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Thread: Grid stoppers with AB2

  1. #1
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    Grid stoppers with AB2

    So i'm about to fire up and trouble shoot my Bass amp that is utilizing Mosfet source followers to drive the output to class AB2. I just wanted to ask what considerations i need to take into account with regards to the grid stoppers. If they are too high of a value will they eat up all the current from my drivers and slow or prevent the grids proper conduction of current? I know I need a decent value just to keep the grids from melting, but the few examples of AB2 PA's i have looked at vary from 470 ohm to 10k ohm, the latter being what I have in there now.

    Thanks for any advice and help.

  2. #2
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    You certainly don't want to go too high, otherwise you lose most of the benefit of AB2. Unfortunately this can be one of those trial and error things. To figure this out on my amp I basically stuck a variable voltage regulator regulator (that will go up to the max voltage you expect on the grid) and a small current measuring resistor (like 10 ohms) on the grid. Assuming the amplifier has the cathode grounded (ie, a fixed bias amp), you can then easily plot the grid dissipation VS input voltage. Depending on the tube, the grid current might be linear or it may draw it in huge blips as the grid voltage increases. I don't really have any experiences with KT88's, so you'll have to find research this. Finally, you can 'calculate' the value of resistor using some guesswork. Ideally since the grid current only flows for half a cycle, you can set limiting resistor lower than the 'safe' rating (assuming you have a safe limit defined). Personally I'd stick a 1K to 2K value on the grid and call it a day (and monitor whether the grids are glowing bright red )

    I haven't tried this with my digital oscilloscope yet but I've been pondering a method to do it via trial and error. You would simply crank your amp to max, multiply the voltage across the grid resistor (divided by the grid resistor value - you can do this later) with the voltage at the grid, and take the average (time integral) of this to get average dissipation. Just keep replacing grid stoppers until you get the perfect value! Due to the limitations of my low end scope (it allows one and one only math function to be running at a time), I'd either need a differential input to measure the voltage across the grid, or isolate the ground from neutral. Or I could dump the data to my PC and do it that way.

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    I assume that you have a power tube like KT88 or GU50.
    For GU50 the G1 max power is defined to 1W, I would assume about the same for the KT88.
    This is the way I have choosen:

    - limit the maximum positive voltage at the Gate of the MosFET via Zener+Diode to not more than about 30V.
    From my experience this is enough positive voltage for AB2
    - supply the drain of the MosFETs with about 30-40V then you don't need a heatsink
    - use a grid stopper - it is more a current limiter- of 100 Ohm (edited, checked the value again)

    This is working quite well in my bass amp (6*GU50, 360W output) in AB2
    Last edited by es345; 09-30-2012 at 11:35 AM. Reason: Value correction
    Steve Conner and Austin like this.

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    Ok I'll try some 1k's for now and do some maths once i have things running smoother. I also am going to add something to drop my positive voltages down a bit I think I have about 80V right now.

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    Tage,
    Just a few thoughts. The higher the gm of the output tube the more susceptible to parasitic oscillation it is and the higher the grid stop you are likely to need.
    So KT88 need higher values than 6L6 etc.
    Like es345 I am a fan of mosfet source followers to drive output tubes. Most of my experience is with Tube HiFI Power Amps but I have noted the following:

    1) Choose the Drain Voltage such that on the maximum positive signal swing on the gate and hence also on the source you have at least 25 to 30V left across the mosfet drain to source. This effectively minimises any modulation of the device capacitance with the audio signal. Probably not critical in a MI Amp but quite noticeable in the smoothness and detail of the top end in a HiFi Amp. No point in going higher on the drain voltage, all that does is increase the power dissipation in the mosfet.
    2) Don't use huge power mosfets, I use ZVN0545A (600mW) when I can or a 2 to 5 Watt rated device when the 600mW is'nt enough.
    3) Direct couple the mosfet source to the output tube grid with just the grid stop in the path
    4) Apply the output tube bias to the mosfet gate.
    5) Dont forget the protection zener (12 or 15V) between gate (zener cathode) and source (zener anode). Right on the mosfet gate and source pins.
    6) DO use a gate stop on the Mosfet, 220 Ohms to 1K will do, Mosfets have lots more gm than tubes and accordingly are even more prone to parasitic oscillations because of it.
    7) The source load resistor should return to a negative rail of at least 3 times the output tube bias voltage (rule of thumb).
    8) If you want to go a little overboard (or if you are building a HIFi Amp) replace the source follower load resistors with current sources, they don't need to be particularly great current sources, the "Ring of Two" bipolar transistors works very well, a high beta, garden variety small signal BC548C for example on the bottom with an MJE340 above it for 300V withstand is what I routinely use. Th Current Source Loads do sound better than simple resistive loads but again that is my HiFi Amp experince talking. You may not notice the difference with a MI Amp.

    OH! and do remember to fit the output tube screen resistors with the resistor body right up against the tube socket pin, just like you do for the grid stop. Screen resistors have a grid stop function as well as their screen dissipation limiting function.

    Hope you find something useful in this.

    Cheers,
    Ian

    This arrangement (source follower drive of the Output tube grid) give one huge benefit which is very often overlooked. Output tubes suffer from a fair amount of grid current noise (random grid currents). With the low impedance drive of a source follower, this noise current is shunted to signal ground and the amp is MUCH quieter. HiFi Amps I've built using this scheme have the "blackest" background of just about any amp I've ever heard, tube or SS.

    I've attached the schematic od one of the HiFi Amps I designed and built to illustrate the "Rave" above.
    Attached Thumbnails Attached Thumbnails bh_fixed_bias_a.jpg  
    Last edited by Gingertube; 10-11-2012 at 05:22 AM. Reason: Added example schemo

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    Thanks Ian, I have looked over your schematic quite a bit, it anlong with es345 gu50 amp design heavily informed my design, although I'm certain my build quality won't match up to your amps, it will work for this amp.

    I have implemented most of your design conciderations, except the constant current source, that will have to be a part of the next ab2 amp i build.

    I wired up some 2.2k grid stops, i'm just waiting on some power supply caps to arrive then time to debug.

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    I get the +30 leaving room for the positive drive plus enough anti-wiggle room to keep things anchored. I'm not sure I follow the -3x bias on Vss though. When you swing past cutoff, it's just that - cutoff. You're not clipping the tubes any harder, the zener protects your gate from -Vgsmax, and the only thing left needing protection is your constant current load - which could be protected with a diode against Vcc < Vee.

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    Just a random thought.

    The purpose of grid stoppers is to introduce a reliable high frequency rolloff filter into the tube itself. The tube has internal capacitances from grid to cathode and grid to plate, and these interact with the driving impedance to form an R-C lowpass filter. It's one of the fundamental limitations on the high frequency response of the tube. The fact that the resistor is a single, lumped value that's not dependent on the tube bias point, gain, age, or phase of the moon also forces the rolloff to be much more predictable and stable, good things for anti-oscillation filtering.

    Grid stoppers work by putting a resistance right at the grid. This both lowers the rolloff frequency of the source impedance and the tube capacitances, but also damps the RF resonance of the L-C formed by the tube capacitances and any wiring inductance. At RF, the distributed L-C tuning can also contribute to an oscillating tube. This is why it's important that it be right at the grid - you don't want to build up distributed L-C in the wiring to the grid from the resistor.

    If you are wanting to use the smallest possible grid stopper, there's another way. Put in the grid stopper you think you can stand with the followers you're driving the grid with, and then increase the grid-cathode capacitance with a real, physical capacitor between the grid and cathode - NOT from plate to grid. The grid stopper forms the real-resistor part of the equation, and the external capacitor, presumably large enough to swamp other varying capacitance values, but not large enough to damage audio response, keeps RF and ultrasonic oscillation at bay.

    This lets the grid stopper have less effect for Class AB2, but still keep its RF-preventative effects. This may be touchier, but is as doable as using a bigger grid stopper and hoping. May take a bit more thought, though.

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    Quote Originally Posted by NateS View Post
    I'm not sure I follow the -3x bias on Vss though. When you swing past cutoff, it's just that - cutoff. You're not clipping the tubes any harder, the zener protects your gate from -Vgsmax, and the only thing left needing protection is your constant current load - which could be protected with a diode against Vcc < Vee.
    From my experience the Mosfet should never go into switch off mode. As they are designed for switching you may have unwanted high frequency oscillations at the point when the Mosfet is switched on again - and that only for a short time. The result is as short distortion peak every time it happens distroying the sound very badly. I have observed this behaviour during the design of my GU50 bassamp. It took me several days until I have catched the issue on the scope (oscillation at the falling side of the sinus test signal) and understood what has happened. The fix was easy - I have doubled the negative supply voltage for the source circuit.

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    Again, just from intuition and not simulation or experience, so long as the tube is complete cut off before the mosfet cuts off, and the mosfet is conducting again before the tube begins conducting, you avoid any switching transients assuming adequate bypassing on Vdd/Vss.

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    I haven't expected this effect as you do not expect, but the reality told me a lesson. I didn't make a complete root cause analyse, but I suspect some RF coupling via the Mosfet powersupply from the nonconducting to the conducting part of the push pull circuit

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    Some interesting discussions here => KT88 AB2 Experiences - diyAudio

    Jaz

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    NateS,
    es345's point is that mosfets can emit a burst of RF as they switch off. That RF radiates into all sections of the amp and will show up on the other side of the push pull. Best (as he says) to make sure the -ve rail is large enough that the MOSFET does not switch off.

    I've seen this on the oscilloscope.

    Cheers,
    Ian

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