Not just an replacement, but an improvement! 

EML 81

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Did you know? Emission Labs was the first company to re-introduce the 81 again, in the year 2012.


Guarantee

The first owner can register the tube at the Emission Labs website, to participate in the 5 years guarantee program.

Description

The 81 was historically intended for use with the Type 50 tube. The 81 is a classical mono diode for relatively high voltage, such as found with large directly heated triodes. Normally two 81 tubes are used for full wave rectification. The heaters of two tubes should be put in parallel, thus the equivalent of a normal dual diode will result. In this configuration It can produce conveniently an output voltage of 500....700 Volt, at maximum 170mA. Other than dual diodes such at 5U4G, a pair of 81 can be used at maximum voltage and maximum current at the same time.

Attention: This rectifier has a 7Volts heater

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 - though for 5U4G here, a pair of mono diodes would give the same configuration. 

Features
  • Hard metal filaments (Note1), series connected.
  • Each tube  is numbered, inside the bulb  with a metal Tag
  • Hand blown, Globe shaped glass
  • Two extra large getters.
  • This tubes is shipped in a high quality gift box.
  • Tube printing with real gold (metal),  red color is burned into the glass. 
  • Gold plated pins,  black ceramic UX4 socket.  (White bottom).  Ideal for the YAMAMOTO tube sockets.
Typical application notes below here, applies for this tube as well.
.
Filament Ratings for one diode
Filament Voltage
= 7.5 Volt (See Note1)
Filament Current 
~ 1.25 Ampere  (See Note1 )
.

Maximum ratings for a PAIR of diodes

IMPORTANT: Please read the notes at the bottom of this data sheet

AC input voltage
700Volts 
DC output current, for a PAIR of tubes
170mA
Copper Resistance of transformer minimum value. Add normal resistor if copper resistance is too low. (so if you have 120 Ohms, you need to add 80 ohms or more)
200 Ohms
First capacitor,  connected to plates
4uF  max
.. .. .

Full Wave Rectification with a pair of 81 tubes.

Capacitor loaded (4uF)

Some examples from
these curves on the right:

Transformer
Voltage
DC Output
Voltage
DC Output
Current
500-0-500V
580V=
40mA=
500-0-500V

520V=

90mA=
500-0-500V
443V=
170mA=
 
450-0-450V
510V=
40mA=
450-0-450V
460V=
90mA=
450-0-450V
382V=
170mA=
 
..

Full Wave Rectification with a pair of 81 tubes.

Choke loaded, for instance
with Lundahl LL1673-10Henry

Some examples from
these curves on the right:

Note, with a choke input, the load regulation is much better, it is approximately twice as good as capacitor loaded.
Transformer
Voltage
DC Output
Voltage
DC Output
Current
600-0-600V
490V=
40mA=
600-0-600V
460V=
90mA=
600-0-600V
440V=
170mA=
 
550-0-550V
450V=
40mA=
550-0-550V
418V=
90mA=
550-0-550V
371V=
170mA=
 
.. .. .
81 Mechanical Data

Size including Socket
(but excluding pins)

140 x 58 mm 
5.5 x 2.3 Inch 

Tube weight:
130 Gram
Shipped weight for a single boxed tube:
400 Gram

Note 1) Good care should be taken when making the design of any DHT rectifier. It must be prevented to have strong current peaks, through the first capacitor, because consequently this flows through the tube anode, and transformer windings as well. The transformer will produce mechanical hum by this, most specially if windings symmetry fails. Also the tube will suffer. For this reason, the first capacitor (C1) should never be larger than stated in this data sheet.   The mechanical transformer hum, and also tube current peaks are greatly reduced by smaller capacitor values, and use higher choke values instead. Although higher capacitor values are at lower cost, using lower capacitors and higher chokes values instead, is always more satisfactory in the end. The result will be: Lower transformer hum, less electrical field radiation into the pre-amp, and more lifetime from the rectifier tube. This is why we recommend using largest chokes. From Lundahl, high value chokes are achievable at the same price as HiFi capacitors, like from a Mundorf or Black gate. So we have to go back to the roots, and use high quality, large value chokes, like in the old days of radio design. For best ripple suppression, increase the choke to any value you need, or even use a C-L-C-L-C circuit, as also advised in the historical RCA data sheet.

Note 2) As a rule of thumb, high voltage power supplies are best build with large size chokes, specially at high output current, whereas low voltage power supplies can be build more easily with large capacitors.


SOME DESIGN NOTES FOR POWER SUPPLIES.

Design note-1

Design note-2


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