Scott 340B strange bias readings.

Discussion in 'Tube Audio' started by Destructor, Jan 13, 2018.

  1. Destructor

    Destructor Super Member

    Messages:
    1,467
    Location:
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    Using an Extech digital multimeter will redplate output tubes if I attempt to set each output pair at 70ma. My old analog meter I could set at 70ma with no problems. Receiver is in top notch working condition and is restored. With my digital meter I set at 35ma, amp sounds good.

    My 340B holds bias very well and only drifts slightly between bias resets. The first time I used my digital meter it read 1/2 of what bias should be using the 2 test points, attempting to set at 70 redplated. If It was truly only set at 1/2 it would sound anemic and unpleasant. It sounds very good at approx 34 ma. It is almost as if it only reads the draw of one tube. My old analog meter has since been smashed in a fall so I have nothing to double check with.

    I have no problems setting bias to spec on my single ended 300B amp with the digital meter. My 340B does not have to get too far off bias before I can tell something isn't right. Currently both test points are holding steady at around 34 ma with very little drift and the receiver sounds fine.

    Any Ideas why I get these readings. The receiver is not a daily driver but I rotate it into duty a couple times a year for a month or more.
     
  2. Dave451

    Dave451 AK Subscriber Subscriber

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    Location:
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    Older current meters act as a dead short when you measure from the top test points, bypassing the 3.3 ohm resistor that grounds both cathodes in the tube pair. Newer digital meters may not do this and you can get erroneous readings when trying to set cathode current by directly measuring the current. An alternative way with your new meter would be to measure voltage at the test points. If you want 70 mA per tube pair, as Scott recommends, then Ohm's law says you should see voltage = current x resistance, or 0.07 A x 3.3 Ohms = 0.231V, or 231 millivolts. I presume your unit is stock with the 3.3 ohm resistors still in place. Why don't you measure the voltage and report what you get?
     
  3. Destructor

    Destructor Super Member

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    1,467
    Location:
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    My 34ma reading corresponds to .27 volts each side. Too high
     
  4. Destructor

    Destructor Super Member

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    1,467
    Location:
    Braintree, Mass
    Readjusted to .23 volts corresponds to around 30ma on my meter.

    The ma reading on my meter gives a reading of a lttle less than half of spec.

    I'll wait a half hour or more then recheck.

    Thank you for the help.
     
  5. petehall347

    petehall347 the brandy coffee man

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    what model number is your meter ?
     
  6. Dave451

    Dave451 AK Subscriber Subscriber

    Messages:
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    Location:
    Philly area
    I was not able to set bias for my 340B by directly measuring current with either of my digital multimeters. If your 3.3 ohm resistors are good, then 231 mV has to be right. I'd set it by voltage. 0.27 volts would translate to 82 mA, or 41 mA per tube. High, but not red plating territory.
     
  7. Dave451

    Dave451 AK Subscriber Subscriber

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    842
    Location:
    Philly area
    You can also check your cathode resistors by measuring resistance between the test points and ths chassis.
     
  8. Destructor

    Destructor Super Member

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    1,467
    Location:
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    I set it at .22 volts and left it there. From the last time I set it I recently noticed bass decreasing slightly and sounding a bit flat. Bass is back along with that nice Scott sound and soundstage. My meter doesn't have a model number on it.
     

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  9. Destructor

    Destructor Super Member

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    Now I can have a beer and listen to the devils music!
     
  10. petehall347

    petehall347 the brandy coffee man

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    time for a better meter . swing meters are more suited to this sort if thing but some digital ones will do the job .
     
  11. Destructor

    Destructor Super Member

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    Yea, I had a nice one with a big needle. This one does seem to measure my single ended amp fine.
     
  12. petehall347

    petehall347 the brandy coffee man

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    i know some meters are only supposed to be used x amounts of seconds in current modes .
     
  13. Tubologic

    Tubologic Well-Known Member

    Messages:
    633
    Use only the VOLTAGE measuring method, as previously recommended. Do NOT use a mA meter because the 3.3 ohms cathode resistor will be in parallel with the meter internal shunt resistance, introducing a considerable error especially on cheap DMM's having high internal shunt resistance on the 300 mA range. (sometimes in the order of 1 to 3 ohms !). The output tubes current being split berween the 3.3 ohms cathode resistor and the mA meter internal shunt the measured value will be LOWER than the actual current through the tubes.
    0.23 Volts DC measured at the TP is OK and equivalent to the (recommended) TOTAL plate current of 70 mA but it doesn't tell you how well the plate currents are balanced between the tubes as there is only one common cathode resistor shared by both tubes, overloading is still possible if there is a serious plate current unbalance. Thus you must also proceed to the DC BAL adjustment according to the manual (removing the phase splitter tubes and adjusting for minimum hum at the amp output), then go back and re-adjust bias and repeat the whole procedure once more if needed.
    You should also check and measure if the 3.3 ohms cathode resistors haven't drifted in value, these are old carbon comp resistors and may have been subjected to many overloads during the amp's life. It's recommended to replace them with new good quality 1% /1W ww resistors.
     
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  14. gadget73

    gadget73 junk junkie Subscriber

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    yes, measure voltage across the resistors. Also make sure the resistors are in spec, otherwise the voltage measurement is not helpful.
     
  15. Destructor

    Destructor Super Member

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    Location:
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    Next time I have it opened I'll replace the resistors and be done with it. I did stumble upon a guitar amp site that mentioned shunting resistors in newer meters. I suspected I could confirm my suspicions if I posted the question. It is a really nice piece of gear. Thanks for the tips.
     
  16. gadget73

    gadget73 junk junkie Subscriber

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    for that spot I'd probably spring for some 1% precision resistors just so you know its good. Hard to hand match that low a value with a typical meter. They just don't have the resolution.
     
  17. Pdow

    Pdow New Member

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    Location:
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    Tobologic you mention a manual.
    Is there a service manual for 340b ?
    Would like to find one. Particularly the parts list that corresponds to the schematic parts.
    Paul
     
  18. Tubologic

    Tubologic Well-Known Member

    Messages:
    633
    Scott never issued complete service manuals with detailed partslists, only "Service Bulletins" which included schematics, simplified alignment instructions, electrical specs, and a short list with specific Scott parts and their factory ordering # (not very useful today)

    These infos are already uploaded here on the AK Database and both versions can be found on this page:

    http://akdatabase.com/AKview/thumbnails.php?album=89&page=3

    afaik, SAMS never issued a Photofact for the Scott 340B
     
    Last edited: Jan 18, 2018 at 1:29 PM
  19. N8Nagel

    N8Nagel AK Subscriber Subscriber

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    I have x-ref'd some of the cap numbers on the 340b parts list, although I would not take that info as gospel as I have yet to actually rip into my unit in anger.
     
  20. Dadbar

    Dadbar Super Member

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    1,821
    Location:
    Waltham & Gloucester MA
    Check the battery on your DVM.
    My Fluke 70 series meter sometimes gets wonky when I am trying to measure unusually inductive loads. I keep a classic Simpson 260 analog meter around to double check weird measurements.
     

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