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Thread: The dying battery simulator

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    Axe-honerated spellcaster's Avatar
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    The dying battery simulator

    I thought this might be interesting to share.....A while back, I read something online that talked about battery sag and its effect on guitar pedals. I found myself thinking about it earlier today, so i went looking. I have no idea whether this is cork-sniffing for pedal addicts, but ir sounds as if it might be legitimate.

    Anyway, here's a simple circuit that lets you construct a "Dying Battery Simulator"...........

    Dying Battery Simulator
    "I know just enough to be dangerous....."

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    Axellent Member Jammin'John's Avatar
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    That's pretty neat.
    Did you get one ?

    JJ
    Tele's & Tweeds.Yes sir.

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    Axetastic doublewah's Avatar
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    Thanks Spell. This actually makes a lot of sense to me. I try not to knock people who can see or hear things I can't.
    Unless they are "under the influence".
    Or trying to sell me something.
    I bought a relic'd guitar because I liked the way it sounded. Then I refinished it because I didn't like the way it looked.

  4. #4
    Axe-honerated spellcaster's Avatar
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    Quote Originally Posted by Jammin'John View Post
    That's pretty neat.
    Did you get one ?

    JJ
    Nope, I don't use pedals anymore. I posted this because I thought the guys that still use pedals would find it interesting.
    "I know just enough to be dangerous....."

  5. #5
    Axetastic doublewah's Avatar
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    This brings something to mind...
    This would be useful to someone designing pedals. It's not exactly something I would want in my pedal-board, but to play around in the studio/workshop. You could apply a lower voltage and if you like that sound, change some resistor values in the pedal to have that sound all the time.
    I bought a relic'd guitar because I liked the way it sounded. Then I refinished it because I didn't like the way it looked.

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    Axe-honerated spellcaster's Avatar
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    I think that article mentions the possibility of building the function into a DIY pedal. It seems like it's something that some people might use. The Ampeg SVT6 PRO that I bought four years ago had a voltage sag pot so you could adjust it within the amp. I didn't care for the effect, at least for bass, but it definitely altered the character of the amp.
    "I know just enough to be dangerous....."

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    Axetastic doublewah's Avatar
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    Yeah, I saw that. I think it is definitely worth experimenting with, if only for the sonic knowledge.

    Current SVT6 Pro's have a Tube Voltage control. Was it called "Voltage Sag" on your amp? Perhaps Ampeg changed the reference on their amp? The author of that article mentions confusion between the term used for amps vs pedals.
    I bought a relic'd guitar because I liked the way it sounded. Then I refinished it because I didn't like the way it looked.

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    Axe-honerated spellcaster's Avatar
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    It's possible that I'm wrong about this. My take on the control in the SVT6 Pro is that it can reduce the voltage being fed to the output tubes, simulating the sag in old tube amps when the power supply can't keep up.
    "I know just enough to be dangerous....."

  9. #9
    Axetastic doublewah's Avatar
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    I think you took it right. I was just curious as to what Ampeg called the control.
    I bought a relic'd guitar because I liked the way it sounded. Then I refinished it because I didn't like the way it looked.

  10. #10
    Axellent Member Jammin'John's Avatar
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    Tube amp sag is WAY different than what the ampeg amp mentioned has. Yes it is a voltage drop but the big deal is the available current drop.
    Sag is one reason I like the 50's Tweed amps.

    There are three main places where sag occurs in a tube amplifier:
    The rectifier:
    If a vacuum tube rectifier is used, sag is generated because of the internal resistance of the tube. Unlike a solid-state rectifier, a tube rectifier exhibits a fair amount of voltage drop which varies with the amount of current passing through the tube. In a class AB amplifier, the current drawn from the power supply is much greater at full power output than it is at idle. This large change in current demand causes the voltage drop across the tube rectifier to increase, which lowers the available plate supply voltage to the output tubes. This lowering of the supply voltage lowers the output power slightly in opposition to the larger input signal, making it act like a compressor. The lowered supply voltage also tends to decrease the available headroom, increasing clipping and changing the operating point of the tube dynamically.

    The transformers:
    The resistance of the high-voltage secondary winding also creates sag. From Ohm's Law, the voltage drop across a resistance is equal to the resistance multiplied by the current flowing through it. This means that there is no voltage drop if there is no current, and the amount of voltage drop goes up linearly with increases in current draw. A typical power transformer B+ winding might have a resistance of 50 ohms - 300 ohms, depending upon the current rating and regulation of the transformer. For example, if the current draw in a push-pull class AB output stage at idle is 70mA total, and it increases to 170mA at full power, there is a change of 100mA in the current drawn through the secondary windings. If the winding resistance of the secondary is 200 ohms, there is a voltage drop of 100mA*200 ohms = 20V in the plate voltage to the output tubes. Likewise, the resistance of the primary winding of an output transformer varies as well, typically 80 ohms - 200 ohms plate-to-plate, depending upon the primary inductance, the transformer power rating, and the rated impedance. This resistance also creates a voltage drop, but the amount of sag introduced is minimal in pentode mode, because the plate voltage doesn't have near as much effect on the plate current as does the screen voltage. In triode mode, there is more sag because the plate voltage has more of an effect on plate current in a triode. The supply sag created by the power transformer resistance lowers not only the plate voltage, but the screen voltage as well, since the screen is nearly always a filtered version of the supply going to the plate. The amount of sag induced by the power transformer winding can be offset if there is a large filter capacitor reservoir to hold the voltage constant during current peaks.

    The filter capacitors:
    The size of the filter capacitors in relation to the amount of current drawn from the power supply also creates sag. The filter caps charge up during the peaks of the AC input cycles, and hold the voltage constant during the "valleys". If the ratio of peak to idle current is high, and the peak current demands are high in relation to the capacitance size, the voltage will sag appreciably during the valleys, creating a lower average voltage. If there is no further filtering, there will also be a 120Hz sawtooth ripple riding on the B+ supply. This normally doesn't induce much hum into the output stage because of the inherent power supply rejection afforded by the push-pull output stage, and the screen supply is usually filtered further with a choke and another capacitor. However, insufficient filtering can induce ripple into the amplifier if the output stage is not well balanced, or if the screen and preamp supplies aren't well filtered.

    The magic behind vintage amps, what makes them sound and feel so good, are their saggy power supplies.{Blues,Rock}

    The magic behind modern amps, what makes them so intoxicating, are their tight power supplies and higher gain preamps. {Metal,Hard Rock,Country}

    Vintage tube amps typically produce about 20% sag, while more modern transistor amps produce about 2% sag.
    Modern attempts to produce sag, such as adding solid state or tube rectifiers, yield sag rates of approximately 5% and 15%, respectively.

    On my old Tweed amps I choose the rectumfrier to give the amp the desired amount of sag.
    5ar4/Gz34 has the least sag at -10dcv they are not used in Tweed amps.
    I use 5u4 to drop 35vdc
    I use 5r4 to drop 50vdc
    5y3 to drop 60vdc

    Sag is ONE of reasons for Tweed amp "feel".

    JJ
    Last edited by Jammin'John; 02-15-2013 at 07:40 PM. Reason: spelling
    Tele's & Tweeds.Yes sir.

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