Rectification with tube power supplies.
This is probably the most misunderstood part of a tube amplifier. It can be a mistake to simply copy something, just because it was talked about, as 'very good'. Cases where good design rules were not respected we find as well with companies of high reputation, as with any other company. Interestingly, we find less of such errors with DIY.
Just to mention some of the most common errors:
- Ignore minimum required DC resistance of the transformer Raa winding. This is a mistake, slowly killing every good rectifier. People are often totally surprized this requirement exists at all, and transformer manufacturers never heard about it anyway. So on the market are many unqualified transformers and amplifiers. Though this is clearly written in every tube data sheet, and good book about tube circuit design.
- Adding 2...20x more than the maximum allowed microfarad as a first capacitor for the rectifier. Just because 'it works', doesn't mean this is not a tube killer. (Also because of the next point)
- Draw excessive surge current from a rectifier tube during cold start. Caused by a low cost choke, which saturates at start up, too big capacitors, or other causes in the amplifier itself, at start up, such as DC coupled tube stages, adjustable bias, and other risky things.
- Combine the above by not measuring peak current, and start up current. Just copy schematics, makes you copy the mistakes too. Just to give the classical warning once more, when you see more than 33uF on a 5U4G, or more than 4uF on a 274B tube, such a schematic is made by an ignorant designer. So this can already by seen from picture files. Whereas transformer Raa Resistance can be measured simple with a low cost multi meter. Yet while peak current is hard to measure, at least you should make a simulation, for instance with 'PSU Designer', which is freeware for DIY, and the best program of it's kind. PSU Designer will work quickly and easy, and simulate peak current with Oscilloscope-like results, at any point of the circuit you select. For the hard core users there is LTSPICE freeware from Analog Devices. Though LTSPICE will take a very long time to understand when you are new to it. You can not see this when using PSU designer, but it is written with LTSPICE.
Do not treat tube diodes like solid state diodes.
To give you some sensitivity for this item, take very good note of the following. When comparing tubes with solid state devices, the elementary difference is: With solid state devices it is more difficult to build such for high voltage, as it is for high current. With tubes, it is reversed. It is difficult to build such for high current, and high voltage was never a problem. We have to realize, when stressing exactly this natural limit, the result will cause problems. So solid state diodes have less of a problem with repeating peak current for 25 years of use, but they definitely will have a reliability problem, with repeating the peak voltage for a life time of 25 years.
With tubes, this above situation is reversed. Tubes have little problems when the peak voltage, but they will develop problems quickly when the peak current is reached continuously. So design considerations are totally another, and it would be completely wrong to apply the design considerations for solid state diodes to vacuum diodes as well, saying 'a diode is a diode'.
With solid state devices we all know, you can not exceed the peak voltage. If you do this only once, they can be dead..
Even greater care should be taken with tubes, since these are devices with limited lifetime, and they are expensive. If you abuse them with peak current at start up, or exceed it during normal operation, lifetime will be shortened severely. We repeat the same as before: As this is impossible to calculate with precision, good designers will measure it with a good instrument, or alternatively over dimension the design ratings. This is in order to stay safely away from the damaging limits.
Many good books have been written about tube power supplies, and of course we can not compress this in a few pages here. Keep three elementary things in mind, to get good tube lifetime: 1) Respect the maximum first capacitor value in the datasheet, and be aware when you use a maximum factor on the one end, you get a compromise on the other. 2) Understand and apply the derating curves, meaning at maximum current, you can not use the maximum voltage, and vice versa. 3) Use the MINIMUM copper resistance needed for a TUBE power supply transformer. Transformer building companies can not know what you need, so without specifications from your side, you get just 'something'. Which is only fine when you need 'anything'. This is so for solid state, but this is not so for tubes. You need to specify the copper resistance to them, and if they can't do the value the tube requires, you need to add the difference by using external resistors.
These rules above, need to be respected to get good tube life, and failure free operation. For maximum lifetime of any object, and sure for electron tubes, keep some distance from maximum limits. That is: maximum capacitor value, maximum voltage, maximum current, and minimum copper resistance.
Low residual hum
With a solid state power supply, the last final bit of hum can be eliminated by using larger capacitors. This is no problem for the diodes, as they do not suffer from transient current. With a tube power supply, the last final bit of hum, is not present because capacitors are too small, but because the choke is too small. Moreover, if circuits are not ideal you would be tempted to tweak components values, but better would be to use an improved circuit. The most commonly used circuit is Circuit #1, as below here, but this is the most unintelligent. In very many cases, simply use circuit #2 already brings an improvement. We call this 'star grounding'. Tough Circuit#2 is far from ideal, and Circuit #4 does the same even better and with less wiring. Circuit #3 shows one of those typical little things, you can observe when somebody does not treat a tube like a solid state diode. This makes sure the left side of the tube heater wire has the same current (and the same temperature by that) as the right side of the heater wire. Whereas Circuit #4 is such a logical and easy thing to do, but people do not understand why, and take Circuit #1, this is really painful. It is allowed to use Circuit #4, also if you do not understand why it is better, as it is free of patent rights. The words are: No ground loop risk, and electrical stray field. Circuit #5 is the ultimate circuit. Only possible with Lundahl chokes. This is the most beneficial circuit ever, for lowest hum. The choke is not only used here as an inductor, but also creates inductive decoupling of the complete rectifier parts from the amplifier circuit. There is no direct connection any more! All hum signals, no matter how they try to flow, must pass a choke now. Please also read the short explanations on the right, below here. After that I hope you understand why we say it is so silly to find Circuit #1 in most 'professional' designs.
Circuit 1: This is the most commonly used. What is not so nice about this circuit, will get clearer when you understand the benefits of the other circuits here.
Design Quality of Circuit1: One Star *
Circuit 2: Added here is a bleeder resistor, a ground connection point, and if needed an external windings resistor. The ground connection point prevents any faulty ground path. This is called 'star' grounding.
Design Quality of Circuit 2: Two Stars **
Circuit 3: This avoids the rectified DC current flows through the transformer heater winding. This will reduce Transformer hum. However circuit #4 is more simplified and will do the same.
Design Quality of Circuit 3: Three Stars ***
Circuit 4: This circuit refers the Choke AC and DC electrical field to ground, where they can cause less problems. Also the ground path for the first capacitor is now forced in a correct way, same as in circuit 3.
Design Quality of Circuit 4: Four Stars ****
Circuit 5: This is how to connect a double coil Choke, such as the Lundahl LL1673 or similar products. Beware the polarity of the connections. (the dots in the circuit diagram here). The importance of this circuit is very high. We have complete inductive separation of the transformer from the amplifier. Definitely, the transformer capacitance from primary to secondary can not inject an AC hum current into the amplifier any more.
Design Quality of Circuit 5: Five Stars *****
This is to empty all capacitors after power off. Using no bleeder resistor is safety issue, as voltage may stay on the capacitors for a long time. Also use of no bleeder is a reason for a sparking rectifier, as a repeated switch on, with partially charged capacitor may trigger a spark. Make sure the resistor discharges all capacitors to less than 40V in one minute. Do not mount the bleeder directly on a capacitor, because this heats the capacitor, and this is not good for reliability.
The special knowledge of tube transformer making has gone lost. Each rectifier requires a certain MINIMUM windings resistance, or the tube will have low lifetime, or even show a white spark. Transformer Raa is measured between the tube Anodes. In many cases the value is too low. This major design error can be fixed by adding an external resistor as shown. This resistor has half the value of the total Raa required. If a tube needs Raa of 250Ω Ohms, and you measure 100Ω only, there is 150Ω missing. So to add this 150 Ohms, you need to serialize two resistors of 75 Ohms. each at transformer winding ends. Alternatively you can add one single resistance of 75 Ohms at the Cathode Tap, which has the same effect. Then, from the tube's view you have Raa of 250 Ohms again.