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Discussion in 'Amps and Cabs [BG]' started by BassMisfit, Jul 1, 2002.

  1. BassMisfit


    Dec 31, 2000
    Irwin, PA
    I have a Peavey Mark111 Combo and i've always thought that the speaker was crap because I could only turn it to like 6 without clipping. But I was talking to an amp technition who told me that it was due to the bias being set incorrectly. I thought I new enough about amps to sove most of my problems but i've never new about Bias. Could anyone give me some insight on ths problem, and also where I could get thi done at?. He told me he could do it but he wasn't cheap. 25$ to look at it and 45$ per hour of work. Any info would be appreaciated.
  2. brianrost

    brianrost Gold Supporting Member

    Apr 26, 2000
    Boston, Taxachusetts
    Your "amp tech" is an idiot :rolleyes:

    Biasing is something that is done to tube amps. Your amp is solid state.

    The amp clipping has nothing to do with the speaker, it has to do with asking the amp to deliver more power than it can. Chief causes are just trying to play louder than the amp was meant to or using extreme EQ boosts which suck up power.

    I have yet to find an amp that stays clean al the way to "10" . This is because the amp designer has to compensate for instruments with weak output levels.

    A Peavey Mark III combo is only a 130 watt amplifier, so it's not going to hard to drive it into clipping.
  3. CS


    Dec 11, 1999
    Find an honest tech
  4. You can bias transistor amps. I have to bias mine. It has a internal Trimpot that sets the gain for the first stage and subsequent stages after that. Sure sure i am a amp tech who built this, so all i do is take out the fuses and put 100 ohm resistors in there and measure the voltage across them. The instructions that came with circuit outline whats a good figure to get.

    So yes you can bias transistor amps, but in commercial "brand name" models, it is rare. The components are usually selected to provide the right gain.


  5. Golem II

    Golem II

    Jan 4, 2002
    Macon, GA, USA
    I would say peavey is the victim of a lot of negative bias :D
  6. I'm with Psycho.

    Amps that run "push-pull" output stages have to be biased: that includes most SS and valve amps. [As it happens, amps that run single ended (Class A) also have to be biased but you don't see them around as guitar amps].

    For push-pull it's necessary to run some current through the output devices to turn them on just a little so that when one is turning off and the other is turning on as the signal swings about a mid point, cross-over distortion is (all but) eliminated.

    I've a suspicion that you might just be driving the Peavey too hard. It's a difficult one, though, because you're clearly convinced the amp is faulty.

    An amp tech would have to do a test to prove it. It'd involve getting the input to output specs from Peavey then checking to see whether or not the system measures up. That is, ?? millivolts in for ?? watts out with the controls set at some point, usually flat .

    Merls, I'm a bit confused by your post. Stage gain and output stage bias are not one and the same. Substituting 100 ohm resistors is certainly one way to check quiescent current of an amp. But thet pot used to make the adjustment doesn't alter the gain of the amp. That's usualy fixed by the value(s) of feedback resistor(s) from the output rail to the input/voltage gain stage.

    Unless I'm missing the point......

  7. brianrost

    brianrost Gold Supporting Member

    Apr 26, 2000
    Boston, Taxachusetts
    Yes, transistors are biased but you don't need to rebias them because you don't replace the transistors all the time like you do tubes.

    Be honest, a guy comes in witjh a 100 watt SS amp complaining it's clipping all the time on "6" do YOU think of a need to check the bias as #1???? How often do YOU actually do rebias jobs on SS amps????

    There's only a few SS amps I can think of with bias pots (SWR SM-400 comes to mind) which means the manufacturer did not think rebiasing would be a common service procedure.
  8. hyperlitem

    hyperlitem Guest

    Jul 25, 2001
    Indianapolis, IN
    i hate to throw in a question in this post, and even more its about guitar tube amps, but i just have to ask. My amp is making funny distorted noises all the time and ive had the original tubes in there for about 3 years now. Im pretty sure its time to change the power tubes. My amp has a feature thats called sumthing like automatic biasing or sumthing. Its supposed to eliminate a tech having to look at it and bias it for you. If it helps the amp is a 2x12 crate blue voodoo combo amp. Im thinkin i just have to buy a new set of groove tubes and throw throw them in, (not actually touching the tubes of course. Am i right?
  9. I've got a bias problem with my daughter's Fender Hotrod Deville 2x12 amp. I have no experience working on analog amps, but figured I'd give it a shot with a tube replacment and bias adjustment. It's got all new 12AX7 and 6L6 tubes, and still has the same problem.

    This amp is a pair of 6L6GC apparently configured as a Push-Pull. Fender provides a 1-ohm bias test point resistor and specs it at 60mV. The problem is I can't get the bias into spec. The lowest it will go is 79 mV with the Fender tubes, and 89 mV with a new pair of Ruby STR. If I leave the Fender tubes in ready for a minute or more, one starts to develop a cherry red hot spot.

    So... I measured all the DC supply voltages, etc etc, and am finding significant over voltages on a number of test points that input to the 12AX7 tubes. Fender calls for voltages under 20 V at TP2, TP8, TP11, but I'm seeing 250 volts here. Worse, the TP27 calls for 97 volts into the 6L6 tubes, but I'm seeing 250 volts or more here.

    All the basic DC supply voltages are correct. It is the inputs to the tubes that are way high.

    Q: is this symptomatic of a failed transformer?

    Link to Schematic and Voltages
  10. It is becoming apparent I'm misreading the Fender schematic somewhere. All my measurements and the full schematics are here.

    For example, TP2 is the X voltage supply to 12AX7 V1A. Pin 1 (plate section 2) is rated for 330 volts. The schematic shows TP2 = 196mV. Actual measurement is 264 volts.

    Link to 12AX7 TP2

    Another point of confusion is the plate voltage for the pair of 6L6GC. They measure 471 V, but the schematic bullet for TP27 shows 94.3 V.

    Link to 6L6 TP27

    I'm really misreading something here. Any ideas?
  11. BassMisfit


    Dec 31, 2000
    Irwin, PA
    Ok, well I guess it's 50/50 weather or not this tech I've found is full of crap. Well i'll probably take the chance and have him try and fix it. The fact that I just got fired (It has to do with me putting a 15" dead sewer rat on the intercom) means it'll probably take me a while to scrounge up the cash, which i'm not thrilled about. Oh well, does anybody know if heat has something to do with disrupting the bias? If so I can have my friend install a fan in the back plate which could solve that problem. Alright well thanks for all the replies they've helped me as well as confused me but it will all turn out for the better. thanks alot.

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