This site is supported by the advertisements on it, please disable your AdBlocker so we can continue to provide you with the quality content you expect.

Welcome to Our Community

Wanting to join the rest of our members? Feel free to sign up today.

5150/6505 Bias Mod Kits

Discussion in 'Backline' started by TheWinterSnow, Dec 31, 2012.

  1. TheWinterSnow

    TheWinterSnow Den Mørke Natt

    Joined:
    Oct 22, 2008
    Messages:
    3,090
    Likes Received:
    2
    Trophy Points:
    38
    Location:
    Sacramento, CA
    The name says it all, I have been trying to find a way to make a simple solution for those who have wanted to bias their beloved 5150s and 6505s but either don't want to pay an arm and a leg. If soldering is not your strong point, I can do the installation cheaper than just about any tech I am aware of, you just have to pay the extra shipping for your amp.

    Sorry no picture, no real need for a potentiometer and wiring harness with two color coded leads coming out of it that will just be put inside the amp.

    You can buy here

    We can let this thread be a source of reviews and general talk about biasing, I know the topic comes up every now and then and there usually tends to be a lot of confusion. Any questions, comments and accusations are more than welcome.
     
  2. Lasse Lammert

    Lasse Lammert HCAF Blitzkrieg

    Joined:
    Feb 12, 2009
    Messages:
    8,424
    Likes Received:
    40
    Trophy Points:
    48
    40$ is quite steep I must say.
    It's one trim pot and a resistor in series=3$.
    Then again, I haven't even seen your product yet:)
    But usually it's just a trim pot and R soldered paralled to that cap, 2 min job and 3$ in parts...but perhaps you found an even nicer solution? Looking forward to some pics!
    But hey, if people buy it, all power to you!
    I'm sure this will help a lot of people with their very cold 5150s...and after all it's the demand that regulates the price.
     
  3. TheWinterSnow

    TheWinterSnow Den Mørke Natt

    Joined:
    Oct 22, 2008
    Messages:
    3,090
    Likes Received:
    2
    Trophy Points:
    38
    Location:
    Sacramento, CA
    I was going to mention about where the price came from, since I had issues in the past with shipping, both to me and the buyer, paypal fees which is something like $2 plus a certain percentage of the sell price, plus the cost of the product itself it adds up quickly. Plus it actually takes me some time to assemble the whole thing and I am not using cheap plastic trim pots. I used fairly expensive high friction pots and like I mentioned regarding the picture actually making a wiring harness, not just insulated cable floating wherever it wants. The idea was to make it so all the installer has to do it drill a hole in the chassis, install the pot, clip out one resistor and solder the color coded leads the filter cap, so my extra effort and extra expenses goes to make it easier for the newbies that want to do it themselves. My current caclulations are putting me at about $15 profit out of the $40 being charged, and considering that I charge $30 for work, if it takes me a half an hour to assemble the kit then it is fairly priced, at least in my opinion.

    If you look at the cost for instillation of a bias mod by most techs they charge upwards of $150, the cheapest I've ever seen is $100 and if they are too far for a drive, you still have to pay shipping, which in the US alone a 5150 will still cost you pretty good.

    Oh and I will have an Installation video and PDF soon
     
  4. Lasse Lammert

    Lasse Lammert HCAF Blitzkrieg

    Joined:
    Feb 12, 2009
    Messages:
    8,424
    Likes Received:
    40
    Trophy Points:
    48
    Cool man, sounds rather interesting, looking forward to some pics.
    Didn't actually mean any offense, didn't know your product would be that thought through, thought it'd just be two cheap parts soldered to one another.
    Good luck mate, happy new year
     
  5. TheWinterSnow

    TheWinterSnow Den Mørke Natt

    Joined:
    Oct 22, 2008
    Messages:
    3,090
    Likes Received:
    2
    Trophy Points:
    38
    Location:
    Sacramento, CA
    Happy new year Lasse! Hope for some more KFM in 2013.
     
  6. TheWinterSnow

    TheWinterSnow Den Mørke Natt

    Joined:
    Oct 22, 2008
    Messages:
    3,090
    Likes Received:
    2
    Trophy Points:
    38
    Location:
    Sacramento, CA
    A big explanation of biasing that is now on my page, maybe it might help some people out understanding what biasing is.

    Tubes can only conduct current in on direction, from the cathode (-) to the anode (+). A vacuum tube is fully conducting when the input is at 0v with respect to the cathode and off when the input is more negative than the cathode, the specific voltage when it shuts off varies by tube type but is generally -4v with 12ax7s and -60v with 6L6s. In order to amplify the positive portions of the wave, the audio signal needs to be imposed with a DC offset that is half way from completely on and completely off, specifically a negative DC offset. This hold true for preamp tubes, however in a push-pull configuration the bias voltage must be so negative that under conditions where no audio signal is present, the bias voltage is so negative that it actaully stops the flow of current and only begins to conduct when a positive signal is present. All negative portions of the audio signal are essentially clipped off.

    A phase inverter is the last preamp tube that's job is to split and invert the signal on one of the outputs. The result is the opposing signals go into their resepctive set of power tubes, one set of power tubes amplifies positive signals, the other negative signals. When one set of tubes is conducting the other set is off because its signal is driving the voltage more negative than the cutoff voltage, the voltage that turns the tube off. The end result is that there is a set of tubes powering the positive portion of the signal and another powering the negative portion of the signal. After is it mixed together by the output transformer it results in the original signal

    The reason that this configuration is used is because it is more power effeicient, delivering more power to the speakers and wasting less power in the amp itself. This means that your tubes last longer.

    There is one flaw just setting the power amps bias voltage to that which would set the tube into cutoff until the audio sigal becomes postive. It takes some time for the tube to respond and turn on when they are off and for some time remain off. This results in what is called crossover distortion which is highly undesired. To remedy this, the bias voltage is adjusted slightly more positive so that all the tubes are just beginning to conduct when no signal is present. For small signal swings, both sections are running at the same time, during the handoff from one set of tubes to the other, eliminating that small amount of time where there is disortion present.

    Unfortunately the tolerances from tube to tube changes and their actual cutoff voltage varies. Becuase of this, you have to caluclate where the tube is operating by measuring its bias current, that is, the slight amount of current that is allowed to flow becuase the tube slightly turned on, or the bias voltage is just above the cutoff voltage.

    The common bias current is typicaly 60% to 70% of the maximum dissipation of the tube. With a 6L6, which its maximum dissipation is 30 Watts, that puts the bias anywhere from 18-21 Watts. The bias current needed to achieve this ranges form amp to amp. The Peavey 5150 and 6505 amps are deisgned to have a plate voltage of 500 volts, but can range from 480 volts to 510 volts, depending on many variables. In order to get the correct current calculation when biasing the amp, you need to check the live plate voltage. With that you can use the equation P=IxV where P is power in watts, I is current in amps and V is voltage in volts. Since we know the power of the tube and will measure the voltage of the plate, we need to calculate the current that we are aiming for so we will use the equation I=P/V. With the 5150/6505 amps being 6L6 amps, we know power is 30W, now lets say that out plate voltage was indeed 500v, then we know that our maximum to not exceed current is 60mA. In order to get our bias current, we can choose anything from 60% to 70% to get our current. I personally recommend 70% which using the equation I=P/V means we are aiming for a bias current of 42mA. The sweet spot for most 5150/6505 users with a bias mod is anywhere from 36mA to 42mA. That is much higher that the typical stock bias that can range from 8mA to 18mA.

    The higher you bias the amp, the more tube sag you will achieve, which adds a level of compression and harmonics at the expense of a tight low end. If you want a tight low end as possible but want to rid crossover disortion the best way to do this is to have a load and meaure across that load with an oscilloscope a sing wave signal that is being injected at the phase inverter and increase the bias until the sine wave looks completely clean and free of any distortions. With a 5150/6505, that will ut you in the 20mA to 30mA area which will mean you will have tighter low end and your tubes will last longer.

    Anectodote: I personally like tube sag, it helps make an amp sound more aggressive and "pissed off" and am willing to go through tubes quicker for that tonality. I will note however taht the date on my tubes are from 2002, I bought my 5150 in 2009 and immediately bias modded it, it has been running at 70% for almost 4 years, it has been a regular weel;y practice amp for band practice at full volume for hours, traveled hundreds of miles for shows, been cranked at shows, in some cases I had to turn the volume up to 7+ on the post gain because some venues don't have a PA for anything other than vocalists and even with a cracked tube, a tube that was cracked when I got it, the tubes still work, so don't think that biasing your amp hotter will dramatically reduce the life of your tubes.
     
  7. MatrixClaw

    MatrixClaw Member

    Joined:
    May 22, 2010
    Messages:
    1,400
    Likes Received:
    0
    Trophy Points:
    36
    Location:
    Mesa, AZ
    Jeeze, you're getting raped on bias mod costs in CA. My tech does them for $50 :eek:

    Still, cool idea for those of us who like to do it ourselves, or have no one else to do it!
     
  8. TheWinterSnow

    TheWinterSnow Den Mørke Natt

    Joined:
    Oct 22, 2008
    Messages:
    3,090
    Likes Received:
    2
    Trophy Points:
    38
    Location:
    Sacramento, CA
    Yeah most small techs do them cheacp but places like Voodoo Amps charge just $100 alone and FJA $150 for a bias mod with an external pot, not counting the shipping and the $100 to $150 charged for just receiveing the amp.

    I am also providing a bit better quality than the typical small tech bias mods which tend to use the cheapest parts available. You do get what you pay for.
     
  9. Lasse Lammert

    Lasse Lammert HCAF Blitzkrieg

    Joined:
    Feb 12, 2009
    Messages:
    8,424
    Likes Received:
    40
    Trophy Points:
    48
    Yeah, 40 for the work is definitely not too much.
    Also thanks for the in depth explanation, interesting!
    I usually like my amps biased rather cold (around 50% plate dissipation), except for Marshalls, and everything crunchy, I like those hotter.
    Small correction/question: isn't it only the 6L6GC that has 30w? I always though most of the regular non-GC 6L6 were 25watts? (Not that that makes a huge difference of course)
     
  10. TheWinterSnow

    TheWinterSnow Den Mørke Natt

    Joined:
    Oct 22, 2008
    Messages:
    3,090
    Likes Received:
    2
    Trophy Points:
    38
    Location:
    Sacramento, CA
    EL34/KT77: 25W
    6CA7: 25W (27.5 per tube in class AB setup)
    6L6/KT66: 30W
    KT88: 42W (50W per tube in class AB setup)

    Also I will update that little bit on typical dissipation numbers, some people do like tehm colder, and 40%-50% is just fine, hell even colder is fine it all comes down to preference, as long as you are not exceeding 100% at idle then it all comes down to what you like.
     
  11. Lasse Lammert

    Lasse Lammert HCAF Blitzkrieg

    Joined:
    Feb 12, 2009
    Messages:
    8,424
    Likes Received:
    40
    Trophy Points:
    48
    Cool.
    When does crossover distortion usually become audible?
     
  12. TheWinterSnow

    TheWinterSnow Den Mørke Natt

    Joined:
    Oct 22, 2008
    Messages:
    3,090
    Likes Received:
    2
    Trophy Points:
    38
    Location:
    Sacramento, CA
    In vacuum tubes I am not sure, it has to do with the forward voltage required to forward bias the semiconductor. That voltage is permanently lost to the signal and that is where the crossover distortion comes from. It is not measured in tubes or at least is never listed in the datasheets. I have not looked at the waveform of a sine wave in a typical 120W 6L6 amp so I can tell you where the average dissipation to when crossover is eliminated. Usually though, it is pretty low, the 5150's stock cold bias is not to far away from where the corssover should disappear as demonstrated on an Aiken Amps Tech page. I think the 120W amp was out of crossover at like 35% to 40%.

    Also with crossover distortion it is only bad at low volumes, since the amount of voltage distortion never changes as the voltage lost never changes regardless of input signal, the total harmonic distortion cuased form crossover distortion drops the greater the magnitude of the input signal. This is why crossover distortion is non-existent in amps that are cranked up to healthy volumes as long as your bias current is greater than 0% dissipation, or Class B.
     
  13. PaoloJM

    PaoloJM Member

    Joined:
    Jul 1, 2010
    Messages:
    94
    Likes Received:
    0
    Trophy Points:
    6
    Location:
    Cork, Ireland
    Every time your hear a cranked Marshall!!
    Some cross over distortion sounds very musical.
    It's what give a cranked Marshall that gritty snarl it has.

    I scoped my JCM800 and there was some cross over distortion happening from around 4 on the master dial. It hits about a hundred watts at that point and begins crossover from there.
     
  14. TheWinterSnow

    TheWinterSnow Den Mørke Natt

    Joined:
    Oct 22, 2008
    Messages:
    3,090
    Likes Received:
    2
    Trophy Points:
    38
    Location:
    Sacramento, CA
    hmm, last time I checked crossover distortion is less audible at louder volumes. I am curious, where did you hear of this concept?
     

Share This Page