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Old 4th March 2004, 01:32 AM   #1
ark is offline ark  United States
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Thumbs up Recitifaction is a pain in the butt? A few lame questions

I'm trying to understand rectification and have a few questions to help me clear up my understanding of this process. You'r comments will be greatly appreciated.


1)Lets say I run 6.3 amps AC into a rectifier tube? What do I expect to see come out the other side? 6.3 amps DC? Less? How do I guestimate the result?

2)Why not rectify the heater supplies to DC?

3)Why can't I pull a switching power supply from my computer and plug it right in to my amp scematic since it has various supplies of DC? Is it clean?

4)What is smoothing? I read a primer on rectification at:
http://www.kpsec.freeuk.com/powersup.htm

I gives the follwing model but i never see this kind of model used in tube amp circuits

AC -> Transformer -> Smoothing (using to Electrolytic Caps) -> Regulator = Regulated DC

Why wouldn't I want to use this model for my B+ and heater supplies?


Thanks a ton for you insights and assistance.
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Old 4th March 2004, 04:58 AM   #2
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Quote:
1)Lets say I run 6.3 amps AC into a rectifier tube? What do I expect to see come out the other side? 6.3 amps DC? Less? How do I guestimate the result?
6.3A seems very much for any normal rectifier tube are you sure you mean 6.3A.

Anyway the input RMS AC current is usually higher then the DC current out for a typical circuit using a rectifier tubes you can calculate with a typical AC current of 1.2 times the DC output current for each transformer leg with capacitive load and 0.785 times if the load use an inductive input filter. BTW, you dont run current into something, current is drawn by the load.

Quote:
2)Why not rectify the heater supplies to DC?
It is done sometimes, especially for preamps but for power amps it is usually not needed when using indirectly heated tubes.

Quote:
Why can't I pull a switching power supply from my computer and plug it right in to my amp scematic since it has various supplies of DC? Is it clean?
It probably dont give enough voltage for use as an anode supply and also give a lot of high frequency disturbance but in principal it is possible to use as heater supply with some extra filtering.

Quote:
AC -> Transformer -> Smoothing (using to Electrolytic Caps) -> Regulator = Regulated DC. Why wouldn't I want to use this model for my B+ and heater supplies?
You use what is needed, nothing more, in my power amp I use only simple smoothing using capacitors but in my preamp I use an electronic regulator in order to lower noise as much as possible.

Regards Hans
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Old 4th March 2004, 08:14 AM   #3
SHiFTY is offline SHiFTY  New Zealand
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I think you are a mite confused here.

The 6.3V that goes into a tube is to power the heaters. This is a separate circuit, all tubes have heaters, powered generally by 6.3V. Just ignore that part for now.

The high voltage circuit is what you should be thinking about. Generally the circuit goes: AC-Transformer-Rectifier-Smoothing section-DC

The transformer steps up the AC voltage, the rectifier creates ripply DC, which is smoothed by a filter capacitor. Very similar to a diode circuit, try googling for a full wave rectifier, you can do it with 1n4007 diodes if you want!

Tube rectifiers and tubes in general are not good for low voltage high current applications.
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Old 4th March 2004, 02:03 PM   #4
ark is offline ark  United States
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Default what about using a regulator?

after smoothing should/could I use a voltage regulator as mentioned in the linked article above?

thanks. this is very helpful
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Old 4th March 2004, 09:04 PM   #5
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Quote:
after smoothing should/could I use a voltage regulator as mentioned in the linked article above?
It depends on what the intended use is of your power supply.

If you can explain what you are building it will be a easier to advice. Usually a regulator is not needed for anode voltage in power amps neither in most ordinary pre-amps. Most of the classical preamps didn't use regulation.

Regards Hans
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Old 4th March 2004, 09:29 PM   #6
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Default Re: Recitifaction is a pain in the butt? A few lame questions

Quote:
Originally posted by ark
I'm trying to understand rectification and have a few questions to help me clear up my understanding of this process. You'r comments will be greatly appreciated.


1)Lets say I run 6.3 amps AC into a rectifier tube? What do I expect to see come out the other side? 6.3 amps DC? Less? How do I guestimate the result?
If you actually mean amps, not volts, and mean to apply this to the plates (as Hans said, 6.3 or 5V is usually put on the heater as appropriate), it'll take a lot of voltage before you'll get anything out. I'm assuming such a setup as raising plate voltage until 6.3A is registered on the AC side.
Note also that even a 5U4 is rated for an absolute maximum of only 4 amperes, for a very short period with long time between. Any more and likely it will strip the cathode coating or melt something that isn't supposed to melt.

If you meant 6.3 volts on the plates, then you will get anywhere from 9V with zero load, to milivolts under a slight load. With a 5U4, you'll never get even 20mA I bet. Even a tungar bulb (now that's something seen little!) has a significant voltage drop and won't function very well here.

Quote:
2)Why not rectify the heater supplies to DC?
Not worth it as most tubes handle AC heater supply just fine. Some do require it for low hum operation.

Quote:
3)Why can't I pull a switching power supply from my computer and plug it right in to my amp scematic since it has various supplies of DC? Is it clean?
First of all, it does not supply all the voltages required by a typical tube amp. Second, to do so you would necessarily have to leave some outputs on the power supply disconnected, something they often don't take kindly to. They're designed for a specific range of load conditions that computers provide.

Quote:
4)What is smoothing? I read a primer on rectification at:
http://www.kpsec.freeuk.com/powersup.htm
Smoothing is what capacitance does. Diodes rectify the AC waveform by clipping the peaks of the sinewave and directing these pulses into capacitors. Sit down with a pencil and a piece of paper, draw the circuit, draw the waveforms, consider what happens during a cycle, it'll become clear.

Quote:
I gives the follwing model but i never see this kind of model used in tube amp circuits

AC -> Transformer -> Smoothing (using to Electrolytic Caps) -> Regulator = Regulated DC

Why wouldn't I want to use this model for my B+ and heater supplies?
Again, regulation often isn't required. Many tube amps are class A, meaning the load on the PSU doesn't change. Regulation is, by definition, a value of how "stiff" a supply is; that is, its apparent internal resistance. Usually rated in percent, such as, say a PSU runs 340V at 50mA but drops to 320V at 100mA full load; that's a difference of 20V, or 6.3% regulation.
The (these days) more common use of "regulation" is as an electronically-stabilized supply, which often includes hum-damping ability. Like DC heaters, it can be useful in sensitive equipment like phono preamps but usually isn't necessary. It's also costly (power-wise) in output stages so isn't used much there.

Tim
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Old 5th March 2004, 12:15 AM   #7
billr is offline billr  New Zealand
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hello

FWIW there is an excellent tool available at www.duncanamps.com which provides a simulation of various combinations of LC, CRC,CLC, rectifiers, both sand state and empty state.

If you play with this you will see the effects that various types of filters have, and the effect that varying the load has.

As for your other points they have been well addressed by previous respondents.

kind regards

bill
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