Traynor YBA-1A mk II: adjustment question

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by traynorbassmate, Oct 5, 2008.

  1. traynorbassmate

    traynorbassmate Member

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    Hello all, i'm a first time poster, I saw another thread about Bias adjustments pertaining to this same amp but did not find any answers to my problem.

    I opened up my friends YBA-1A Mark II to have a glimps inside. Everything works but the pots needed a cleaning and I was curious to see if any mods had been made - this amp's history is a bit shady with its current owner.

    I read that old Traynor amps never had bias adjustments, but I found a trim pot inside the chassis which apparently controls the screen voltage.

    Please see Yorkville's website:

    http://www.yorkville.com/default.asp?p1=6&p2=121&p_id=116

    and select the YBA-1A from the drop down list to view its schematics. The later versions mention "Note 7: Adjust R7A to allow 4 VDC across R25 (470ohm) screen resistor"

    Please see my pictures: (1) full view, (2) R7A pot, (3) R25 (top) and R38 (bottom)

    [​IMG]

    [​IMG]



    [​IMG]


    The instructions say to adjust the voltage across R25. When the pot is fully clockwise (max) I get a reading of 1.5 VDC across R25 (green-green). Out of curiosity I also measured from R25 - R38 (Red-Red). At this location, I was able to get a 4VDC reading with the pot in its center (~12 o'clock).

    Am I doing something wrong? I wouldn't expect the schematic instructions to be incorrect...any hints/tips/tricks regarding this adjustment?

    Thanks!!
    Adam
     
  2. skipm45

    skipm45 Member

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    Denver
    Per your post I pulled down the YBA-1 schematic and there is indeed no bias adjustment shown, However I've read that there was a bias adjustment in theis amp.
    That said, to bias this amp you would apply a (-) voltage (from the negative supply consisting of D1, C28, R38, C29, and R39) to the output tube grids at the R34 / R25 junction.
    The screen voltage is fixed by R30, and in this type of design is the "B" supply voltage ("A" being the OT center tap, "C" being the PI plates etc.).

    To bias this amp you can use a tool like this:
    https://taweber.powweb.com/biasrite/br_page.htm

    and this bias calculator:

    http://www.webervst.com/tubes1/calcbias.htm

    Plug in the correct values in the calculator,hook up the bias-rite (or similar tool) and dial in the bias adjustment pot.


    HTH

    Skip
    www.skipzcircuits.com
     
  3. traynorbassmate

    traynorbassmate Member

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    thanks for the suggestion - however Screen voltage and bias are two separate things, are they not? I'm not really interested in adjusting the bias with an external tool and since this amp has no adjustment i'm not willing to invest in adding one.

    My problem is the discrepency between the schematic note (4vdc) and my reading (1.5vdc) across R25. Just wanted someone to double check my measurment procedures.
     
  4. skipm45

    skipm45 Member

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    Sorry if I wasn't clear. Frankly I'm as confused by their description as you are.
    Yes, Screen voltage and bias are two seperate things entirely.

    A simple and cheap way to bias this amp would be to install a 1 ohm 1% resistor in series with pin 8 of V4 and V5 (2ea) . Then a direct voltage measurement across the resistor would equal cathode current (in mA).

    The Weber bias calculator can then be used to determine the correct bias point based on plate voltage. Simply measure the voltage and tweak the pot to dial it in.

    Skip
    www.skipzcircuits.com
     
  5. Dave C

    Dave C Gold Supporting Member

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    Keep scrolling down the schematics , there are more than one and only the first that uses 7027 tubes shows no bias pot...the rest show the R7A bias pot using EL34/6CA7 tubes. I wouldn't use their biasing method, they are measuring screen current by reading voltage drop across the screen resister and hoping the resister is accurate and both tubes are fairly well matched. I wouldn't trust it.

    I'd install the 1 ohm (1% tolerance) resisters to pin 8, use a bias probe, use OT voltage drop method or OT shunt method. Their idea of measuring the screen current over both tubes at once seems to me to be akin to inflating your cars tires by measuring the bumper height with no regard to spring sag or equal size tires on each side......or maybe I've just never seen or used this method before.
     
  6. phsyconoodler

    phsyconoodler Member

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    I agree.Install a 1 ohm 3 watt resistor from pin 8 to ground,replacing the ground wire already there.Then simply measure the millivolt reading across that resistor.It translates directly to milliamps via the 1 ohm resistor.
    Then you can adjust the bias pot shown in the second picture,and you are set to use either 7027's,6L6's.
    Looking at the age of that amp,it would be adviseable to change the electrolytic filter caps and the bias supply caps.
    They are a good sounding amp when working properly.Keep a close eye on the tube sockets.They are prone to carbon tracing because of tube failures.Guys often just popped any old tube in those amps and never checked bias,and as a consequence,blew out tubes and ate screen resistors.
     
  7. phsyconoodler

    phsyconoodler Member

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    What exactly is accuracy where a tube amp is concerned?So you are a few milliamps off either direction.As long as it isn't loafing or biased too hot,what is all this 'accuracy' you are fretting about?

    There are other methods of checking bias and they all work.
     
  8. traynorbassmate

    traynorbassmate Member

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    Thanks for the suggestions everyone.
    The amp is off my test bench, back together and back in my friends possession. So everyone PLEASE stop using the word bias ;)

    I had ZERO intention of going through the trouble of biasing this amp (I'm not the kind of person to fix something until it breaks. Knowing me, i'd probably break it before the end of the day, and it wasn't my amp to begin with. Fret not! i'm reading a book about valve amps). Furthermore, the amp was running on mid-80's JAN 12ax7's and Sovtek EL34/6CA7's which have been in the amp for at least 10-15 years, and it sounds great, LOUD, with volume set at 2 or 3 (daisy-chaining channel 1 to 2). I only took a crack at it to clean the pots and tube sockets, and have a looksy for curiosity's sake.

    If Pete Traynor at the time did not include a bias adjustment or a kit to add one later on, and the amp runs...well... I can only use the term 'workhorse' to accurately describe it, as it has been since day 1, then i'm really not gonna bother unless a problem arises. I just wanted to test/adjust the only 'user adjustable' segment of this stock amplifier, which was Screen Voltage. Every single post brought up the word Bias when this adjustment has nothing to do with it. I was simply curious as to why my reading was way off across R25, (maybe caused by a bad resistor, diode, or capacitor along the way??). I understand that this method and others, it seems, are not 100% accurate but I'm not really picky. A ball park figure is good enough at the moment.

    As I said, i'm reading a book about this wonderful world, maybe later i'll be able to understand what you guys are saying. But I've learned (woohoo!) that screen voltage and bias are not the same thing.
     

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