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Ampeg PF-50T owners: Where do you keep the bias set?

Discussion in 'Amps and Cabs [BG]' started by cazclocker, Apr 23, 2017.


  1. cazclocker

    cazclocker My social skills are rapidly dwindling.

    Oct 24, 2014
    Newton, Kansas
    I just scored a nice deal on a 6-month old PF-50T, and I've been playing with the bias setting on the back panel. The manual says you can set it for a fat tone and short tube life, or you can set it for a thin-sounding tone and long tube life... all from going by the red/green LED's.

    I can hear the tonal difference depending on where I set the bias pot. There are a few things I don't get, though.

    1. Is the bias pot supposed to rotate 'round and 'round instead of a definite stop and start point, like on a typical volume or tone pot?

    2. How long could I expect my power tubes to last if I adjust bias for "better tone, just before the red LED lights up" (according to the manual, page 12), assuming two 4-hour gigs a week?

    3. What the heck is bias anyway? I know it's a setting peculiar to tube amps, but I always thought it was an adjustment done by a service technician.

    4. Does the bias setting drift over time, or will it stay where I set it?

    You can tell I'm new to this stuff, I just need some 'splaining. Thanks in advance.
     
  2. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    Much like a car, bias sets the idle or operating point of the tubes. All tubes require a bias. The adjustable bias on your amp is for the power tubes. On the other tubes, the design is such that they self bias. This is the difference between what is called cathode biased and fixed bias. Your amp's power tubes are fixed bias. But let's not complicate things, I mention them just in case you have heard these terms.

    What does it really mean? Think of bias like a string being tuned. When you tune, you are setting the tension on your string. Too much or too little tension and the string doesn't sound right. When the string is in tune, the notes sound right. A bias voltage or tension is applied to the tube, it balances the operating point. Like the tension on your string.

    When you play a note, the operating point swings and that can shift the output of the tube into compression, distortion, or even shut it down. The bias is set so that this swing point is optimal in terms of the tube's sonic performance.

    By changing the bias voltage, you control how much current is flowing through the tube at idle. Too much current wears down the tube quicker, but it is being driven harder and sounds good when a note is applied. Too little current, the tube is not being driven hard enough and it can sound thin.

    1. Bias pots normally have stops. Some are multi-turn, other are single turn like a volume control.

    2. It depends on how hard you play and how loud the volume is turned up. It you drive your car with the brakes on, they will wear down quickly. If you drive with the brakes down harder, they will wear down even quicker. How long the tubes will last depends on the amp design and how it is played. Tubes can last a year or many years. I set the bias so that the amp sounds the best both at lower volumes and whn it is being pushed. Just make sure that it is within the safe operating zone. Use the LED's as a guide. BTW, bias should be set with the amp warmed up, unplug your instrument. The LED colors mean nothing when you are playing.

    3. Transistors are biased just like tubes. It is not just a tube thing. When a tech checks out a SS amp, they should be checking the power stage bias, jusy like they would do on a tube amp. The LED's provide a simple meas of adjusting the bias so that you do not have to go to a tech. A tech can still use test equipment to optimize the bias setting. This can be better than relying on the LED's.

    4. Yes bias does drift with time. Your amp bias will change as the tubes wear down. Check the bias after the first few months, then every six months to a year if you are using the amp regularly. Your bias will also change if the line voltage changes. At some venues, the wall voltage will dip when the load of all the amps and lights are present. This can affect the way that the amp sounds. You don't have to go nuts adjusting the bias at every venue, although some players like to. The amp is designed to operate on a range of voltages, but it can affect the performance of the amp if it is not optimal.
     
    SirMjac28, cazclocker and kobass like this.
  3. cazclocker

    cazclocker My social skills are rapidly dwindling.

    Oct 24, 2014
    Newton, Kansas
    Thanks, b-o-t. You've explained everything really well... I'm fairly certain I understand now.

    Just one point.... #2...The manual says to play my bass through the amp for at least 20 minutes to allow everything to get warmed up. Then, to set the bias, the manual says to turn down all the controls on the bass while leaving the amp controls untouched. But it sounds as if you're recommending that I physically unplug my bass from the amp to set the bias. I guess unplugging the bass is electrically the same as turning the bass's controls all the way down. Is that the case?
     
    beans-on-toast likes this.
  4. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    Bias sould be set with no input. Because an instrument signal (which is AC) will swing the bias. Turning down the bass is pretty much the same as no input. But unplugging is better because with no plug in the jack the input is shorted. With an instrument plugged in there is always the chance of a small amount of hum, that AC again.

    The amp has to be warm when the bias is set. This heats up all the components, when they are not up to temperature, they can drift around a bit. So then the amp is warm, it reaches what is called a steady state. This has to be done in playing mode, standby off. When in standby only the tube heaters are turnd on, not the high voltage. When the high voltage is on, the amp is fully on, passing signal through. They suggest 20 minutes but longer is fine. Playing through the amp will heat it quicker.

    In general, I set the bias, then wait and check it about half an hour later. I had a SVT-VR that used to take an hour to stabilize. Sometimes it drifts a bitif you check it later. As I said, you don't need to go nuts over micro setting the bias.

    To be consistent, I set the tone controls flat and turn the volume down all the way when setting the bias.
     
    cazclocker likes this.
  5. cazclocker

    cazclocker My social skills are rapidly dwindling.

    Oct 24, 2014
    Newton, Kansas
    Ah, alright. I see.... thanks so much for your explanation, I appreciate it.
     
    beans-on-toast likes this.
  6. cazclocker

    cazclocker My social skills are rapidly dwindling.

    Oct 24, 2014
    Newton, Kansas
    b.o.t., one last question....when you turn the volume down to set the bias, do you turn just the master volume down, or the master AND the gain as well? Or does it make a difference??
     
  7. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    I make it standard operating procedure to turn the gain and master down all the way when setting the bias. It is more about consistency in the procedure, even if it doesn't need to be done.

    Does it make a difference? Do the following test to see. Unplug the instrument cable from the input of the amp. Turn up the volume and master all the way. If you hear an increase in hum, then turning them down will make a difference when setting the bias. You don't want any signal going through the amp when the procedure is being performed. If you don't hear a difference in the level of the hum, it doesn't make a difference with your amp.
     
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  8. cazclocker

    cazclocker My social skills are rapidly dwindling.

    Oct 24, 2014
    Newton, Kansas
    Thanks much, beans. Your crystal-clear explanation are exactly what I was looking for.
     
    beans-on-toast likes this.
  9. username1

    username1

    Dec 28, 2005
    alberta canada
    I set the bias on my pf50t right in the middle of the green range, works great for me,
     
    Jefenator and cazclocker like this.

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