, many of the examples of individual biasing schemes I could find had the controls and/or test points located underneath the receiver chassis, which I try to avoid at all cost. As I get older, these things get heavier and more cumbersome. Fixed umbilical cords to separate external boards or monitoring boxes can address that, but make removal of the unit a hassle from a cabinet when necessary. Therefore, anything I can do to either eliminate removal, or minimize the maneuvering of these things when removal is required is a blessing for sure!

    With these design and installation goals then framing the project, I thought I'd share the approach I used to achieve them, as but one more offering to include in the stable of available options when these types of circuits are installed in Fisher receivers.

    The circuit containing the 4 pots, 6 resistors, and 4 caps, along with the 5 test points (one being ground) for use when making adjustments are all contained on a small piece of perf board measuring 1" tall X 2&13/16" wide. The board requires 10 leads to connect it into the 800C, which was conveniently handled by a 9 conductor shielded cable, with the shield acting as the 10th (ground) lead. The leads do not need to be shielded, but it provided a convenient way to neatly bundle the 10 leads required to install the board.

    The test points are nothing but insulated closed-end wire connectors bent into an "L" shape to form a convenient point for mounting the board to the chassis as well as a test point for the circuit, with similar connectors bent into the same shape but mounted on the back side of the board behind the test points to allow for a connection to the test point. The insulated barrel of the wire connectors forming the test points are then affixed to the chassis with hot glue. I know. Cheezy. But in practice, it works quite well, and is in keeping with my effort of being only minimally invasive when necessary, and only when necessary. This time, it wasn't necessary, and all work can easily be reversed without a trace of the installation remaining. The pics show all of this rather clearly.

    The final results turned out very well indeed. Located just behind the Reverb connection jacks on the top of the chassis, the bias can now be measured and adjusted quite easily at will, even when installed in a cabinet. The wiring from the board enters the chassis through one of the ventilation holes drilled around one of the can caps, and is secured underneath by a wire restraint.

    Pics include:

    1. The front side of the board during construction. You can see how compact it is, yet contains everything needed to properly adjust the output tubes for optimum operation.

    2. The rear side of the board during construction. The wiring is close quarters, but hardly the tightest quarters.

    3. The assembly was finished by attaching the flying pigtail lead that will connect the board into the circuits of the 800C. Each twisted pair represents the wiring to a specific output tube: one lead connects to the cathode terminal to establish test point operation, while the other is the adjustable grid bias supply source from the board for that same tube. Individual 10 ohm cathode sampling resistors are installed at each output tube socket, as are individual 100 ohm Screen Stability resistors as well.

    4. A close up of the board mounted in place. It was my first hot glue experience so the job is not the neatest, but does the job of mounting the board down quite admirably none the less. The center test point is a ground point for the negative meter lead, while the test points on either side of the ground test point represent a channel, with the test points electrically connected/arranged in the same order as the output tubes themselves appear on the chassis. For long term memory of the test point/output tube relationship, the two most left terminals are color coded green, with a green dot then marked on the chassis between the two most left (left channel) output tubes. Red indicates the same information for the right channel tubes and test points.

    The control centered above a channel's pair of test points is that channel's DC balance control. It is first adjusted for 0.0 vdc between the two test points below it. The control next to each channel's DC Balance control is that channel's Bias control, which is then adjusted for the target cathode voltage from either of that channel's test points to ground after the DC Balance control is adjusted. The range of the controls are such that the least negative bias voltage that can be applied to either tube in each channel is -13.5 vdc, preventing any catastrophic tube damage from setting the grid bias voltage too low.

    5. A rear view of the finished installation, showing how the flying pigtail lead snakes around the Reverb jacks, making them still fully usable if needed, before entering the chassis through a ventilation hole.

    The design allows for a 3 volt spread over the range of the DC Balance control, and another 3 volt spread over the range of the Bias control. This nicely accommodates the rather wide spread of the tubes installed in one channel, while still allowing an easy enough adjustment of the two more closely matched tubes in the other channel. The output tubes are now all happily idling at 30 ma each.

    In the final post, I'll show some underside shots of how the board's pigtail lead is connected into the wiring of the receiver, and a quick scribble of the circuit I developed for this project.

    Dave
     

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    Last edited: Jul 1, 2014
  2. gadget73

    gadget73 AK Subscriber Subscriber

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    Nice execution. I like the bias/balance setup better than just straight up bias adjustments. I may have to see if I can figure a way to stick something like that on my Sherwood. It has absolutely no adjustments or means to monitor whats going on in there. The Bogen DS-225 is the same situation.
     
  3. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    And of course, that's exactly the same situation with the Fisher receivers as well......

    Dave
     
  4. ferninando

    ferninando super tek Subscriber

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    Could a schematic be provided for this neat bias addition?
    It looks to be similar to the ""IBAM"" CIRCUIT, done awhile back on the X100's,
    which I implemented. This one looks to be more advanced with mounting externally and test points added.
     
  5. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Should be able to get it up in the morning.......

    Dave
     
  6. bhamham

    bhamham AK Subscriber Subscriber

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    Elegant solution, Dave!
     
  7. audiodon

    audiodon AK Subscriber Subscriber

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    Dave keeps chipping away at the improvements in these Fisher units . . . to the benefit of us all.
    I also prefer bias/balance over only bias.
     
  8. larryderouin

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

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    CHEEZY? Is Scarlet Johannsen cheezy in the Chanel commercial ???? NO WAY! As much as I like the IBAM on my 800-C's and Sansui 1000A, I'm finding it harder to "get inside" to adjust everything as i age, so anything that is done that can safely adjust the bias and balance, make it easier to implement, and keep it closed up is a DEFINATE PLUS! Like Don said, Dave keeps chipping away with improvements for ALL FISHER's and we all benefit. Now THAT's AUDIOKARMA!!!!!

    I like the idea of getting the tubes balanced to 0.00vdc between the pair. I can get close with the IBAM but it does take time and effort "hotswapping" the meter leads (which I don't really like doing). This can / will take the "hotswapping" away from the circuitry, cutting down the chances of an "OOOOPSY" and injury to the unit and the
    owner.

    Thank you Dave!

    Larry
     
  9. ferninando

    ferninando super tek Subscriber

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    Right Larry. I built the IBAM for my X100B per the extensive thread on another AKr's unit.
    I becomes evedent real qwik how nice it would be to have the bd external rather than under chassis. It's OK when the bottom cover is off and you have 4 DMMs to use to keep trak of the settings, which are a bear to dial in but doable.
    Now I'm thinking of repositioning my IBAM bd to external with test points.
    Thank you ever so much Dave!
    Schematic? just to see what has to be diff. for the X100B.
     
  10. gadget73

    gadget73 AK Subscriber Subscriber

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    personally I'm not a big fan of multiple meters for bias adjustments, unless you can calibrate them somehow to verify they read the same. Seems a little silly to try and match things to a tenth of a milliamp on the meter display, when they may not be reading the same value at all.
     
  11. ferninando

    ferninando super tek Subscriber

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    Yeah gadget, but ya do the best ya can with what ya got.
    At least I do and never had a problem yet.
    I checked all my dmms(5) and they read within 20% of any spec fisher ever gave, but more like 1-3% at worst across the 4. No complaints here.
    Maybe not good enuf for some folks tho and thats OK by me.
     
  12. dsndblm

    dsndblm Well-Known Member

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    I'm interested. Very nice! Looking forward to more info.
     
  13. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    More Pics -- 800C DC Bias/Balance Modification

    First, for you late nighters or early risers, I went back and edited my initial post to include more detail about the pics provided that will help to answer some basic questions that might arise, so I wanted to alert you to that.

    Beyond that, the underside shot is really quite uneventful, as one of my goals is to leave a unit looking as much Fisher produced as possible after I'm done, as it did before I started.

    The usual work has been performed by way of installing matched 10 ohm cathode resistors and 100 ohm Screen Stability resistors at each output tube socket, so that the new tubes can operate worry free. I always use pin #1 of the 7591 sockets as the new daisy chain screen buss terminal for the Screen Stability resistors to tie into at each socket.

    For now, the bias leads from the new DC Bias/Balance board tie into the existing grid return resistors with very secure and safe air connections. This unit is a work in progress with more work ultimately planned for the area, so they will be more properly addressed at that time. For now however, the connections represent no concerns even over the long term.

    The original bias wiring has been left in the unit, as have been the original -17 volt bias tap components (5.6K, 15K, and .1 uF) -- although the end of the 5.6K resistor that attaches to the heater feed terminal of the heater/bias supply has been lifted from that terminal, so that the old network is no longer powered. It is this same heater feed terminal that the supply lead to the new DC Bias/Balance board attaches to.

    Pics include:

    1. The finished look on the back side. No, it hardly looks like it was part of the original build. But for the ease of accessing the new controls and test points the installation provides, the neatness and practicality of it all trumps maintaining original external appearances in my book.

    2. The under side view hopefully shows how well the modification's wiring just nestled right in with the original wiring, drawing little attention to itself or the work done. Here you can see the new 10 ohm cathode resistors, the 100 ohm Screen Stability resistors, and the new connections to the grid return resistors.

    After it enters the chassis, the pig tail lead is first secured by a cable clamp that is sandwiched between two nuts installed on a longer screw installed that secures the back left mounting tag of the left OPT when viewed from the rear. To effect a good ground connection, the shield of the pig tail is then soldered to the nearby mounting lug of the T-strip servicing the grid resistors for the left channel output tubes. The yellow lead from the pig tail snakes its way over to the DC heater/bias supply to apply power to the new DC Bias/Balance board.

    3. The new (to me) Westinghouse output tubes are quite happy with the new arrangements. They are used, but still have a considerable amount of life left in them.

    4. I drew the circuit out on a note pad and took a pic of it. If the resolution is not adequate, I'll scan and re-post it later, but I think this will suffice. I notice I left an arrow off the wiper of the right channel DC Balance control, but no doubt you get the gist of the circuit.

    5. Finally, I now have a wonderful 800C that performs as good as it looks -- and it performs exceedingly well in all aspects now. The original volume control even still tracks quite nicely at all volume levels. When it all works the way its supposed to, it really is unmatched when the total package is considered. It was an amazing unit to produce in its day, and remains just as amazing today.

    BTW -- Even though the 800C has a powered center channel output feature, unlike so many units with that feature, this unit does not change absolute polarity for any internal signals or external signals provided to selector switch inputs. Just some extra info I meant to pass along earlier.

    With this unit really coming into its own now, are there any recommendations as to who might build a quality aftermarket copy of a Fisher cabinet to properly dress up this looker? The few real ones that are currently available on the auction site look pretty rough!

    Dave
     

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  14. ferninando

    ferninando super tek Subscriber

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    PERFECTLY GOOD SCHEM, DC.
    clean design with 1 adj for 2 tubes. That is a plus.
    everything is all balanced.
    Dave, U-DA-MAN!!
    Thanks.
    One question: where do U get ""closed end wire connectors""? Never heard of such.
    Are they necessary or can U use plain crimp connectors? I got gobs of those.
     
    Last edited: Jul 1, 2014
  15. eduarsan1

    eduarsan1 Luis Eduardo Sanchez Subscriber

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    Dave

    Antony Saliba from JNTS Products mades my cabinet with a good shape! Call him at 973-2277287. Not affiliation with him,
     
  16. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Luis -- Thank-you!

    Ferninando -- Thank-you as well! I hope you can find it useful. As for the connectors, it is much more likely my description that is at issue here. These are the everyday crimp connectors you mention, but instead of having an open end (appearing as a "U"), the ones I used are enclosed, appearing as an "O" on the end of the connector. With the enclosed type, you have to place the screw through the hole of the connector to attach it, rather than simply slipping it onto an appropriately loosened screw. You can use either type. I just fine the enclosed type seems to remain in place more firmly over time.

    Dave
     
  17. ferninando

    ferninando super tek Subscriber

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    Ah ha. I got it.
    I know U and O connectors. O being the ''enclosed'' type.
    I will be doing some modification on my X100B IBAM in near future.
    Thanks again dc.
     
  18. larryderouin

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

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    Which side of the pot (bias) does the wiper connect to, or does it make a difference? That always confused me with 2 leads and 3 connections. Can you show the lines that connect and or don't connect by use of dots at the connections? It looks to me that it all connects at each line, or is it me just being overly dense in this friggin +95*/95%humidity (heat index of 103*) in the Mid Atlantic!

    Larry
     
    Last edited: Jul 1, 2014
  19. ferninando

    ferninando super tek Subscriber

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    I would connect it to the top of the 10KR. All depends on which way you want to turn it, CW or CCW, to increase bias I.
     
  20. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Larry -- Your assessment is correct in spite of your heat and humidity levels! All the lines DO intersect. When I draw a schematic, if one line does not intersect with another, one line will be showing as "jumping over" the other, using a symbol similar to an "n" or upside down U to show the jump. If two lines pass through each other without any such symbol -- as they do on this schematic -- then they do intersect.

    As for which side of the pot do you connect the bias control wiper to, that is purely a preference based on what makes the most sense to you.

    For me, it is connected to the most counter-clockwise terminal, so that turning the control clockwise reduces the resistance of the control in the circuit. So connected, rotating the bias control clockwise then reduces negative grid bias voltage to the tubes, causing the voltage at the TPs to increase, and current flow through the tubes to increase as a result. Turning the control counter-clockwise increases resistance, allowing more negative bias voltage to go to the output tubes, the voltage at the TPs drops, meaning that less current is flowing through the tubes.

    If you connect the wiper to the other terminal of the bias control, its action will be exactly backwards from the above description.

    Open a cool one for me buddy!

    Dave
     

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