Not just an replacement, but an improvement!

These tubes use a very fine wire grid, with a gain of 18x for the 20A and 20B, and 28x for the 30B. It is recommended to use these tubes with a SE transformer, or choke loaded. When you choose a plate resistor design, it can be done, but you must still have the high voltage over the  plate resistor. For this reason a SE transformer or plate Choke is the most practical way, and gives lowest distortion also. 

 

The 20A, 20B and 30A tubes are based on older principles, where high gain tubes have much wider plate distance. This is still the best way to make a very linear DHT high gain tube. However this results in higher working voltage, and larger size. This was not done any more after 1935, when miniaturization of tubes started to become important. With the new production tubes we make today, miniaturization is no requirement. So for high gain tubes, we can now focus on lowest possible distortion. We build these tubes again the original way, with wide plates. Check plate picture here. 

Distortion for small signal has been measured and is below 0,08% at 10 Volt output signal, at 400Volts plate voltage. For larger signal, distortion depends on the way the tube is used. Please note that when one DHT tube is driving another, some part of the odd distortion of the second tube will be eliminated by the first tube. This harmonics cancellation is only with triodes, and they must be similar tubes, like drive a 300B with one of the tubes of this series.

For lowest possible mechanical noise with (any) high gain DHT pre-amp tube, use high quality tube sockets that do not pass the chassis vibrations to the tube. Chassis vibrations can come directly from the mains transformer, or by air from the loudspeaker. The best sockets for this are probably the Yamamoto Teflon, because Teflon is a flexible material. Use tight fitting Teflon tube dampers such as the Duende Criatura. They make the best damper for the EML tubes. They can be fit very nicely on the tube top, or one on the top and one on the bottom.

• The EML 20A / 20B are  universal  and stabile tubes, combining high gain, with low output impedance. At a gain of 18,  there is no comparable directly heated tube, that offers such a low  plate impedance.  This makes it possible to use the 20B as an output tube. When using a step-up input  transformer, no additional tubes are needed , so it is possible to construct a complete HiFi amplifier with just one 20B tube. 
• As a driver tube,  when using a plate resistor, 20A an 20B can work with high voltage circuits for an 845  using a high power supply voltage of around 800 Volt.   For supply voltages around 400Volt the use of a plate choke is recommended to have sufficient plate voltage remaining over the tube. 

These tubes will often produce a blue glow on the glass.  That is because of the wide plate distance. The fluorescent light pattern changes with the voltage or with the signal. These light effects are normal for this type of tube.  It must not be confused with gassy tubes, which  produce a purple-blue cloud  inside the plates.    High resolution image here.

• Gold plated, Wolfram Grid
• Center tapped filament
• Slow Start filaments, for best lifetime
• Hard metal Construction (Note1)
• Two extra large getters
• Hand blown Glass bulb
• Anti-microphonic plate- and grid  suspension
• These tubes are shipped in a high quality dual box
• Tube printing with real gold (metal),  red color is glass burned into the glass 
• Machined, solid tube base pins for highest contact precision. YAMAMOTO tube sockets highly recommended. 
• Gold plated pins,  black ceramic socket.  (White bottom.)

EML 20A / 30A

High. Res.Images: 
Front - Side 
Bottom. - Day Glow

....Curves will be uploaded later

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EML 20B
High. Res.Images:
Front - Side - Night Glow
Curves

• Note 1)  Hard-metal filaments prevent filament breakage with DHT Tubes.  This material has a higher melting point than the classical nickel wires. Wolfram grids  allow the most precise grid wire distance, because wolfram is an extreme hard metal.   This ensures better grid  precision, which gives uniformity and better linearity at the low end of the tube curves
• Note 2)  Individual Test data, such as:  Matching Data, Grid Current, Vacuum, Filament Current, etc.,  are on the Certificate that comes with each individual Tube.  Each tube is numbered, inside the bulb  with a metal Tag
• Note 3)  Tube Curves:  Made on the The Sofia Curve tracer, with courtesy of AMR, Wisconsin, USA.