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Is that really close enough?

Discussion in 'Shock Brother's DIY Amps' started by RollingBender, Jan 26, 2021.

  1. RollingBender

    RollingBender Tele-Afflicted Vendor Member

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    I’ve been messing around, tweaking my latest build...
    https://www.tdpri.com/threads/super-lead-type-build-see-how-many-boxes-i-can-check.1059658/

    I am using @robrob bias calculator page. Running the numbers, 50% max dissipation has me at 20ma. I connected my multimeter to ground and the cathode pin on one 6BQ5 and adjusted the bias knob until I hit 0.020vdc (=20ma).

    Multimeter still on and connected, I started playing a bit of guitar. Gain on both channels cranked, volume about half way up. Loud, crunchy, and glorious 70’s hard rock tones!! I look down at the multimeter display screen as I’m playing and see that it is now reading an average of 0.040vdc with spikes up to 0.050vdc on loud low full chords.

    40ma ~ 98% max dissipation and 50ma ~ 123% max dissipation.!!!

    So, is “set it at 50% at idle and you are good to go” really good enough? Am I interpreting these observations correctly?
     
  2. robrob

    robrob Poster Extraordinaire Ad Free Member

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    Yes, you are fine. The Valve Wizard says 80% idle for push-pull is ok so at 50% you're running cool, like many 1950-60's amps from the factory.
     
  3. dan40

    dan40 Friend of Leo's

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    Yep...it is called the idle bias because we only want to measure the current draw when the amp is at idle. When you start to play and push the amp a bit, the current draw will shoot up quite a bit. Even if it exceeds 100% on a loud power chord, it only has to supply that current for a short period of time so the tube's do have time to rest a bit. B+ voltage will also sag a bit when pushing the amp hard and this in turn causes the dissipation to drop slightly.
     
  4. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    There is also screen current added to the plate current when you measure with the 1 Ohm resistors. So, you are fudging the numbers. Once you take off the screen current your 40mA figure could equate to ~83%PD and the 50mA figure could equate to 104%PD. (Actual % may vary from my example.)

    As dan40 says you will not be exceeding the 100%PD for long. The tubes can handle that in short bursts.
     
    Last edited: Jan 27, 2021
    dan40 and sds1 like this.
  5. David Barnett

    David Barnett Doctor of Teleocity

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    That's how Class AB works...
     
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  6. RollingBender

    RollingBender Tele-Afflicted Vendor Member

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    @Lowerleftcoast do you have the maths behind this? I would like to learn more. This is very interesting and I would like to learn how I would properly measure accurately what is going on when the amp is pushed. An amplifier version of a dynamometer!!!
     
  7. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Before we go off in the weeds, tube bias is different for every tube. Tubes are like snowflakes no two are alike. The tubes will run in a wide variety of situations. The main thing is to have them not redplate. Next we'd like to have them sound good. I think the best dynamometer is your ears. Dial in the bias by ear and then check to see if the bias is in a sustainable range.
    To make this easier on me, I will defer to robrob as he has already provided an excellent resource with his tube bias calculator. Rob pointed out that his bias calculator has the equations listed, and it does.
    If you scroll down on his site, you will come to: *How to measure bias. The Output Transformer Resistance Method.* This method provides the information to calculate the current for the *plate* only. Since we want the *Plate Dissipation*, this is *one* method to get that number. It uses Ohm's Law (V=I x R) to derive the current from the voltage drop and resistance. *V* is the voltage drop across the resistance of the OT. *I* is the current we are looking for. *R* is the resistance of the OT coil from the CT to the plate. Rearranging the equation to find current we have I=V/R.

    There seems to be confusion about how to measure the voltage drop. The most accurate way is to measure voltage directly over the resistance. In this case from the OT CT to the tube plate. The meter will detect only a few volts, typically less than 5 VDC. Most multi-meters will provide the voltage with decimal points. Caution must be used, the voltage on the CT or at the plate is several hundred volts. It is at or near B+. The meter is only showing the difference between those two high voltages.
    One bit of confusion stems from the indirect measurement of the voltage drop. Measuring the high voltage at the CT and subtracting the high voltage measured at the plate. Technically this is the same as measuring the voltage drop. An inaccuracy occurs though. Most multi-meters do not provide decimal points when measuring ~400 volts. Subtracting the plate voltage from the CT voltage essentially rounds off the measurement, which makes for a less accurate bias measurement.

    A multi-meter can measure current directly. Measuring the current across the OT is a riskier venture. When this method is used there is ~400 volts on both probes when the high voltage is on one probe. If the probes of the meter slip and make contact to the wrong point, the current measured can be well past the ability of the meter. The meter will fry and there still may be ~400 volts on both probes.

    Measuring at the Cathode:
    Measuring the voltage drop across a 1 Ohm or 10 Ohm precision resistor on the cathode makes the Ohm's Law calculation very easy to perform. With a 1 Ohm resistor V=I. If 34mV is measured, the calculation results in 34mA of current. Measuring at the cathode measures both plate current and screen current. To more accurately determine plate current, Rob approximates the screen current as 5.5% and subtracts that from total current. This is only accurate for tubes that actually have a screen current that is 5.5% of the total. Since Rob's bias calculator covers many different tubes, his approximation is conservative for most tubes on the list.
    Measuring at the cathode is easy. It doesn't have the meter probes on high voltages. It provides a conservative approximation of plate current, so the amp should be in no danger of redplating. With a little engineering it can be designed into the chassis so the bias can be checked without pulling the chassis. Many shock brothers use this method. After-all close enough is good enough when biasing an amp.

    To more specifically answer your question, ime it would be difficult to measure the peak bias condition of your amp just using a guitar and watching the multi-meter. Perhaps you could adjust the amp to redplate with normal guitar playing and then back off a few %.

    For the approximation I made in post #4, I subtracted 17% for screen current. I chose 17% very arbitrarily. (I recently measured an amp with 6V6 tubes to have 18% screen current.) I do not have enough data with your amp to determine the screen current. The 17% figure was just a guesstimate. Maybe you will find the plate current and get a more accurate number for the screen current on your 6BQ5 tubes.
     
  8. 2L man

    2L man Tele-Meister

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    When carhode current increases the anode voltage decreases and when tube power is a multiplication of them it is fine to see much higher cathode currents.

    Open this calculator

    https://www.vtadiy.com/loadline-calculators/loadline-calculator/#calculator

    Then you can select the tube and tab push pull box when default is SE. Then add voltages, bias current and OPT impedance and you see immediately what your loadline looks. When you add headroom you see its power and what kind of distortion there comes. Look what effect to g1 and screen voltages have each others and how "knee" height, which is current, changes.

    Bias current also changes distortion. It is difficult to say how true it is to actual amp but it is possible see what direction they go. When A-class SE power amp bias is adjusted we can see how effective operative voltage increases when loadline starting point moves to the left and its because OPT being a coil can store energy which shows as a voltage when it "releases" current. Same feature is on AB amps as well which effects how far it operates mostly A-class and the point where amp changes to B turns loadline steeper up when only one tube drives OPT impedance.

    Also how beam tetrode drive signal needs to become positive for full power drive (power at 0 g1) but using higher screen voltage less drive is needed. This is the difference on datasheet tables when you see AB1 which has lower output power and AB2 which output power comes higher when tube(s) are driven to positive g1 voltage.

    Loadline is just a tool and In real life when tubes see impedance which changes depending freaguency there is no line anymore. Current/voltage signal fills some kind of vaque oval are both side of line but as a tool to tune tube circuits it works great.

    Or reply your data here and I do it. I always use it when I see other builders data which seems different what I have used myself.
     
    Last edited: Jan 31, 2021
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