power supply will eat up rectifier tubes?
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seavote
Well-known member
thanks to everyone. i have learned quite a bit. i cant say i understood everything (but i'll reread,research,think etc. and i'll get most of it)but i did understand alot. i'll go with prr's layout it seems to take all rules of thought into consideration. i'd like to ask another newbie question. what designates one filter stage from another? a voltage drop? can a 0 ohm resistor designate another stage? i wouldn't think so.
HotBluePlates
Member
Let answer your first important question first...
You must know Ohm's Law: Current = Voltage / Resistance.
It is usually written this way for technical reasons... You can almost always directly control voltage or resistance, but you can't directly control current. You do that by varying the voltage or the resistance.
That said, you can rearrange this equation with algebra to put it in whatever form is most useful. For voltage drop, that form is Voltage = Current * Resistance. What that says is that you know a voltage on one side of a resistor. If you know the current flowing through that resistor, then the voltage on the other side is "dropped" (reduced) by current*resistance. If you put one lead of your meter on either side of the resistor, you will be able to measure this voltage drop.
What designates one filter stage from another?
Well, the easiest thing to look for is a cap to ground. Between that cap and the next downstream, you will see either a resistor or a choke. Either is there to assist the cap in reducing ripple on the power supply, or said another way, to get rid of a.c. while keeping d.c. One common means of deciding what the resistor value should be is to decide how much lower voltage you need at that supply point, as compared to an earlier point. This resistor is best termed a "power supply decoupling resistor", but in guitar amps, it is commonly called a power supply dropping resistor. That's because they typically use the highest voltages available for the output tubes, slightly lower voltage to feed the driver or phase inverter, and lower still for the preamp. All along the way, a separate filter stage is used for each stage, with a decoupling or dropping resistor. Again, the resistor value is usually sized to drop the supply voltage to the value desired.
Now the power supply is often seen by newcomers as a way to light up the circuits doing the "real work". However, do be aware that the signal travelling through the intended circuit is developed from, and travels through the power supply. The goal of using separate filter stages (aside from setting the desired voltage), is to maintain a low impedance to these signal currents, and to keep them from travelling through the power supply where they might interfere with another stage's signal. Hence the term "decoupling" resistor. The goal is to decouple the signal at one stage of the power supply from another stage.
What the deisgner would do is to estimate what the lowest likely signal would be, then size the cap and resistor combo to where the cap provides a very low impedance to ground at that frequency, while the resistor provides a very high impedance to that signal. Basically, to get the signal current to pass through the cap to ground, rather than let it get upstream to another part of the power supply.
Usually, any reasonable value of capacitance/resistance accomplishes this goal, and we just worry about setting the stage voltage. I'd suggest looking at schematics of tube gear to see what "reasonable" values are. The only other thing to say is that where a lot of current is flowing through the power supply, small resistors are used with large caps (remember voltage = current * resistance. Big current with big resistance = big voltage drop; make the resistor smaller for less voltage drop, make the filter cap big to keep the filtering effect). When relatively small current flows, bigger resistors are used (small current * big resistance still equals relatively small voltage drop).
You must know Ohm's Law: Current = Voltage / Resistance.
It is usually written this way for technical reasons... You can almost always directly control voltage or resistance, but you can't directly control current. You do that by varying the voltage or the resistance.
That said, you can rearrange this equation with algebra to put it in whatever form is most useful. For voltage drop, that form is Voltage = Current * Resistance. What that says is that you know a voltage on one side of a resistor. If you know the current flowing through that resistor, then the voltage on the other side is "dropped" (reduced) by current*resistance. If you put one lead of your meter on either side of the resistor, you will be able to measure this voltage drop.
What designates one filter stage from another?
Well, the easiest thing to look for is a cap to ground. Between that cap and the next downstream, you will see either a resistor or a choke. Either is there to assist the cap in reducing ripple on the power supply, or said another way, to get rid of a.c. while keeping d.c. One common means of deciding what the resistor value should be is to decide how much lower voltage you need at that supply point, as compared to an earlier point. This resistor is best termed a "power supply decoupling resistor", but in guitar amps, it is commonly called a power supply dropping resistor. That's because they typically use the highest voltages available for the output tubes, slightly lower voltage to feed the driver or phase inverter, and lower still for the preamp. All along the way, a separate filter stage is used for each stage, with a decoupling or dropping resistor. Again, the resistor value is usually sized to drop the supply voltage to the value desired.
Now the power supply is often seen by newcomers as a way to light up the circuits doing the "real work". However, do be aware that the signal travelling through the intended circuit is developed from, and travels through the power supply. The goal of using separate filter stages (aside from setting the desired voltage), is to maintain a low impedance to these signal currents, and to keep them from travelling through the power supply where they might interfere with another stage's signal. Hence the term "decoupling" resistor. The goal is to decouple the signal at one stage of the power supply from another stage.
What the deisgner would do is to estimate what the lowest likely signal would be, then size the cap and resistor combo to where the cap provides a very low impedance to ground at that frequency, while the resistor provides a very high impedance to that signal. Basically, to get the signal current to pass through the cap to ground, rather than let it get upstream to another part of the power supply.
Usually, any reasonable value of capacitance/resistance accomplishes this goal, and we just worry about setting the stage voltage. I'd suggest looking at schematics of tube gear to see what "reasonable" values are. The only other thing to say is that where a lot of current is flowing through the power supply, small resistors are used with large caps (remember voltage = current * resistance. Big current with big resistance = big voltage drop; make the resistor smaller for less voltage drop, make the filter cap big to keep the filtering effect). When relatively small current flows, bigger resistors are used (small current * big resistance still equals relatively small voltage drop).
[quote author="adamasd"]That was a great explanation on the working of rectifier tubes. The most complete explanation I have ever seen.
Thanks,
adam[/quote]
Simulation shows all that PRR states, so even with SS rectifiers I always add a little current limiting resistance the PSU. It saves the transformer, the rectifiers and the caps.
analag
Thanks,
adam[/quote]
Simulation shows all that PRR states, so even with SS rectifiers I always add a little current limiting resistance the PSU. It saves the transformer, the rectifiers and the caps.
analag
