DESIGNING RECTIFIER CIRCUITS
Typical application
|
Left Chart for following Capacitor loaded tube,
with these values:
- Capacitor input filter. Value chosen 10uF here
- Transformer DC Resistance 170 Ohms for curves 1...6
- Transformer DC Resistance 230 Ohms for curves 7...8
|
Left Chart for following Choke loaded tube,
with these values:
- Choke input filter
- Transformer DC Resistance= Uncritical
- Capacitor value 16uf here.
|
|
About these Charts:
These charts are graphical design tools, saving the trouble of calculations.
Both charts show some degree of derating, meaning you can not have maximum current and maximum voltage at the same time. As the choke loaded circuit is not so hard on the tubes, you will see derating is less at very high current. So the maximum of 265mA can be used up to 350V DC output. This is definitely not possible with a capacitor loaded circuit. Though capacitor loaded circuits perform good at low to medium current, like at 100mA you can get 600V, where choke loaded will only do 470...540V depending on the choke inductance. This is only from the view of convenient use. From the view of good tube life and smallest hum field radiation, choke loaded is preferred, unless you work at very low current, like below 75mA.
From the right choke loaded chart it can be seen, the maximum current of 265 can be used to generate 350Volt DC, whereas the capacitor loaded circuit at 350V can only so 230mA. Yet when generating 480VDC, the limit is 150mA only of choke loaded, or 200mA capacitor loaded. It pays off to check, if you can fulfil the needs with a choke loaded circuit, and if yes, this would be the better circuit to take. So both circuits can do 480V at 150mA, and choke loaded will sure do this at less disturbance to sensitive circuits near by.
How to use these Charts:
- Choose the required DC Voltage on the Vertical axis
- Choose the requires Output current on the Horizontal axis
- You have to be in the white Zone. Otherwise 5U4G can not be used. (or use two in parallel)
- Take closest curves 1...8 for the transformer AC voltage, or estimate one curve in between.
- Note, the curves on the right chart have a smaller angle. (they are more horizontal). Meaning output voltage depends not much on actual DC current.
- Note, this is official RCA information from the RCA tube manual 1954. You can download the 1954 and other manuals from 4tubes.com.
|
These graphs are from the historical RCA data sheets. When designing new circuits yourself, be aware a tube rectifier is more difficult to use than a silicon diode. Very roughly, the electrical model of a tube rectifier is like an ideal diode, with a resistor in series. You need to limit the peak value at any circumstance to prevent defects, and keep it as low as possible for best lifetime. Since peak current is difficult to measure, a more practical way is use graphs such as the one on the left here. All in the end, you will notice you need more than a quick glance on the data. You will always end up with some sort of de-rating. Such as: At maximum voltage, you can not draw maximum current. At maximum current, you can not use the maximum first capacitor. Maximum tube life will not go together with maximum load conditions, etc. Mistakes come from now knowing how these compromises must be made, or copy circuits from the internet, made by others the same way.
Choke loaded circuit.
 The best for the tube, and for low hum field radiation, is a choke loaded tube. This will force almost DC current through the diodes. This may sound unbelievable, but it works like this : The inductance of the choke is a constant current user, or constant current supply, either way, depending if the transformer is charging the circuit, of if the circuit is discharging itself into the load. When the inductance is large enough, the DC current change very little within one AC cycle. So a signal almost like DC will flow trough the choke, and for that reason through the entire circuit. This includes the rectifier diodes, and even the transformer windings. The engine storage buffer for this is the choke itself, which will generate the required voltage, to keep the DC current flowing. Like this, the whole circuit carries DC current. The function on the second capacitor is to prove an L-C filter to reduce residual hum, but AC current peaks of the capacitor are small. Much to the contrary of an C-L-C circuit, which has a very heavy pulsed charge current into the first capacitor. This current with C-L-C circuit can radiate a magnetic field, saturate the mains transformer at it's peaks, and also wear out the rectifier diodes much faster. The much more gentle L-C circuit. can load a mains transformer up to 100% of it's specified ratings, wheras C-L-C circuits can load a transformer only up to 66% (or the transformer will produce audible hum + magnetic hum field)
We much recommend you to have a look at the original RCA data sheets, winch content is too extended to quote it here, however this is the one and only reference for designs with a long tube life. Designs with mistake in it, will initially work. So the fact it "works" does not prove you will get long lifetime.
Please note it can not be the intention here, to explain how to design a good circuit. However we try to tell some things here, that we know are sometimes not looked at very well. Also we encourage you to read the original old data sheets. from RCA, General Electric and Sylvania, and Telefunken. You can download these and a lot more s from 4tubes.com. So look there for datasheets, and the famous RCA handbook
Some important things to remember (unsorted)
1) Fuse Protection:
To protect the rectifier, a slow fuse must be used. If choke loaded, the fuse must be from the output of the DC voltage, to the rest of the circuit. If capacitor loaded, the fuse must be to the transformer center tap. (So where there is a wire to the transformer center tap, inside this wire must be a fuse inserted)
2) The ideal application of ANY rectifier tube, all brands, is Choke Loaded.
A choke loaded rectifier circuits will give better performance in many ways, however it's function is often misunderstood, and for this reason not often used. However we recommend a choke loaded circuits with first priority always.
Advantages of a choke loaded rectifier circuit, vs. capacitor loaded are following:
- Transformer HV winding can be used up to specified maximum output power, instead of 66% de rated value for capacitor load. This is so for all transformers, any brand. Otherwise heavy mechanical hum may appear. In other words: A 100 Watt transformer winding may be loaded only with 66 Watt of capacitor loaded, or 100 Watt if choke loaded. Choke loaded circuits are almost a resistive load for the transformer, whereas capacitor loaded circuits cause impulsive load (with rattling noise).
- Longest lifetime of the rectifier
- Less AC field radiation from the wiring
- Very lower internal resistance of the output voltage, when above 1/3 of maximum output current. This will make the DC voltage independent of the current drawn, within 10%. Capacitor loaded rectifier circuits provide no load regulation, and drop the output voltage rapidly at higher current.
- No need to deal with transformer windings resistance
3) If Capacitor loaded, you must have a minimum required copper resistance of the transformer winding.
This is an old design rule, obligatory for any 5U4G, EML or other brand. If capacitor loaded, the first capacitor must be chosen at or below the maximum value in this data sheet. At EML we adapted to the same values from old data sheets. So there will be no doubt about those values. The minimum resistance is specified for the complete winding. (So not measured from the center tap). In case you use a transformer with too low copper resistance, you need to add one series resistors in each HV winding connection of the transformer. Then, with those two resistors in series, you re-measure the transformer winding, and the result must be as follows:
Raa, value, for Curves 1...6: minimum 170 Ohms
Raa, value, for Curves 7...8: minimum 230 Ohms
If you ignore this design rule, tube damage will result. Also in many "professional" amplifiers, this design rule is not used by designers who do not read the historical data sheets Tube damage can result as a white spark inside the tube at switch on, filament material can chip off, or the tube life will be much reduced. With most amplifiers, the transformer winding is directly connected to the tube socket, and no protective series resistors are used. In most cases, the transformer resistance can be conveniently measured by a specialist, directly at the tube socket, when the rectifier tube is removed first.
4) Never operate the tube in the red area of the graph, above.
Note the graph has a white and a red (pink) area. Operation in the red area is strictly forbidden. Going to the limits is possible, but maximum tube life will not result from this. The "70%" limit for long-life operation applies also here.
Note, when studying the graph, you will see a dotted line, on the right upper side. It looks like a corner of the graph is cut off here. This cut off piece will be larger when the first capacitor is larger. So now, it is specified for a first capacitor of 10uF in this graph. The best way to prevent problems, is not the maximum value capacitors, and use simply a bit higher transformer voltage and larger chokes to get the required result. This will not give less efficiency of the rectifier circuit ! It will however make any type of rectifier tube last longer. Even the opposite effect can be seen, when people over-rate the first capacitor in an attempt to get lowest possible hum. In several cases, over-rating the capacitor will even increase the total hum of the amplifier. Elementary design rules say, you stabilize a high voltage with a large choke, and low voltage with a large capacitor. With maximum value capacitors, the capacitor charge pulses get extremely high, causing hum field radiation into the pre-amplifier wiring and tubes. These charge pulses have a kind of "bad sounding" wave shape, and smallest hum field radiation from this, can become audible if strayed into the pre-amp circuit anywhere. Two design notes for this are at the end of this data sheet. When designing your own circuit, you should really read those notes.
|
