Setting bias question

Discussion in 'Fisher' started by walyfd, Oct 10, 2013.

  1. walyfd

    walyfd Well-Known Member

    Messages:
    718
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    Can anyone give me the dumbed-down version of checking/setting bias on an X202 amp? The instructions say to set the tape switch to RECORD, but it has no RECORD switch. It also says to set the "selector to appropriate input position" which would be??????

    And what is the preferred load resistor to use at the speaker terminals?
     

     

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  2. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    I don't have the official Fisher instructions for an X-202, but it sounds like you are using those supplied by Fisher, which typically requires an IM distortion test set to set (in part) the output tube balance control. Without a distortion analyzer, following a portion of Sam's instructions would be the next best thing:

    1. In each channel, the cathodes (pin #3) of the output tubes are connected together, and then connected to ground through a jumper provided. These jumpers need to be removed in each channel, allowing a 10 ohm cathode resistor -- also installed from from the cathodes to ground in each channel -- to then be "in the circuit".

    2. With the jumpers removed, connect your dvm across these 10 ohm resistors (black to the chassis, red to the pin #3 terminals), and adjust the output tube bias control in each channel for a reading of .60 vdc.

    3. If you have no analog FET or VTVM, then using headphones or sensitive speakers, adjust the output tube balance control for minimum hum in each channel with the volume at minimum.

    These controls are interactive within each channel, and between each channel, so perform the adjustments until both the minimum hum and .60 vdc settings are achieved in each channel at the same time.

    When finished, reinstall the jumpers previously removed in each channel.

    If your B+ voltages are running high, then you might want to set the bias for .55 volts in each channel, but otherwise, the procedure remains the same.

    I hope this helps!

    Dave
     
  3. walyfd

    walyfd Well-Known Member

    Messages:
    718
    Location:
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    Thanks. It helps but I still am not sure what function/source the amp should be in for best results on the bias--Aux, Tape Play, Phono...

    Here's the procedure from the Fisher manual.
     

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  4. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Walyfd -- As long as the volume is set to minimum, it makes no difference what any of the control functions are set to when adjusting the output tube bias and balance controls. The point is to make sure there is no signal passing through the amplifier when these adjustments are made.

    The reference to the selector switch setting, mode switch setting, tone control setting, and tape monitor switch setting (in the Record position), etc., is all for making the Phase Inverter adjustment. These controls need to be properly set, to make sure that when the IM test signal from the distortion analyzer is injected into one of the high level inputs, it will in fact make its way through the chosen channel it is connected to because:

    1. It is connected to a high level input that the selector switch is set to, and

    2. It is only going though the channel being tested as designated by the mode switch setting, and

    3. That it in fact makes it through the designated channel to the output, and is not stopped by the tape monitor switch being set in the wrong position, and that

    4. The test signal is not significantly altered by tone controls that are not set to their flat setting.

    Since you are only adjusting the output tube bias and balance controls, the settings of these function controls do not come into play. Just make sure that the volume control is set to minimum when making your adjustments.

    I hope this helps!

    Dave
     
  5. walyfd

    walyfd Well-Known Member

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    718
    Location:
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    AH! Yep, that clarifies things. Thanks!
     
  6. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    Walyfd; Look for a wire from each tube pin 3, that are soldered together (ends overlapping.) (With the unit on it's side, transformer on the low side, the jumpers will be in a vertical plane, between each pair.) DeSolder them so they are separate. (That's the jumper). I had the same problem with NAPTOWN's X-202 18 months ago. Then do the adjsutments, and when done, solder them back together.

    Like Dave said, no signal thru the unit, I use AUX with no source connected. Volume to minimum.

    Find the jumpers and take pictures, and note the jumpers on the pictures, Then write out your procedure step by step, using the pictures and following the FISHER and SAM's instructions. I find this helpful the 1st few times with a new piece of gear. Use the pictures and the written instructions as a dry run, and go thru it a couple times until you are comfortable with it. then do the procedure on the unit.

    Larry
     

     

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  7. walyfd

    walyfd Well-Known Member

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    Thanks again. I'll be getting back to the project today or tomorrow as the Hershey car show was a washout and I came home early.

    Once set on one side, would swapping the left and right tubes require another bias and balance adjustment or would it still be within spec?
     
  8. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    At minimum check the bias and balance. If the tubes are matched, it may be close enough to leave as is. But if not you'll have to do it again. It's cathode biased so swapping them back and forth shouldn't be as critical to the bias as it would be with a fixed bias unit. You WILL have to adjust the DC Balance in any event.

    Just to be on the safe side, anytime you swap output tubes around, adjusting for optimal performance is recommended.

    Larry
     
  9. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Agree with Larry, except (if I may), the X-202 is very much a fixed bias design.

    This is a topic that causes much confusion unless you have the correct "perspective" on what the term "fixed" means as applied to bias systems. Today, many folks are of the belief that if there are no controls to set the bias for the output tubes, then it is obviously "fixed". True enough, but that is not what the term fixed is relating to in this instance.

    In a cathode bias design, there is a cathode resistor that elevates the cathode voltage to be more positively than that of the grid -- making the grid therefore more negative than the cathode, just as any bias system does. The size of the cathode resistor determines the bias voltage developed, which in turn determines how much current the tube draws when no signal (quiescent) is present. When a signal is applied, the tube then draws more current, and useful power output is produced. However, as more current is drawn by the tube, more bias voltage is developed across the cathode resistor, causing the original bias voltage developed under quiescent conditions to be anything but fixed. This fact has a useful result in allowing the quiescent current draw to remain largely the same across a variety of tubes that might be installed, since a tube prone to drawing more current will develop more bias voltage to reduce the current draw, and visa-versa for tubes that tend to draw less current. This is why cathode bias designs are often called "self" bias designs.

    On the other hand, by definition, fixed bias designs relate directly to the bias voltage required to establish a given current flow under quiescent conditions. Making it fixed allows a tube to develop more power output, at less distortion, than an equivalent cathode biased design will. To make it fixed, the cathode resistor has got to go, since the effects noted above cause the quiescent bias voltage to be anything but fixed when a signal is applied. To make it truly fixed then, the cathode must therefore effectively be grounded. That means that to have any bias at all then, the grids must actually be supplied from a negative voltage supply to keep them negative with respect to the cathodes which are now grounded. But now, to cause the tubes to all draw the same identical current flow under quiescent conditions, that means there must also be some means of adjusting the negative grid voltage supplied to each tube to achieve this. That is why fixed bias amplifiers so often employ all the variable controls to effectively set the bias voltage (and therefore the quiescent bias current) for each output tube individually. The controls do allow the bias voltage to be varied to each tube for the purposes of setting the correct quiescent current flow BUT, the take away point is that once set, the bias voltage applied to each tube remains absolutely fixed REGARDLESS of the signal level applied. As was shown for cathode (self) biased designs, that is hardly the case.

    I hope this helps!

    Dave
     
  10. walyfd

    walyfd Well-Known Member

    Messages:
    718
    Location:
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    Well, I'm still doing something wrong. The jumpers are removed; load resistors on the speaker terminals; volume at minimum; function in AUX; BUT, exactly where is the voltmeter supposed to be connected "across the resistor"? I don't want to poke around too much in there with it ON or my bias will be permanently adjusted...
     
  11. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    With your voltmeter on its lowest DC voltage scale, connect the black lead to any convenient place on the metal chassis. Using a clip on your red meter lead (or using a jumper clip lead), connect it to pin #3 of either output tube in the channel you wish to adjust, and then adjust that channel's bias and balance control accordingly. Then, move the red meter lead to pin #3 of one of the output tubes in the other channel, and adjust that channel's controls. You will need to move the red meter lead back and forth between the two points of connection in each channel until all adjustments are optimum. Since the resistors in question are connected between pin #3 of the output tubes in each channel and ground (the chassis), this measuring process will result in you measuring the voltage across these resistors as required for setting these controls properly.

    Good luck with it!

    Dave
     

     

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  12. walyfd

    walyfd Well-Known Member

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    After I CAREFULLY reread everything, it made total sense! I GOT IT! Bias is dialed in at 0.6, all four output tubes have that beautiful orange-red and electric blue aura! I still want to recheck the DC Balance tonight. I was trying to set that with the meter but got no real changes in the numbers so I'm going to do the speaker method. I had it running for a good hour last night and it never sounded better!

    As for the Phase Inverters--do you think I should leave them where they are (bias changed a lot) or go with the suggested half-way points on the pots?

    I can't thank you guys enough! I really never thought I'd be able to recap this thing without the knowledge and help on this forum!
     
  13. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Glad it's working well for you! As designed, there is no good way to use your meter to set the DC balance control. Down the road, a simple modification will in fact allow you to do that, but for now, use sensitive speakers or headphones to adjust the DC Balance control for minimum hum.

    Remember that any adjustment to these controls will require a quick reset of the output tube bias controls, and that if you change the bias control in one channel, then recheck the bias control setting in the other channel as well.

    The Phase Inverter control is best adjusted with proper test equipment, but without those facilities, can be set half way as Fisher suggests. If you're more adventurous, there is a way to set them fairly accurately using your ohm meter as well.

    Dave
     
  14. walyfd

    walyfd Well-Known Member

    Messages:
    718
    Location:
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    Since I seem to be on a roll, what's the ohm meter procedure? I'd like to get this back in the cabinet and leave it there for longer than two weeks at a time for a change. Having the phase slightly out will bother me.
     
  15. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    Putting it back in it's cabnet -- Harrumph. Now what would you want to go and do something practical like that for? :)

    Actually, the Phase Inverter adjustment has nothing to do with signal phase or speaker phase as we commonly think of. Rather, it has to do with the Phase Inverter stage's inherent job that it has to do.

    The power amplifiers in your Fisher are what is known as "push-pull" amplifiers, which employ two output tubes in each channel, both sharing the production of power produced. Most of the time, they share this responsibility equally, one at a time, back and fourth so to speak. But they do it with each tube operating perfectly out of phase with each other. They do this so that when their signals are recombined in the push-pull output transformer, any harmonics of the original signal developed in the process of amplification are largely canceled out, leaving only the fundamental signal again. In other words, a very low distortion signal is produced in the secondary winding of these transformers to drive your speakers.

    Since the output tubes operate out of phase with each other, they require a drive signal to them that is out of phase with each other as well. That's where the phase inverter comes into the picture. It supplies two out of phase drive signals -- one to each output tube, so that the push-pull action takes place in the output stage. But to make the output tubes share the production of power equally, they need equal, output of phase drive signals presented to them -- and that's where the phase inverter control comes in. It is used to adjust the two out of phase drive signals developed in the phase inverter stage so that each is of the precise amplitude relative to each other, to create maximum distortion cancellation in the push-pull output stage. That is why it is best to adjust this control with distortion measuring equipment.

    Without such equipment, one very good compromise setting is to adjust the phase inverter control, so that it is providing exactly equal (in amplitude) drive signals to the output stage. This will be a good close setting, but not necessarily the best setting, since slight differences in output tubes and related components can (and usually does) require the drive signals to be slightly different to in fact produce minimum distortion in the output. Still, a setting of equal drive is hardly a bad position to use.

    Fisher changed the circuit of the phase inverter used in the X-202 a number of times, so at the very least, I will need the serial number of your unit, while a good under the hood shot of these circuits would help as well.

    The ohm meter procedure is not listed anywhere by Fisher, or otherwise. However, it is a method I've developed and used to help other employ to very good satisfaction.

    Let us know!

    Dave
     
  16. walyfd

    walyfd Well-Known Member

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    Location:
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    Of all the things I never bothered to carry around is the serial number. I'll get it tonight and report back.

    And why put it back in the cabinet--I'm afraid I'm going to drop it one of these times...
     

     

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  17. dcgillespie

    dcgillespie Fisher SA-100 Clone Subscriber

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    And there ya go. That's as good an excuse as any just to leave it out!

    Dave
     
  18. walyfd

    walyfd Well-Known Member

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    718
    Location:
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    That's what the 500B is for. All-in-one and more easily portable...

    My SN is: 317678.
     

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  19. larryderouin

    larryderouin Turn it UP, POP? PLLUUEEEZZZZZEE Subscriber

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    walyfd; Bets are that last "8" is actually a "B". I've never heard of a FISHER serial # going as high as 300,000. Nor even as high as 100,000. 31767B makes more sense in the whole scheme of all things FISHER.

    Larry
     
  20. walyfd

    walyfd Well-Known Member

    Messages:
    718
    Location:
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    I think you're right. The panel with the number is really corroded and I can hardly make out more than one number. The "B" could indicate the console version of the amp.

    I tried the DC adjustments again with speakers and I don't notice any change in turning the pots. I did turn the Phase Inverters closer to mid position and turning the Phase Reverse switch made a far less noticeable change in sound.
     

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