Not just an replacement, but an improvement! 

EML 274A / 274B
Mesh Version

Last Revision: June 5th 2011


Note, Emission Labs was the first to re-introduce the 274A again, in the year 2001.

This Data sheet. is about the 274A and 274B, which only difference is the tube base. These tubes are a full wave rectifiers of a special kind, and electrically equivalent to  the original  Western Electric historical tubes. Our tubes feature a series connected filament for both diodes together. The series connection  makes the middle of the filaments to be virtually grounded, when  using a filament supply winding with a center tap. This tube is can supply 140mA with a normal C-L-C filter.   The 274A and 274B are designed specially for Audio purposes, and is optimized for use with a small size first capacitor. With this tube, you will have only normal rectification current  flowing in the rectifier circuit, and no heavy current spikes.  This will result in less hum radiation into of the preamplifier, an improved sound compared to low impedance rectifier tubes for industry applications,  or solid state rectifiers.  Also the 274 is specially suited for applications with relatively high DC voltage. The new ceramic sockets we use, are a good help for this, since these give no leakage, and do not age under high voltage.

For ultra low ripple,  it is recommended to use the Lundahl LL1673 dual coil choke in low CMR configuration. In this configuration, there is virtually no field radiation from the choke. (See  link to circuit diagram, at the bottom of this page). 

  • Slow Start ( 2...7 seconds)
  • Filaments are series connected, for best symmetry of the two diodes inside.
  • Two extra large getters
  • Each tube is numbered, inside the bulb with a metal Tag
  • Two extra large getters, flashing the complete tube bottom
  • These tubes are shipped in a high quality box
  • Tube printing with real gold (metal), red color is burned into the glass
  • 274A has machined, solid tube base pins for highest contact precision. Ideal for the YAMAMOTO tube sockets. 
  • Starting May 2011, we have started to ship the 274B-mesh with the new ceramic socket, with five pins. From Yamamoto, the Octal 8/5p Teflon socket is recommended. This is an octal socket, but has five pin holes only. It is specially used for this rectifier. When working with Octal sockets, you will find the Yamamoto an amazing top class product. For this rectifier you can take the special octal version with five holes only. Like this you can never put in a non-rectifier tube by mistake. You can also use normal octal sockets of course.
Typical application 
Filament Ratings
Filament Voltage
= 5 Volt (AC or DC) 
Filament Current 
~ 2.4 Ampere 
. .

Maximum ratings.
possible simultaneously
Take one maximum number, you must de rate another
READ all notes at the bottom of this Data sheet.

AC input voltage
First capacitor,  connected to plates
DC output current
Typical Conditions 
for C-L-C filter  See: Note 2 
DC output voltage
Total rectified current
First capacitor,  connected to plates
10 Henry 
Second capacitor 
max 20uF

For a total (C1 + C2) capacitance above 24uF, a limiting series resistor must be used, so the peak in-rush current through the choke is limited to the saturation current of the choke, or max 600mA, whatever comes first. Otherwise tube damage can result. MUST MEASURE. Can not be calculated in a reliable way. For lowest hum, this resistor is in series with the choke. So you get a C-LR-C filter. In this way 200uF is possible.

274A Mechanical Data
Tube Size including Socket: 
140 x 58 mm 
5.5 x 2.3 Inch 
Tube weight
130 Gram 
Shipped weight for 
double box with one pair
730 Gram 
274B Mechanical Data
Tube Size including Socket: 
147 x 55 mm 
5.8 x 2.7 Inch 

Filaments: 8 + 2
Plates: 4 + 6 

Single Tube weight 
125 Gram 
Shipment weight
400 Gram

  274A / 274B
Large Image
Plate Detail

274A Base
Large Picture

UX4 Base

274B Base
Large picture

5-Hole Octal Base




















  • Note 1) Hard-metal filaments prevent filament breakage with DHT Tubes. This material has a higher melting point than the classical nickel wires. 
  • Note 2) To prevent large charge current peaks, the first capacitor (C1) should NOT be larger than 4uF. If the input capacitor is too large, this will result in heavy AC charge current through this capacitor. This is not good for the rectifier tube, and also the AC capacitor current peaks may cause hum radiation into the preamplifier.  With the given C-L-C values in  table, the rectifier circuit will work  without any trace of hum.  For ultimate filtering,  with oversized components, you  will have best  results by increasing the choke.  Do not oversize capacitor C1, this  will not give better  results. 


    Application Note AN-01

    Old Design Info- Still valid!

    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 presented 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: Adding 100's of microfarad as a first capacitor for 5U4G, too low DC resistance of transformer Raa winding, or draw excessive surge current from a rectifier tube during cold start. Caused by a low cost choke, which saturates at start up, or other causes. Combine this 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 questionable. So this can already by seen from picture files. Wheras 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 free to use for DIY. 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.

    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 becomes difficult to build such for very high voltage. High current was never a problem. 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, but they definitely have a reliability problem with exceeding the peak voltage. With tubes, the situation is reversed. Tubes have little problems when the peak voltage is exceeded occasionally, but they will develop problems quickly when the peak current is exceeded. So design considerations are totally another, and it would be completely wrong to use design considerations for solid state diodes for tube diodes as well, saying "a diode is a diode". The answer to this can only be: "a data sheet is a data sheet".

    With solid state devices we all know, you can not abuse them with the peak voltage. If you do, catastrophic failure rate will rise sharply. As this is impossible to calculate with precision, good designers will measure it with in instrument, or alternatively over dimension the design ratings. This is in order to stay safely away from the damaging limits.

    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 in 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.

    Summary: 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. Wheras 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 *****

    Bleeder Resistor: 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.

    Windings resistance. 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 center tap, which has the same effect. Then, from the tube's view you have Raa of 250 Ohms again.

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