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Do you think your tube amp sounds better after it as been biased using an oscilloscope?

Discussion in 'Amps and Cabs' started by gunslinger, Oct 9, 2018.

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  1. jchan

    jchan Member

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    Sorry, makes no sense electronically and has nothing to do with biasing tubes!
     
  2. jchan

    jchan Member

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    Perhaps you can explain how visualizing a signal on a scope is telling you anything about how to bias tubes? And, how visualizing a signal is an objective test!
     
  3. gulliver

    gulliver Member

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    It will not sound better ... but hot-damn, it will look so much cooler.
     
  4. ontariomaximus

    ontariomaximus Member

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    I tell people all the time, don't get too hung up on biasing. An AB amp will draw the appropriate amount amount of current depending on your playing. How it sounds at idle doesn't matter a whole lot, because you are not playing. There is a wide range of acceptable current, just don't be at the extremes.
     
  5. grumphh

    grumphh Member

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    No. Asking this question you have proven that you wouldn't understand it anyway. :)
     
  6. jdel77

    jdel77 Silver Supporting Member

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    [​IMG]
     
  7. slagg

    slagg Member

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    the sweet spot is the sweet spot,does'nt matter how you get there.
     
  8. zenas

    zenas Member

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    I'm not going to get into the biasing debate going on here.

    But I get what you're saying in general about some otherwise great techs working on tube amps, expecially vintage ones.
    Just bought a BFSR from a guy that took it to the same tech that fixes his recording equipment, for "crackling". Probably a dirty input jack or something a little pot cleaner took care of.
    However had he taken it to someone that's more familiar with vintage amps he might have found out a few more things wrong with it, that need to be addressed.
    I looked inside before I bought it, a few parts changed over the years, but the electroyltics are OEM and bulging. It doesn't hum and since everything works someone not familiar with it might think it's OK.
    The seller was happy with it's tone on 7 so of course it's ok.
    Those of you that know what I'm talking about are raising an eyebrow to that "on 7" part right now.
    Yeah it's the quietest Super Reverb I've ever heard !
    Had to put it next to my SFSR last night for a little side by side. The 7 is about 3ish on the healthy one. Yes volume numbers very but I also tried it on 10, still not that loud. Set that SFSR on 4 and the neighbors a mile away would be pissed.
    Anyways the moral of the story is, use a guy that knows something about what he's working on.
    I won't even plug that amp in again until it gone through. Had the guy brought it to me I would've told him what it needed and gave him the choice, fix it right or take it somewhere else. Talking with the seller I think he would've had it done right if he'd been informed. Real nice guy.
    Oh he sold it because it was too loud. Lol
     
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  9. killer blues

    killer blues Member

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    I agree. I always bias to specified current then adjust with my ears.
     
  10. ripgtr

    ripgtr Member

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    Question - did it get back to normal loud when you replaced the caps?
    Reason I am asking, I played one that did that, back in the 70s. We went to Ohio to record an album, 5 guys in a van, so we didn't take our amps. Used the house amp. Engineer said about the SR not to use it because it had no volume, but I was able to crank it and get a really good OD sound with the amp not blasting. Probably had it on 6, and on one song, my headphones fell off and I couldn't barely hear it. Sounded good on the record though. I've always wondered why that amp was so quiet.
     
  11. duderanimous

    duderanimous Monsterpiece Fuzz - Fuzz Master Extraordinaire Silver Supporting Member

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    Everyone know this method is only applicable to the Mesa Rectumfryer series, right?
     
  12. jchan

    jchan Member

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    Agreed
     
  13. Schafrocks

    Schafrocks Member

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    Please enlighten us.
     
  14. Dave_C

    Dave_C Supporting Member

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    Traditional scope biasing leaves amps running way too cold. Best way to do it is to adjust cathode current so that plate dissipation is somewhere between 50-70% of max for the tube in question and use your ears to find the sweet spot. Some amps, like Dumble styles, are super sensitive to bias and will vary in tone all over the map as wall voltage (hence B+) changes. That's why I bias my amps using a variac for the range in AC voltage that my Furman AC regulator puts out. That dramatically reduces tone variation from venue to venue.
     
  15. grumphh

    grumphh Member

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    "No" is a word that is to hard to understand for you?
    Given the above i am pretty sure that any "enlightenment" would be wasted on you too :)
     
  16. Teleplayer

    Teleplayer Silver Supporting Member

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    Proctoscope works better.
     
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  17. Schafrocks

    Schafrocks Member

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    Haha. Didn’t think so.
     
  18. HotBluePlates

    HotBluePlates Member

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    If you know electronics well enough to know how to operate an oscilloscope, then you know it is a measuring device with horizontal marks denoting time and vertical marks denoting voltage. Both are calibrated, unless you are performing a test that is qualitative rather than quantitative.

    Amp output power can be measured on the scope, using a sine wave generator and a dummy load. It's easiest to see peak-to-peak voltage across the load resistor on the scope. The voltage is read directly off the scope screen graticule, and power is calculated as:

    RMS Power Output = ((Volts p-p)^2) / (4 * Resistance)​

    The measured power output can be compared to the rated power output. If the amp is biased too hot, the output waveform will distort before reaching rated power. That's because "hotter bias" is smaller bias voltage, and the tube distorts earlier because the control grid is driven positive with less applied signal. Output tubes have "transconductance" or "milliamps (plate) / volts (grid)" and smaller input signal therefore means less output plate current swing.

    If the amp is biased too cold, crossover notch can be observed. However, the tube would have to either be biased beyond cutoff, or have a very small drive signal applied. Driving the tube (even biased near cutoff) with a very large signal will mask the crossover notch.

    In between too hot & too cold, there is a broad range of acceptable bias voltage where power output changes only a little. And this matches the experience of those who "bias by ear" who find a wide range of useable idle current points doesn't overheat & the sonics are mostly unchanged. The ones who "bias for more distortion" are idling there amps hotter, meaning smaller bias voltage, and less output before distortion. Which is why they also note their amps distorting at a lower volume knob setting.

    ________________________

    There can be a good deal of fiddling back & forth between the bias pot & signal generator level if the tech really wants to set up the amp for maximum clean output power.

    ________________________

    You'll have to understand electronics and how tubes work to follow the above. And as I mentioned earlier, a power output test akin to the above is the best check of output tube health. Such is stated directly in the Radiotron Designer's Handbook, 4th Edition, in the section on tube testing.
     
  19. Jim Kelley

    Jim Kelley Supporting Member

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    I don't know what tradition you're referring to. What I do know is that it allows me to actually see whether the amp is biased Class A/B or not, and it prevents me from biasing either too hot, or too cold. The Goldilocks thing. But if you can't make it work for you, then by all means don't use it.
     
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  20. jchan

    jchan Member

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    I believe the formula is :
    RMS Power Output = ((Volts p-p)^2)/(8 * Resistance)

    Anyway... I understand you can make those visualizations with an oscilloscope, my point is that assessing "waveform will distort" and "crossover notch" are subjective views of the waveform. Measuring Vpp is not subjective, rather, assessing waveform distortion is. Plus the fact that tube output varies somewhat with frequency so using 1 frequency isn't the most accurate either.

    The point of biasing is to setup the operating point (grid control for optimizing operating point) that is a best compromise for performance and tube life. That is simply done by using bias probes or other methods to adjust the system to between 60-70% of max power. And, to have the push pull circuit working properly and therefore to have matched tubes with plate currents within 10% or less of each other. This type of adjustment is similar to biasing a FET or transistor in a circuit and is a quantitative vs qualitative method that involves qualitative visualizations of distortions, cross-over distortions. No method is perfect.
     

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