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SVT-II problem...no reading between bias point 2 and ground

Discussion in 'Amps and Cabs [BG]' started by Whoopysnorp, Sep 12, 2008.


  1. Whoopysnorp

    Whoopysnorp

    Feb 5, 2006
    I recently noticed that my SVT-II's volume was considerably lower than it should be (as in I'd turn it up past halfway before it started to get kinda loud, and it would be pretty heavily overdriven by that point). I examined the bias today and found that I'm not getting any reading between ground and bias test point 2. Between ground and point 1 I was getting around 50 mA, and it's reading the same between test points 1 and 2 (it's supposed to read 0 +- 0.01 there, according to the instructions). I moved the tubes around to see if the problem would follow them, but to no avail. Does anybody know what could be wrong?

    By the way, what do the two bias test points/adjustments correspond to, anyway? Is one of them for the push side and the other for the pull side, or what?
     
  2. Bassmec

    Bassmec

    May 9, 2008
    Ipswich UK
    Proprietor Springvale Studios
    Well I don't know much about the SVT 2 but it stands to reason to have
    provided two bias controls one will be for push and the other for pull.
    My old SVT had six.
    Change the grid bias resistors out for vishay ceramic wirewounds anyway and measure again.
    Sounds like you have either hardly any push or hardly any pull.
    Check it out with a scope it will be all hills and hardly any valleys.
    Or the other way up.
    Good luck mate!
     
  3. Whoopysnorp

    Whoopysnorp

    Feb 5, 2006
    Does anybody know where I can get a schematic for this amp?
     
  4. You have a either a MAJOR tube or amp failure. No schematic should be necessary to find and fix it, so by asking for one, you've shown that you have no business inside the amp. Take it to a professional who has the proper expertise and equipment to repair your amp.
     
  5. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    SAme idea, not as direct as PBG...... Schematic is no problem, I can help with that, but I suspect you might be better off staying OUTSIDE the chassis....

    BTW, you should have measured "millivolts" on the bias point, not "milliamps".
     
  6. Whoopysnorp

    Whoopysnorp

    Feb 5, 2006
    It's just as well; the boards are mounted with the components facing upwards so you have to undo a ton of stuff to get them out of there. I'll just haul it to a shop.
     
  7. Hoss

    Hoss

    Jan 9, 2006
    Vienna, Austria
    By the way: What does this Voltage stand for? I am supposed to dial in 75mV for both "sides" (is one for them for the right triplet and the other for the left triplet or is it indeed for push and pull?), but what if I dial in - lets say 150 mV?

    Is it then biased hotter and puts out more power?

    I ve never seen an SVT with SIX bias controls?
     
  8. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    The bias sets the idle current in the set of tubes to the right amount. Enough to avoid "crossover notch" (a poor transition from push to pull), and NOT enough to burn up the tube from too much power.

    150MV would be trying to cook the tubes.

    2 bias controls explains why the output tubes must be matched...... that way they all share current evenly. In reality, only each bank needs to be matched within itself, but generally people buy 6 matched, because they come in pairs.

    "One matched tube" has little meaning unless you have a "grade number"..... i.e. you want another "grade 3 " tube. Even then, an older "grade 3" is not the same as a brand new "grade 3".
     
  9. Hoss

    Hoss

    Jan 9, 2006
    Vienna, Austria
    Thanks Jerrold!

    So the voltage adjusted equals the current-flow thru the tubes (against a 1 Ohm Resistor)? Like if I adjust 72mV for the left triplet (=push?) then its 72mA at something like 700Volts for three tubes, making 24mA per single tube, if they are perfectly matched?
     
  10. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    In the SVT-CL the resistor is larger, not 1 ohm. Each tube has a 10 ohm, meaning the total is about 3.3 ohms, which is why the bias reading is larger than the old SVT.
     
  11. Hoss

    Hoss

    Jan 9, 2006
    Vienna, Austria
    Hi Jerrold,

    I am writing about the old SVT, not the CL. ;) I am asking because if my guess is right and each tube is biased at 24mA, then even with the high plate voltage at 670 Volts this is a rather conservative setting, dont you think?

    I got this BIAS tutorial with a formula calculating the BIAS current: http://www.tube-town.net/diy/bias-setup-e.pdf

    Following this formula and assumed the plate voltage is 670V (I havent measured that yet) than I got the following results:

    SED 6550: min. 26mA/ max. 36mA
    EH 6550, SED KT88 and JJ KT88: min. 31mA/ max. 44mA

    This would make a min. current of 93mA and a max. current of 132mA per side for the JJs, the SED KT88 and the EH 6550/KT88. Thats a bit more than the suggested 72mA... :confused:

    PBG did send me a very detailed Bias tutorial (Thanks for that!) and I am sure that this is the OPTIMUM procedure for biasing an SVT, but for this I would need to measure distortion with a scope, signal generator, etc...

    So if I dont have all this stuff, can I just APPROXIMATE to the max. output (while not frying the tubes) with using this calculated values?

    Thanks a lot!!!! :cool:
     
  12. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    The suggested bias point is the best one in general.

    If you look at the suggested bias, the minimum settings are all about 1/2 tube dissipation. the max are all about 0.7 x dissipation.

    Given that an old SVT has a no-load voltage which may be closer to 725V, especially at a high mains voltage, 72 ma total gives 17.4W dissipation, vs 35W, or right "there" with 1/2 of allowable dissipation.

    Then also, "matched" tubes are just "fairly close" to each other. There is a range of acceptable difference. So among the three, one may be "hogging current" and be dissipating significantly more than the others.

    The "cooler" bias allows for variations and drift of matching over time, drift of actual tube characteristics over time, etc.

    Your choice, I know what I'd do
     
  13. Hoss

    Hoss

    Jan 9, 2006
    Vienna, Austria
    Thanks, Jerrold! :)
    Good point, the imperfect matching of tubes. :meh:

    I think I am gonna take a compromise of 60% of tube dissipation.
    At 725 Volts plate voltage that would be 34mA per tube with the SED KT88 which makes 100mA(V) per side.
    I will switch between the suggested 72mV and 100mV and then simply check out if theres a noticeable difference in sound.

    I am wondering why there hasnt been a thread like that before. Or may I just havent found it? I know there are loads of 70s SVT-Users here. Am I the only one trying to optimise its bias settings? Ok... :smug: except of Psycho Bass Guy who uses his pro tools like scope or whatever...
     

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