Ampeg PF-50T - Setting Tube Bias

Discussion in 'Amps and Cabs [BG]' started by GregPantelides, Feb 15, 2017.

  1. GregPantelides


    Jul 18, 2016
    Good Day,
    I just picked up an Ampeg PF-50T and ordered their PF-112HLF cab which should be arriving tomorrow. I've read through all the product material referencing tube biasing. I'm wondering, how does one know when to bias the tubes? Ampeg's owner's manual indicates this is to be done when changing tubes and also as they age. I understand the reason for biasing new tubes but how should one rebias the tubes as they age? What are the signs to look for and sounds to pay attention to?

    The literature also indicates that for the best tone but shorter tube life one should "under bias" the tubes. On the PF-50T this would occur just before the red light comes on. Is their any tonally perceptible difference in doing this?

    Best Regards,

    -Greg P

    P.S. I'm super excited to get this amp up and running. It's the perfect small venue/recording amp i've been waiting for someone to make. :)
  2. BassmanPaul

    BassmanPaul Inactive

    It’s a new amp leave it alone. :)

    Seriously though, the bias should be checked when changing output tubes which you will not have to do for a long while.
  3. GregPantelides


    Jul 18, 2016
    That's what I figured. Thanks for the quick reply :) :) :)
    BassmanPaul likes this.
  4. popcat


    Jan 7, 2016
    You can adjust the bias setting and listen for yourself, you won't damage anything.

    Amp Bias Explained | Pro Guitar Shop
    Under-biased (otherwise known as biased too hot) is a state when the idle current is set too high. This causes the signal to distort earlier, decreasing headroom and can also cause the tube to exceed the plate dissipation at certain operating conditions causing the plates to overheat and fail, thus shortening tube life. An under-biased amp can produce a sound that is slightly louder and may sound punchier and fuller at the expense of headroom. Keep in mind this may be a subtle effect. An over-biased (biased too cold) amplifier will cause the audio signal to deteriorate quickly and sound thin, possibly sterile. Over-biasing may also decrease tube life but not as drastically as under-biasing since it is running the idle current too low.
  5. BassmanPaul

    BassmanPaul Inactive

    Err Not true. I repeat it’s a new amp let it be!
    bobyoung53 and cflat like this.
  6. beans-on-toast


    Aug 7, 2008
    Ampeg talks in the manual about playing with the bias settings to suit your tastes. A cooler setting will prolong tube life, a hotter setting will allow more grind but the power tubes will wear down a bit faster. I prefer cleaner settings with a bit of distortion when pushed so I tend to set my amps up on the conservative side.

    You will notice the effects of bias settings more at higher volume levels. My best advice would be to experiment at higher volume levels and see which you prefer. As long as the bias is not into the red.

    It is a good idea to check the bias when you receive the amp or as they said when the power tubes are changed. The bias will change with changing power line voltages. The voltage can drift during the day and from one site to the next. You don't have to go nuts re-biasing the amp though. It was designed to work over a range of voltages. If the amp sounds off, you can check the bias. I like to check the bias when the tubes are new. After about a month of use, then every six months or a year. It depends on how much you are using the amp.

    A couple of comments, things that they don't say in the manual. The amp needs to reach a thermal steady state with no drifting before the bias is set. Play through it for at least a half hour first, the longer the better. When setting the bias, do it with no input. I unplug the instrument. To be consistent, turn the volume down and set the tone controls flat. If you set the bias and recheck it an hour later and it has changed, either the amp was not warmed up enough or the line voltage has drifted. As I mentioned, not a big deal though. Always adjust bias by turning up clockwise to the desired bias point. If you overshoot the setting, turn it back counterclockwise and then turn up to the bias point. The reason for doing this is that the potentiometer has mechanical slop in it, called hysteresis, and applying clockwise pressure when setting the position helps minimize this. When you are playing, don't be concerned with what the LED is doing. The readout is only meaningful with no signal input.

    When buying power tubes, deal with a place that provides a well matched set. If the set is not matched, there will be hum and more harmonic distortion.
  7. GregPantelides


    Jul 18, 2016
    I followed the instructions in the manual and adjusted it for better tone. I played through it for about 30 minutes and removed the bass from the input before adjustment. Sounds fantastic.

    I figured just like I check the downforce on the stylus of my record player and adjust it periodically makes sense to check and adjust the bias too. :)
    keyofnight and beans-on-toast like this.
  8. popcat


    Jan 7, 2016
    Yes, it is true. The adjustment range is designed accordingly, you are not going to cause a failure by playing with the bias and checking for differences in sound. Leaving the setting at some extreme may shorten tube life over time.

    Yes, do not have anything plugged into the input jack(s) when setting the bias. Also, checking it once in a while is a good idea; even new out of the box.
  9. BassmanPaul

    BassmanPaul Inactive

    I stand corrected in this particular case but this cannot be said of the vast majority of fixed bias tube amplifiers out there. It also means that the bias setting has a very limited spread and quite possibly may not have the range to bias properly every tube set that might be installed.
    bobyoung53 likes this.
  10. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa Boogie, Development Engineer-Genzler (pedals), Product Support-Genz Benz
    It's important to understand that terminology can vary since the bias voltage on the grid is negative, increasing the bias voltage in the strict sense means making less negative which increases the conduction angle and results in more quiescent current through the output.

    The terms under-biased and over-biased can refer to the resultant current, and an over-biased amp can mean that the amp is biased too "hot" (excess quiescent current flowing in the output stage). It's critical to understand exactly what is being indicated otherwise costly mistakes can be made.

    The above applies to tube amp output stages, (typical) solid state linear output stages use a positive bias voltage (base more positive than the emitter on the NPN device, but base more negative than the emitter on the PNP device), so increasing the bias voltage appears more logical when considered from the positive rail device.

    Before doing ANY adjustments on an amp, be sure you really understand what is being said, and have the means to correctly measure the result (such as the bias current sense LEDs on the amp being discussed). Mistakes can be costly, and in the case of solid state amps especially, can be virtually instantaneous.
    Rich Fiscus and BassmanPaul like this.
  11. Bent77


    Mar 6, 2013
    1. feed voltage constant at 121VAC (has been each time I've tested)
    2. I biased the tubes about a month ago, to run them a "little" hotter. I may not have had it warmed all the way it wanted

    So it was quite fuzzy. That was fun for a day. Went back to set the bias as described above.
    Now the Red light comes on before the Green light.

    If I set it there, It sounds warm and punchy with both the gain and volume at the 11 o'clock setting
    Either one of those at full, and it produces fuzz, but nothing ridiculous

    I'm inclined to believe I made the power tube(s) mad, and that throwing a fresh set (re: don't have currently) of 6L6 tubes in there and start over.

    What say ye?
  12. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa Boogie, Development Engineer-Genzler (pedals), Product Support-Genz Benz
    I say that a proper analysis should correctly identify the cause, which may (or may not) be the output tubes. Only then will the proper solution be known.

    you may be on the way to creating new problems... for example leaving an otherwise properly working amp alone.
  13. beans-on-toast


    Aug 7, 2008
    I know what you meant but just to be clear, the amp requires 6L6GC power tubes, not a pair of 6L6 tubes.
    BassmanPaul and Bent77 like this.
  14. Wasnex


    Dec 25, 2011
    IMHO it's pretty important that you let the tubes warm up fully before doing the final bias setting. The idea here is tube current will drift until the tubes are fully warmed up, so setting the bias too soon will not provide valid results. Often tube current increases throughout the warm up period. So if you set the amp for high current before the tubes reach operating temp, you may cause a melt down.

    From the manual (Page 12.):

    Red comes on, then green. Tubes not properly matched. Set slightly before green comes on, obtain matched tubes when possible.

    This is the condition your describe.

    Since the PF50T only has two output tubes, I would expect it to produce hum if the tubes are mismatched significantly. The hum should sound like sort of a dull low frequency roar, and the loudness will increase with how much current is passing through the tubes and also with the severity of the mismatch.

    I have not seen a schematic for the PF50T but I believe the bias sense circuit is just a simple Boolean logic circuit that senses and compares the current of each output tube.

    In the SVT CL for example, the cathodes of each output tube are connected to ground with a 1 ohm resistor, and the bias sense circuit keys of the voltage drop that occurs when current flows through the resistors. The voltage drop across each resistor is proportional to the current flow through the associated tube. Since its' a 1 ohm resistor, a voltage drop of 1mV represents 1mA of current.

    I believe this is how the PF50T circuit works:​
    There is a range of current levels that are deemed acceptable for the output tubes. When the current in both tubes meets the lowest level in the range, the green LED turns on. This is the "AND" function; in other words both tubes most meet the condition for the green LED to light.

    When the current in one of the tubes exceeds the safe range, the red LED lights. This is the "OR" function. The red LED will remain lit as long as at least one of the tubes is pulling excess current.

    When the red LED comes on first, it indicates one tube is pulling too much current before both tubes are pulling the minimum current required to turn the green LED on. In other words, the OR function in the circuit is satisfied before the AND function.

    If the amp is not producing hum, the tubes may be fine and there maybe something wrong with the bias sense circuit.
    Bent77 likes this.
  15. Bent77


    Mar 6, 2013
    Solid explanation and thank you

    It does have a low level hum/hiss

    I’ll get some 6L6GC tubes in and start over
  16. BassmanPaul

    BassmanPaul Inactive

    At last!! :D
    beans-on-toast likes this.
  17. Something I noticed when I replaced tubes and set bias on my SVT-CL, is that there was a small range of green light when I got the bias to that point. There was also a slight hum. As I continued to move the control within that green range I could hear the hum fade and then start to come back. I ended up setting it for minimal humm, which is close to silent.

    Next time I do this I will set the head backwards on the cab so I can better hear the hum when I make the adjustment.
    Bent77 likes this.
  18. beans-on-toast


    Aug 7, 2008
    Always turn the pot to the bias point in a clockwise direction. If you overshoot, turn back, then clockwise up to the point. This is because there is always some slop in the mechanical pot. Going is one direction keeps pressure on the mechanism. The technical term is mechanical hysteresis.

    Unlike the SVT-VR, the CL does not have a balance pot. This pot balances the output level of the two trios. with the CL, if the power tubes are perfectly matched each trio is the same. But getting a perfectly matched set is rare. The cathode currents might be matched to within 10% or better if you are lucky. A mismatch between the trios can result in more total harmonic distortion which is heard as hum. Some matched tube set tubes have numbers on each tube or on the box. If there is, group each trio together so that the numbers on each group are as close as possible.

    When setting the bias, there can be a magic spot within the green range where total harmonic distortion is minimized. Keep doing what you are doing. With some power tube sets, you might not hear any hum at all.
    Bent77 likes this.
  19. Good to know, thanks. It was a well matched set that went in.
    Bent77 and beans-on-toast like this.
  20. Bent77


    Mar 6, 2013
    Thanks for reminding me the obvious to read the manual :D

    I referred to it again today and added the new 6L6GC tubes and started over. All seems normal at this point

    Appreciate the info everyone!
    Wasnex likes this.
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