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Thread: how hard is it/how much of a pain is it to bias an amp

  1. #1

    how hard is it/how much of a pain is it to bias an amp

    I've lived in the world of cathode biased amps, but I'm curious about the 2 channel H amps. They're fixed bias, but have ports on the back of the amp for biasing. Having never thought about, or attempted, biasing an amp, what's the hassle factor? Since you don't have to remove the chassis I imagine it's relatively low, but I don't have any idea what the process entails. I do notice it always says "take to a qualified tech" and while I realize that's some legal speak, I also think, "hassle."

    Thanks!

  2. #2
    It's not difficult, especially on an amp with bias jacks. It's a good idea to have someone knowledgeable walk you through it if you don't know what to do.

  3. #3
    Quote Originally Posted by LSchefman View Post
    It's not difficult, especially on an amp with bias jacks. It's a good idea to have someone knowledgeable walk you through it if you don't know what to do.
    Thanks, Les. So if I love an amp with bias jacks I'll try not to let that be a big factor in the decisoin to buy or not.

  4. #4
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    I literally just got back from having a pro bias a non PRS amp. He agreed to teach me. Even though the chassis of my amp had to be removed, the procedure was quit simple even to an electronic virgin like me.

  5. #5
    Quote Originally Posted by David Eaton View Post
    Thanks, Les. So if I love an amp with bias jacks I'll try not to let that be a big factor in the decisoin to buy or not.
    Definitely don't let it be a big factor. There are advantages to an amp that can be biased; you have a wider range of tube choices, for one. And an amp with jacks on the outside of the chassis has an advantage because you don't have to remove the chassis to do the biasing.

    With many amps that don't have the jacks, you have to use adapter sockets along with a multimeter, and sometimes dismount the chassis, which is slightly more of a chore; but on an amp with bias jacks, you only need a multimeter and a screwdriver. Assuming that you do it correctly, it's really as easy as changing strings.

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    Senior Member slowro's Avatar
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    I was under the impression that it was pretty dangerous to do anything to an amp. Is it exagerated?

  7. #7
    deus ex machina
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    Quote Originally Posted by slowro View Post
    I was under the impression that it was pretty dangerous to do anything to an amp. Is it exagerated?
    No, it's not exaggerated. Most of the 50W amps that Doug and his team build have between 450 and 500 Volts DC on the plates. Any job that requires pulling the chassis should be left to someone who knows how to safely discharge an amp's power supply, as the power supply capacitors in many amps remain charged after power has removed from the amp.

    However, with that said, biasing an amp with bias jacks is not difficult or inherently dangerous as long as one knows the amp's approximate plate voltage. The bias jacks do not provide that reading. One either has to pull the chassis or us a bias jig to read the plate voltage. As stated above, pulling the chassis is best left to someone who knows what they are doing.

  8. #8
    Quote Originally Posted by Em7 View Post
    However, with that said, biasing an amp with bias jacks is not difficult or inherently dangerous as long as one knows the amp's approximate plate voltage. The bias jacks do not provide that reading. One either has to pull the chassis or us a bias jig to read the plate voltage. As stated above, pulling the chassis is best left to someone who knows what they are doing.
    What I've done in the past is call the manufacturer, and simply find out the number they set the bias to when the amp is shipped.

    It's never been suggested that I determine the plate voltage. I'm not against determining the plate voltage, by the way, I've just never been told to do it. But I don't monkey around inside amps, setting the bias is as technical as I get.

    Though I once built a Heathkit amp, it was a very, very long time ago.
    Last edited by LSchefman; 09-27-2012 at 04:23 PM.

  9. #9
    deus ex machina
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    Quote Originally Posted by LSchefman View Post
    It's never been suggested that I determine the plate voltage. I'm not against determining the plate voltage, by the way, I've just never been told to do it. But I don't monkey around inside amps, setting the bias is as technical as I get.
    We set an amplifier's idle plate dissipation when we bias it; therefore, we need to know the plate voltage. A generally accepted rule of thumb is to bias amp such that the tubes are idling at 70% of their rated dissipation (the most accurate way to bias an amp is with a signal generator and an oscilloscope). For example, an EL34 is a 25W tube. Seventy percent of 25W is 25 x 0.7 = 17.5W. With a plate voltage of 450 Volts DC, we would bias the amp such that each tube drew 17.5 / 450 = 0.039 amps (39 milliamps) of current. In reality, we can usually go as low as 60% of plate dissipation ( 25 x 0.6 / 450 = 33 milliamps) without introducing crossover distortion; therefore, any reading between 33 milliamps and 39 milliamps will get the job done.

  10. #10
    Quote Originally Posted by Em7 View Post
    We set an amplifier's idle plate dissipation when we bias it; therefore, we need to know the plate voltage. A generally accepted rule of thumb is to bias amp such that the tubes are idling at 70% of their rated dissipation (the most accurate way to bias an amp is with a signal generator and an oscilloscope). For example, an EL34 is a 25W tube. Seventy percent of 25W is 25 x 0.7 = 17.5W. With a plate voltage of 450 Volts DC, we would bias the amp such that each tube drew 17.5 / 450 = 0.039 amps (39 milliamps) of current. In reality, we can usually go as low as 60% of plate dissipation ( 25 x 0.6 / 450 = 33 milliamps) without introducing crossover distortion; therefore, any reading between 33 milliamps and 39 milliamps will get the job done.
    Gotcha.

    So I shouldn't just call Doug Sewell at PRS and say, "I'm putting in new tubes, where should I set the bias?"

    OP, please ignore my original response. I may not have been doing it correctly.

  11. #11
    Member dsenoj's Avatar
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    I bought a 2 Channel H at the Experience and spoke to Doug between sessions and asked about biasing it. He told me to set it to 30mv plus or minus 5mv. He said that the amp could handle EL34 tubes as long as the bias is adjusted to the same 30mv plus or minus 5mv.

  12. #12
    All PRS amps should be BIASed at 30mv +/- 5. which makes it very easy to change tubes. All of their Fixed BIAS amps have the jacks on the rear panel.. some have the trim pot on the rear panel and some like the 2 Channel amps have the pot on the bottom of the chassis.
    ZebraPRS
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    Can anyone recommend a youtube video that actually shows a PRS amp with biasing jacks on the rear being biased? It would prove invaluable to actually watch someone with a multimeter biasing the amp. I mean, I understand the theory...but, to be honest...I'd really appreciate a walk through of what you do once you stick that multimeter's probe into the bias jacks.

    thanks!!

  14. #14
    Quote Originally Posted by docbennett View Post
    Can anyone recommend a youtube video that actually shows a PRS amp with biasing jacks on the rear being biased? It would prove invaluable to actually watch someone with a multimeter biasing the amp. I mean, I understand the theory...but, to be honest...I'd really appreciate a walk through of what you do once you stick that multimeter's probe into the bias jacks.

    thanks!!
    I would do this but my flip just crapped out last week... I have done it and it is very easy...
    ZebraPRS
    Guitars: 1967 Gibson Byrdland; 1995 PRS Custom 24; 20?? PRS Custom 24 Custom Zebra Stripes, 2009 25th McCarty NF, 2014 Brushstroke 24, 2013 PRS HBII,
    Amps: PRS Original Sewell 50 watt #43 with Paisley 2x12 cab; PRS Pre-Prod 2 Channel C with 2x12 DB Cabinet, PRS Custom 10 watt head

  15. #15
    Quote Originally Posted by docbennett View Post
    Can anyone recommend a youtube video that actually shows a PRS amp with biasing jacks on the rear being biased? It would prove invaluable to actually watch someone with a multimeter biasing the amp. I mean, I understand the theory...but, to be honest...I'd really appreciate a walk through of what you do once you stick that multimeter's probe into the bias jacks.

    thanks!!
    Here's a video from PRS showing final test of the amp before putting it in the cabinet, including checking/setting the bias:

    http://www.youtube.com/watch?v=NNB0Pn-b3qE

    It's really not difficult.

  16. #16
    deus ex machina
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    Quote Originally Posted by zebraprs View Post
    All PRS amps should be BIASed at 30mv +/- 5. which makes it very easy to change tubes. All of their Fixed BIAS amps have the jacks on the rear panel.. some have the trim pot on the rear panel and some like the 2 Channel amps have the pot on the bottom of the chassis.
    Most of the 50-Watt amps run with a plate voltage of around 460 VDC. The BIAS jacks are wired such that they convert a current reading to a voltage reading via a pair of 1-Ohm resistors.

    With that said, tweaking the bias control until one's meter reads 30mv is fine for EL34s, but that reading is not even close to optimal for 6L6GCs running in class AB1. An EL34 is a 25-Watt tube. A 6L6GC is a 30-Watt tube; therefore, bias levels need to be adjusted up by a factor of 1.2 with 6L6GCs. Thirty 30 ma is less than 50% of the rated plate dissipation of a 6L6GC at 460 Volts. To get the best tone out of 6L6GCs at 460 Volts DC, one should really tweak the bias control until the meter reads between 39 and 46 millivolts (sixty to seventy percent of plate dissipation).

  17. #17
    SuperD Boogie's Avatar
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    To kick this great thread back to life...

    In my observations, manufacturers (especially Mesa/Boogie) tend to bias their amps on the extremely cold side. Tubes last longer and customers complain less. But I agree that the optimal setting seems to be at the upper end of the range, as mentioned, around 70% dissipation. That's where power tubes feel better to me.
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