DESIGNING NEW CIRCUITS
Left Chart for following 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
Right Chart for following values:
- Choke input filter
- Transformer DC Resistance= Uncritical
- Capacitor value 16uf here.
How to use the Charts:
- For HIGH VOLTAGE at LOW CURRENT, choose Capacitor input, such as 125mA / 620V DC is possible. (AC = 550-0-550)
- For HIGH CURRENT at MEDIUM VOLTAGE, choose Choke input, such as 225mA / 440V DC is possible. (AC = 550-0-550)
- Choose the requires DC Voltage on the Vertical axis
- Choose the requires Output current on the Horizontal axis
- Must be in white Zone
- Take closest curves 1...8 for the transformer AC voltage, or estimate one curve in between.
- Note right chart has quite horizontal lines at high DC current, meaning output voltage depends not much on actual DC current
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 a tube rectifier is like a silicon 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.
The best for the tube, and for low hum field radiation is a choke loaded tube, so an L-C circuit. This will force DC current through the diodes, and AC current peaks are almost gone. It is quite hard to understand how this works. Also this is unpopular because it works at higher transformer voltage. An L-C circuit has the magic of load regulation too. So at higher current draw, the voltage will drop not very much. Still a choke costs more than just a capacitor. The first compromise is often to go for a capacitor loaded tube anyway, so a C-L-C circuit. However de rating is higher here. So look in the graph on the left, and you see that at 150mA at 650V is in the red zone, it is not allowed. At 650V you can only draw 80mA with curve #8 (550-0-550V transformer), or even only 28mA at curve #7. Note, this is official RCA information from the RCA tube manual 1954. You can download the 1954 and other manuals from 4tubes.com.
The following information is for a pure capacitor loaded tube, so even without a choke. For any other circuit, 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. However, since a tubes have such a thing as lifetime, any design errors will result in white sparking, and/or reduced lifetime. Not just EML rectifiers, but any rectifier, new, NOS or used.
1) Find the required transformer voltage, when you know the required DC voltage, and DC Current.
For this, select the output DC Current on the horizontal axis, and the output DC voltage on the vertical axis. In the graph there is the green dot, where the arrow points at. This is an example for 175mA @ 350 Volts. From here move vertical up and down, and you find the lines of 350V~and 400V~. You have to estimate the line that would be at the indicated point. Here, that would be at 365V. The required transformer is 365-0-365V
2) At a specific transformer voltage, find the relation between output voltage and output current.
For this you see eight curves, numbered 1...8. The upper curve (#8) is for a transformer of 550-0-550V~. For instance at 150mA output current you find 600V= at the vertical axis. For transformer voltages that are in-between, you need to print this graph, and pencil in the required line.
Note, from the graph it can be seen, that curves 1...6 may be used at full 225mA maximum current. The curves 7 and 8 have some de rating, which means you can not use the full current of 225mA any more at this high DC voltage.
DESIGN RULES FOR 5U4G RECTIFIER CIRCUITS
Please note it can not be the intention of a data sheet, to teach any user how to design a good circuit. So it is assumed you already know how to do this. However we try to tell some things here, that we know are sometimes forgotten. Also we encourage you to read the original old data sheets. from RCA, General Electric and Sylvania, and Telefunken.
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
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.
Raa, value, for Curves 7...8: minimum 230 Ohms
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.