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. 


    Design note-1

    Design note-2

    Rectification with tube power supplies. This is probably the most misunderstood part of a tube amplifier, and we see many errors made, even with companies of high reputation. Better is to follow the basic rules yourself, and not simply copy something. Also do not treat tubes are solid state diodes. Tubes do not like transient current, whereas solid state doesn't like transient voltage. So design considerations are totally another. Many good books have been written about tube power supplies, and of course we can not compress this in a design note 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 do not know this, and do not care. So 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 yourself with external resistors.

    So these three 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. In order to achieve low residual hum, the last final bit of hum, is not present because capacitors are too small, but because the choke is too small, and because the circuits is not optimized. The most commonly used circuit is Circuit #1, below here, wheras in very many cases, simply use circuit #2 already brings an improvement. Wheras Circuit #4 is such a logical and easy thing to do, but people do not understand why, and take Circuit #1. Circuit #5 is only possible with Lundahl chokes, but this is very beneficial for lowest hum. The choke is not only used here as an inductor, but also used for inductive decoupling of the first Capacitor from the whole rest of the amplifier circuit. So any unwanted path for AC current (hum signal) becomes virtually impossible.

    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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