Cold bias vs Hot bias?

Discussion in 'The Workbench' started by The Dose of Harmony, Aug 20, 2019.

  1. The Dose of Harmony

    The Dose of Harmony Well-Known Member

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    Hi guys !!

    i have heard two different opinions on how cold or hot bias affect you breakup and headroom in your amp?

    one from Uncle Doug and another Bad ass tech ?

    but they contradict each other

    one says that cold bias its gonna reduce your headroom and the breakup and clip sooner than hot bias ?
    and the other the other way around
     
  2. pedecamp

    pedecamp Well-Known Member

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    I've found that 70% dissipation is a good happy medium, so for a 50 watt Marshall thats 38 mA or 96 mA for a 100 watt amp. My opinion is your amp has headroom or it doesnt, adjusting your bias isnt gonna add or detract headroom to an amp, at least not enough to make a difference, its the amp circuit that dictates how much headroom you have. Get a volt meter and a biasrite and do some tests yourself to see.
     
  3. The Dose of Harmony

    The Dose of Harmony Well-Known Member

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    I run all my amps within range at 65 to 70 % , i have the bias probe from euro tubes and i check them often its just that was very weird to hear two very wise guys to differ in that particular topic!
     
  4. pedecamp

    pedecamp Well-Known Member

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    Some people are hair splitters and can hear a difference in the tiniest differences, I just dont bank alot on it and dont think it matters enough to say a tone is different enough or not by splitting hairs. One guy may say setting bias up or down a hair is earth shatteringly different, I just dont hear it enough to split hairs over it. So its not to say either of those guys are wrong, it just doesnt make a whole lot of difference to me, everybody hears things differently. Challenge them both and see what they say, tell them what the other guy said, maybe one of them got their facts wrong or didnt say it right!
     
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  5. purpleplexi

    purpleplexi Well-Known Member

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    Before the internet people got great tones by using their ears. If I've learned one thing from all this it's that the tone one person loves may not sound good to you. And vice versa. In fact some people have gotten truly excellent tones by just doing exactly the darned well opposite to what everyone else says.
    Not me though I'm rubbish at this stuff.
     
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  6. anitoli

    anitoli Well-Known Member

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    It goes both ways. Too cold and the tubes break up, too hot and you run out of headroom too early and the tubes saturate. What i did discover when trying Ampmads o-scope version of biasing is that the tubes bias up alot cooler than when using cathode resistor method.
     
  7. The Dose of Harmony

    The Dose of Harmony Well-Known Member

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    I am about to star recording engineering next month besides that , i am building up a small studio and i am already hook up with modding amps so i want to understand and have better idea of happens with the bias too cold or to hot from the electrical point?

    I found this video and I would like all you guys to tell me about it you can find the topic on minute 4:13 on this video he says the opposite!!

     
  8. FourT6and2

    FourT6and2 Active Member

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    Plug your guitar in and play as you adjust the bias trim pot and see for yourself. It's not gonna damage anything as long as you don't redplate the tubes. It isn't a huge difference. As long as you hot enough to avoid crossover distortion and you're below 70% of max plate dissipation, it's fine. Set the bias to wherever it sounds good.
     
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  9. ampmadscientist

    ampmadscientist Well-Known Member

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    Start the reactor. free Mars!
    Cold bias sounds like ass.
    But the tubes last longer.
    Cold bias has much more distortion and the headroom is gone from the start.
    Cold bias works like a noise gate. The output is shutting off when there is no music playing, therefore you hear less noise.

    upload_2019-8-20_16-29-57.jpeg Waveform of cold amplifier. The distortion appears long before amp has reached maximum output. (is why it sounds like ass)

    upload_2019-8-20_16-31-37.png Clean sine wave from hot amplifier. The output should remain clean (no distortion) until output has reached maximum.


    Hot bias has much less distortion, and all the headroom that is available. (there is no magic way to increase headroom beyond original)
    The tube life is shorter with heat.
    Of course hot bias sounds better, but there will also be more noise. (because the output is turned on all the time)
    When there is no music, you will hear the noise because the output is still "on."
    The output does not shut off when no music is playing.
     
    Last edited: Aug 20, 2019
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  10. Nik Henville

    Nik Henville Well-Known Member

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    .
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    @purpleplexi and @FourT6and2 both speak the truth.
    I have been playing glass bottles for five decades plus, and always set 'em up using my ears and a guitar. Punters cant hear 95mA versus 96mA, but they CAN hear good sounding against crappy. I just wind the bias down to where it sounds like shit, then up to where those bottles begin to glow, and then set it to someplace in between that sounds good to my ears.
    .
    .
     
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  11. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    We would need to read the articles or postings of both.

    The bias point is relative to the signal so all that is bullshit unless you are person that either likes it biased on the colder side or the hotter side.
     
  12. The Dose of Harmony

    The Dose of Harmony Well-Known Member

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    well guys after testing it my self on two different amps the JMP and TSl 100

    looks like Uncle Doug and Ampmadscientist are right

    Hot bias more Headroom less distortion
    cold bias Less Headroom more distortion ,clipping
     
  13. The Dose of Harmony

    The Dose of Harmony Well-Known Member

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    you were right and Uncle Doug I tested it myself.
     
  14. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    That sounds all good but can you give everyone details for each amplifier like the hot and cold bias settings along with all amplifier control settings and settings of any equipment in the signal chain?
     
  15. The Dose of Harmony

    The Dose of Harmony Well-Known Member

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    Guitar directly to both amp

    Amp settings all eq at noon

    On the JMP plate voltage was around 357 v


    hot settings went to 62 ma
    Cold settings went to 26 ma
    I wanted get cooler but the pot did not give me more

    I notice that when it was cold the pre amp knob was more sensitive

    it sound it good man i wanted to tested for longer time but it was getting late


    On Tsl 100 PV was 448

    Hot 58 ma
    Cold 28 ma

    This one took me more time when you lower the other pair and those pots are very sensitives

    The cold settings were more noticeable on tsl , sounded very rare
    On the Jmp you have to pay more attention

    Next time i will do it i am gonna record but everybtime i need to record some stupid update or the computer brakes or something fucking kill my plans


    By the way on low setting there is like a dead band you go down and you don feel it but suddenly its there


    Please if anybody does the test let me know
     
  16. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    So do you like the cold biases at 26 and 28 per tube? You like crossover distortion?

    I cannot stand that stuff. But everyone has there own opinion.

    What do you guys think?
    To me that is to close or into crossover distortion for those amplifiers.

    Valve/tube speaking:

    A hotter bias does not give more headroom. It is the opposite. It just provides a clearer more detailed signal up front. A hotter bias drops the B+ a little and places the incoming signal closer to that level which actually decreases headroom of the power tubes. The thing is the power tubes are now putting out more power, quicker, so you end up lowering preamp signals and turning down the overall volume which in turn keeps the signal cleaner.

    A colder bias gives more headroom but when bias point is too low the operation gets into crossover distortion. The bias is colder and the B+ goes up a little giving the power tubes more headroom. But too cold and the bias point is too low putting out less power and causing the user to up the volumes for more signal which in turn makes the signal dirtier.

    Still I cannot stand to go too close to crossover region. The amplifiier looses some detail and bottom frequencies do not sit right.
    My EL34 at around 460VDC stay around 38-41mA. I really do not like going belwo 34mA much less 28mA.
    My 6L6GC/5881 at around 470VDC stay around 28-31mA. I tried lowering my Bassman to 24 and even 22mA and it just starting loosing detail and sound like ass compared to a good bias level. (Yes 6L6 types bias much lower than 6CA7/EL34 types.)

    Moral of the story is that you are altering the entire signal and amplifier when biasing at different levels. It has less to do with power tube headroom but rather the entire amplifier's headroom.
     
    Last edited: Aug 21, 2019
  17. The Dose of Harmony

    The Dose of Harmony Well-Known Member

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    Sorry man i am still work and i was not clear and typing fast I liked when it was hot!

    I tested more the Jmp on the hit side because sounded good

    I am not an amp tech i am just staring to get into amp mods and understanding how amps work i just bring in the questions and you guys are the experts
     
  18. MarshallDog

    MarshallDog Well-Known Member Premium Member

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    Thats exactly what I found a couple years back but I thought it was very subtle at best. Its a thick hair of difference IMO! So, I just do 70% dissipation based on the voltage.
     
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  19. The Dose of Harmony

    The Dose of Harmony Well-Known Member

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    I was wondering if you saw the video that posted?
    And let me know if he is wrong where its his mistake
     
  20. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    The video on the single ended amplifier, I did not.
    I will later when I have more time.
     
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