59 Bassman LTD - bias procedure?

Discussion in 'Amps and Cabs' started by JimEff, May 5, 2009.

  1. JimEff

    JimEff Member

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    I bought a used 59 Bassman LTD :drool
    I've got a bias probe, I've got a multimeter, I know how to use it, and I know how NOT to shock myself to death :huh

    Does anyone know what bias current I adjust to, with the 6L6 tubes?

    Btw, any tube suggestions for the Bassman? (JJ, Tung Sol, etc)

    I'm wondering why Fender doesn't provide this biasing info in the manual....
     
  2. JimEff

    JimEff Member

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    Bump. No '59 Bassman LTD owners out there?
     
  3. sqadan

    sqadan Member

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    Free bump on this... as I'm curious too.

    I have a Bassman LTD and would like to get rid of the Groove Tubes soon...

    I've heard good things about Winged C / SED 6L6's BTW.
     
  4. JimEff

    JimEff Member

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    I guess the amp-tech union wants to keep it confidential:rotflmao
     
  5. deepbluez

    deepbluez Member

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    I'll tell you what has worked for me, with my understanding that a safe, maximum bias current for the bassman is roughly 40ma per tube -

    Clip ground lead from meter to any point on chassis; clip positive lead to bias point shown in photo below. The indicated current is for both tubes, so divide by 2. (60mv could be 30ma per tube or any combination that equals 60 - you'll need a bias meter to precisely measure each separately)

    I set bias to what sounds best to my ear - while not exceeding the safe maximum - and generally end up somewhere around 31 - 32ma. Hotter than that, for me, rarely sounds better. Be careful...and when in doubt, see a tech.

    Oh yeah, try a RCA 5R4-GYB rectifier tube in this amp for some warmth and a sweet, earlier breakup!

    [​IMG]
     
  6. JimEff

    JimEff Member

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    Thanks Deepbluez. Is that a typo? (see your statement that I've quoted above). Don't you mean "60mA" not "60mV"? I assume we are measuring DC milliamps here, right?
     
  7. JimEff

    JimEff Member

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    I've heard a tube vendor say that the Bassman RI can be biased as high as 42-44 mA for a "hot" setup. ???
     
  8. deepbluez

    deepbluez Member

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    Yeah, typo it is Jim.

    At 42 to 44ma your tube life will be shortened considerably, methinks. Then again, if that gets you where you want to be tone-wise, have at it!
     
  9. PeeVee

    PeeVee Member

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    Even though Deepbluez agrees it's a typo, I don't think it is. In this case, the kathode of the 6L6 is grounded via a 1 Ohm resistor. The Voltage that drops over this resistor equals the current that flows through it (Ohm's law: Voltage divided by current equals resistance. When resistance = 1, the Voltage will be equal to the current).

    When measuring bias using a 1 Ohm kathoderesistor, you should be measuring mV's, not mA's. If you wanted to measure mA's, you would have to put your meter in series with the cathode (or anode).

    Note that you are not measuring true bias (= anode current) but you are measuring the total current that flows through the cathode which is the sum of the anode current and the grid current. A healthy tube will draw about 10% via the grids so when you measure 44 mV (meaning a total current of 44 mA), the anode will draw about 40 mA and the grid will draw about 4. In other words, when you measure via a cathode resistor, you can adjust for 10% extra.

    The correct bias current is depending on the anode Voltage. In a typical reissue Bassman, this will be somewhere between 400 and 450 Volts. In a class AB push pull amp, you bias a tube at 60% - 70% of the Pa max (the max power the anode is allowed to draw). For a good 6L6GC tube (e.g. SEd, JJ) the maximum anode power is 30 Watts. If you bias at 60% of this power, you should divide 18 (Watts) by the anode Voltage e.g. 450 Volts. In this example you should bias at 0,04 Amperes = 40 mA. Considering the fact that you are not only measuring the anode current but the sum of anode plus grid, you can safely add 10 percent so 44 mA would be OK here. Note that biassing at 60% power is on the safe site, many people bias at 70%. While there is hardly any difference in sound, but lower bias will increase tube life drastically, I always bias at 60% but if you want a "hot" biassed amp, you could go up to 47 mA (70% of 30 Watts divided by the plat Voltage). The lower your anode Voltage (also often referred to as "B+"), the higher bias current is allowed. I like amps that run at much much lower Voltages (my DIY low power tweed twin is running on 320 VDC); in these amps, the bias current can be around 60 or 70 mA.

    Assuming the Bassman does not exceed a plate Voltage of 450 VDC, the 42 - 44 would not even be "hot", it would be the normal / correct setting where the 6L6 is biassed at 60% of max dissipation (take into consideration that you measure 10% extra). 44 mV against the 1 Ohm resistor would mean a anode current of around 40 mA which is fine.



    You ARE measuring mV's.
     

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