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Biasing Output Tubes

Discussion in 'Amps and Cabs [BG]' started by bucephylus, Apr 1, 2006.


  1. bucephylus

    bucephylus Supporting Member Commercial User

    Aug 18, 2002
    General Manager TecPadz LLC
    I have seen a number of posts discussing the importance of biasing output tubes to get the correct plate current, and I understand the concept. However, I pulled down the schematic for the T-E Quatra/Hexa heads, and it appears the plate bias is fixed by resistors that are hardwired in. They appear to have set the bias voltage at -42V and it appears fixed.

    When re-tubing, are you supposed to solder in new bias resistors, re-valued for the plate current? That seems ridiculously complicated. If that were required as SOP, they should have just put in trim pots.

    What am I missing here? Thanks.
     
  2. Tom Crofts

    Tom Crofts

    Mar 15, 2001
  3. bucephylus

    bucephylus Supporting Member Commercial User

    Aug 18, 2002
    General Manager TecPadz LLC
    Thanks. I'm certainly aware of that thread. I was intending to ask the question relative to output tube biasing for tube amps in general. I am trying to get a feel for the importance of the plate biasing beyond what the manufacturer wires in. Is it a "must have" mod to add trim pots for tube changes?

    I went ahead and plugged in a set of 6 matched Sovtek 6550WE's from Tube Depot, and it sounds excellent. I just want to make sure I'm not doing something stupid that will burn the tubes out in a week.
     
  4. You are 100% correct. In ANY nonadjustable fixed bias amp, if the bias need adjusting you have to swap resistors until you get the value that will give you the voltage drop for the proper amount of bias voltage. It's funny because my VA350 is almost the same amp as the Hexavalve, but it DOES have trim pots for bias adjustment.

    A manufacturer's reasoning for using a fixed resistor is usually twofold: first, the amp should not be damaged by user or incompetent "tech" set bias that is too low and second, the value they select is usually pretty conservative in order to prolong tube life and cut down on warranty claims. Yes, this does cut down the amount of output power, but they can get away with using lesser quality tubes. (cough... Mesa Boogie... cough)
     
  5. bucephylus

    bucephylus Supporting Member Commercial User

    Aug 18, 2002
    General Manager TecPadz LLC
    Thanks very much for your usual expertise, PBG. I assume this also means that the ouput will lean towards "clean" as opposed to distorted?

    This simplified setup is OK with me. I'm very satisfied with the clean tone and certainly don't need any more volume.
     
  6. Generally, no, but it is also possible that if the static value is too low then the tubes will run hotter and "cleaner." Up to the point of output transformer core saturation (usually a non-issue in high powered bass amps because their OT's are so big and have plenty of inductance) the more plate current you can get out of a tube, the more clean power it will produce, but if you exceed the emission limits of the tube (cheryyplating) then the tube will have greatly reduced life.
     
  7. bucephylus

    bucephylus Supporting Member Commercial User

    Aug 18, 2002
    General Manager TecPadz LLC
    Thanks, I think I understand - the more bias potential to a point, the higher the clean signal to noise ratio. I'll have to bear that in mind. As things stand with this particular amp, it sounds great. Without installing the trim pots, I'll never be able to know what kind of sonic difference is possible. Anyway, thanks for the food for thought.
     

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