Announcement

Collapse
No announcement yet.

Why only half-wave rectifier on bias?

Collapse
X
 
  • Filter
  • Time
  • Show
Clear All
new posts

  • Why only half-wave rectifier on bias?

    Why is the bias on tube amps only a half-wave rectifier? I typically only see a single diode, dropping resistor and capacitor.

    Why not a full wave rectifier, like a bridge rectifier, with a high value capacitor to smooth out the ripple?

  • #2
    Cost, and full wave not really needed.
    Half wave works just fine and amp makers are penny pinchers, or the bean counters are in charge....
    If it ain't broke I'll fix it until it is...

    Comment


    • #3
      Agree completely. There is almost no current drawn from the bias supply, so there is no need for extra filtration and smoothing.
      Education is what you're left with after you have forgotten what you have learned.

      Comment


      • #4
        KOC uses full wave on some of his amps where bias comes from the high Voltage secondary. It helps lower the impedance of the bias supply which is important when you have a high B+ and want individual pots for each of four output tubes.

        When bias comes from a secondary tap, there is no option for full wave unless you want another tap or a floating winding like the Traynor YBA-1.
        WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personnel.
        REMEMBER: Everybody knows that smokin' ain't allowed in school !

        Comment


        • #5
          Even amps with a separate winding often go half wave. for example Peavey. 5150, MAce VT, etc.
          Education is what you're left with after you have forgotten what you have learned.

          Comment


          • #6
            Originally posted by loudthud View Post

            When bias comes from a secondary tap, there is no option for full wave unless you want another tap or a floating winding like the Traynor YBA-1.
            ^^^ This is the primary reason.

            A secondary thing to consider as that a half wave rectifier will produce ripple with a fundamental frequency equal to the power supply i.e 50 or 60Hz. Guitar speakers are generally very inefficient as such frequencies making the ripple less audible. If your were to double the frequency for the same ripple amplitude the speaker will certainly produce more audio output.

            A simple improvement for bias hum in a PP tube power amp is to add a bias balance pot.
            Last edited by nickb; 10-23-2020, 09:12 PM.
            Experience is something you get, just after you really needed it.

            Comment


            • #7
              Originally posted by nickb View Post
              A secondary thing to consider as that a half wave rectifier will produce ripple with a fundamental frequency equal to the power supply i.e 50 or 60Hz. Guitar speakers are generally very inefficient as such frequencies making the ripple less audible. If your were to double the frequency for the the same ripple amplitude the speaker will certainly produce more audio output.
              Plus the ripple should be cancelled out in push-pull anyway if it is reasonably balanced, or is that what you were hinting at with the balance pot comment?

              "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

              Comment


              • #8
                Originally posted by g1 View Post
                Plus the ripple should be cancelled out in push-pull anyway if it is reasonably balanced, or is that what you were hinting at with the balance pot comment?
                In a perfect stage the common mode gain would be zero. At idle the common mode signal is the bias ripple and so the output would be zero. A balance pot allows you to adjust that common mode gain closer to zero. Reality is of course more complicated, but that's it in a nutshell.
                Experience is something you get, just after you really needed it.

                Comment


                • #9
                  Originally posted by nickb View Post

                  In a perfect stage the common mode gain would be zero. At idle the common mode signal is the bias ripple and so the output would be zero. A balance pot allows you to adjust that common mode gain closer to zero. Reality is of course more complicated, but that's it in a nutshell.
                  This may seem like common sense... but...
                  At which point do you look for the balance to occur? I assume scoping the output of the PI, where the output coupling caps meet the original bias resistors?
                  "'He who first proclaims to have golden ears is the only one in the argument who can truly have golden ears.' The opponent, therefore, must, by the rules, have tin ears, since there can only be one golden-eared person per argument." - Randall Aiken

                  Comment


                  • #10
                    Just use your ears, balance for lelast hum.
                    Education is what you're left with after you have forgotten what you have learned.

                    Comment


                    • #11
                      Originally posted by Enzo View Post
                      Just use your ears, balance for lelast hum.
                      cool... the simple route!
                      "'He who first proclaims to have golden ears is the only one in the argument who can truly have golden ears.' The opponent, therefore, must, by the rules, have tin ears, since there can only be one golden-eared person per argument." - Randall Aiken

                      Comment


                      • #12
                        The traditional technique is to balance for minimum hum at idle - in the olden days for PP output stage, when speakers weren't too sensitive, that was done by inserting a small signal transformer in the B+ feed, and listening to ear-phones connected to the secondary of the small transformer.

                        Nowadays with super-sensitive soundcard spectrum analysers, you can balance for a large signal condition - which will then take in to account PP stage tube dynamic differences - but you'd have to be keen

                        Comment

                        istanbul rus escort
                        pendik escort
                        sex vidio
                        altyazili porno
                        antalya escort
                        beylikduzu eskort bayan eskort bayan escort antalya sirinevler bayan escort
                        gaziantep escort
                        atasehir escort
                        antalya escort bayan escort atakoy
                        izmit escort
                        ankara escort
                        Sex ?zle
                        Working...
                        X