bias dilemma

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by mlynn02, Jul 14, 2005.


  1. mlynn02

    mlynn02 Member

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    i'm trying to bias my super reverb and i'm having an interesting issue. i just bought some matched 6l6's from mike at kca and i've got a weber bias rite to measure the current. with the old tubes in, the 1st one measures at 40.1 and the 2nd one measures at 28. and then with the new tubes in i get similar numbers. why are they so different? what is going on and what should i do about it?

    thanks,
    matt
     
  2. Swarty

    Swarty Member

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    Swap positions and see if the specs follows the tubes or stays at the sockets.
     
  3. mlynn02

    mlynn02 Member

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    the numbers were a little different when i swapped the tube sockets. but i also swapped the leads on the bias rite, so it could be that too. however, the tubes are still not measuring together, they are about 4 ma apart (10-15%).

    presumably, if the tubes are truly a matched pair, they should measure equally regardless of where the bias is set. right?

    what kind of things can cause two matched tubes to measure different currents in the amp?


    thanks,
    matt
     
  4. Swarty

    Swarty Member

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    40 and 28 would be a mismatch of 12. If it is 4mA, that is matched in my book.
     
  5. TheAmpNerd

    TheAmpNerd Member

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    Is it is a weber bias rite?

    If you put the tubes in, then what you need to do
    if it is a silver face amp... is up in the chassis
    behind the choke (the smaller of the three trannys)
    is a pot that you adjust with a screw driver.

    It is probably set up to be a balance pot,
    that will balance the output tube bias.

    Balance it out until both tubes read the same
    on your bias rite.

    You should be okay.

    Now, if your readings on both tubes just
    increase or decrease, then it could be
    a challenge inside your amp on the
    output tube socket resistors, the final coupling
    caps or the voltage divider resistors.
    (Yes even though you can have perfectly
    matched tubes...this can affect bias).

    So try that and then let us know what the
    readings are, we'll gro from there.

    Good luck.
    (edit added)
    Also know that Mike over at KCA tubes is a stand up
    guy, so you don't need to worry about that, he'll
    treat you right.

    BTW..you typically want your output tubes to match
    within 1 mA of each other.
     
  6. Wakarusa

    Wakarusa Member

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    As our friend from Texas notes, we have to know what year Super Reverb you're working with in order to know if it has a balance or a level bias adjust.
     
  7. TheAmpNerd

    TheAmpNerd Member

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    Hey Todd,

    I don't know if I mentioned this or not.
    Your web site is coming along nicely.

    Good for you!
     
  8. mlynn02

    mlynn02 Member

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    the pot causes all of the tubes to change in the same direction. it's a '69, but it's been blackfaced.

    does that help determine whether it's bias or balance?
     
  9. Swarty

    Swarty Member

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    If it causes all tubes to move in the same direction it is a bias pot.
     
  10. TheAmpNerd

    TheAmpNerd Member

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    Do you have any other tubes?
    If you place them in the amp
    how do they read?

    Also, I'd give Mike a call and work this through
    him.

    It is one of the things,

    Amp
    Tubes
    Bias Rite.

    You can measure the resistance of pin 8 (and
    the other plug end)on both BiasRite sockets.

    If same, then BiasRite is okay.

    Then if you have a known good set of tubes
    try them. what are their readings in each socket.

    Thinking back to what you said. ONLY change one thing
    at a time. If you move tubes around, only do that,
    don't change anything else (swapping BiasRite leads).

    You didn't say what the readings were after you
    moved the tubes to different sockets either. Just
    that they got closer.

    Good luck, you and Mike will figure it all out.
    Keep us informed.
     
  11. hasserl

    hasserl Member

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    Considering that the Bias Rite uses 1 ohm resistors on the cathode pin to measure the voltage drop across, it is rather difficult to measure this on both probes to compare. The typical DMM cannot accurately measure 1 ohm. Heck, a 1 ohm resistor w/ 1% tolerance theoretically could result in a difference of 20% between two different probes. And you couldn't tell that by measuring the resistance with your ohm meter, because the sensitivity at 1 ohm is not great enough. Unless you've got a very expensive meter, which most of us don't.

    That's why I don't use a probe to measure cathode current, IMO the accuracy is questionable. I prefer to measure plate current.
     
  12. justonwo

    justonwo Member

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    KCA NOS tubes matches them to within 3 mA, so 4 mA isn't that far off.

    KCA Tube Info
     
  13. Blue Strat

    Blue Strat Member

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    Sounds like a bias "balance" pot. Guess we'll have to wait until he reports back.
     
  14. TheAmpNerd

    TheAmpNerd Member

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    Wow

    Matched to 3mA? Well I guess the secret is out folks.

    I'm anal when it comes to amps and tubes.

    All my stuff I do a 24 - 48 hour burn in and match
    to with in 1 mA. I only do this during the winter.

    Hasserl, yeah I've got a low ohms meter too.
    Figured he could at least see if there was
    a problem, like a faulty resistor or solder
    joint causing a mis reading.

    How is your preferred method of measuring
    plate current accomplished?
     
  15. hasserl

    hasserl Member

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    I use a probe that shunts the plate current thru the meter.
     
  16. TheAmpNerd

    TheAmpNerd Member

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    Oh, just like that?

    Don't you have to do anything else?

    Or is your amp set up for that already?
     
  17. hasserl

    hasserl Member

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    No, you don't have to do anything else. It has nothing to do with the amp, you could do this on any amp. It's a probe, just lke the cathode current probes, but it shunts the plate current thru the meter. You use it the same way as a regular probe, but you set your meter to read current not volts.

    Some people do not like doing this due to the exposure to high voltage / current. That is why the cathode current style probes are more popular.
     
  18. Wakarusa

    Wakarusa Member

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    There's also the possibility that he's got imbalance in the output transformer.

    If his description of the bias control is accurate (everybody moves the same way), then it's a level, not a balance. If the tubes exhibit the same behavior when swapped left for right and vice versa (but don't swap the bias meter sockets) I would look to the OT.
     
  19. Wakarusa

    Wakarusa Member

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    Thanks man. Still have a huge amount of work to do tho'. Still surprises me how much of an amp company got nuthin' to do with designing circuits.
     
  20. John Phillips

    John Phillips Member

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    I don't like this method.

    It's quick, but has slightly questionable accuracy, and it's DANGEROUS - because the meter is set to read current, its internal resistance is very low, and if you slip with the probes or connect them to the wrong pin you will cause a direct short on the plate voltage - via the meter and possibly the OT depending on what touches what, to ground or somewhere else, like the filament winding in the PT. At the very least you'll blow fuses or damage the meter, the the worst you may seriously damage the amp. Even if you don't make that kind of mistake, it's not 100% accurate since bypassing the OT primary affects the circuit conditions and increases the current draw slightly - and in some circumstances can make the amp unstable and oscillate, which not only screws up the readings but could cause damage.

    The cathode-current (bias probe/built-in resistor) method is at least safe, but includes the screen current too, so it is also not accurate.


    But there is a truly accurate method which is not too dangerous. Instead of bypassing the OT primary with a current meter, read the voltage drop across it. This is a lot safer, since the meter has very high resistance, and unless you actually short two pins together with the probe tip you can't do any damage.

    Then, once you've measured the voltage drop, turn the amp off and measure the DC resistance of the primary. Check both sides, they may be slightly different. By dividing the voltage by the resistance, you have the true plate current. Obviously this is a little slower and involves a calculation - although you only need to measure the resistance once, and you can do the calculation backwards to aim for a specific voltage drop if you want a particular current.


    It's also important to remember that with either 'transformer' method, you need to be very careful since you're measuring at points which are at the full plate voltage above ground, and even if you don't short out the amp you could potentially electrocute yourself.

    FWIW, I have given myself a few nasty shocks over the years (as most techs have), and once turned a quite nice multimeter into a small firework while metering primary current directly. And I'm quite careful and experienced. Accidents happen.
     

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