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the new EL34 Parallel Single Ended

by Andrea Ciuffoli - 2004

contact me for any problem or question: webmaster@audiodesignguide.com
 

Design targets:


As output transformer has been choiced the Lundahl  LL1627/185mA with secondaries connected in B configuration to have a primary load impedence of 2.3K on 8ohm load. so each EL34 will have a load of only 2300 * 4 =  9200ohm.

The LL1627/185mA have a primary inductance of 9H and the EL34 have a nominal plate resistance about 910ohm and four 910 / 4 = 227.5ohm so in theory the low frequency cut-off will be egual to Ra / ( 2 * pi * 9H) = 4Hz (-3db).

Using the LL1627/185mA each EL34 will be set with a bias about 185 / 4 = 46.25mA that produce at 430V an anode power dissipated of Pa = Va * Ia = 430V * 46.25mA = 20W that is very less than EL34 max power.

You could request without any problem to Lundahl a custom LL1627/220mA or a 240mA to increase the Ia to 55mA or 60mA per each EL34 but there is no advantage.


Schematic



I have all gate of output tubes at the same bias volatge because I have used a matched quad by



but if you want to use spare output tubes without any match you should use a  separate bias trimmer and a separated interstage capacitor per each output tube on the gates and a 10 ohm 1/2W resistor on each cathode to measure the current of a single tube.
The complete schematic to implement the separated grid voltage could be found here.

Before to insert the EL34 tubes in the sockets switch on the amplifier with only the two little tubes (12AX7/ECC83 and 5814/ECC82) and set the trimmers to get about -50V from the grids of the EL34 and the ground.
After switch off insert the EL34 and switch on the ampl. to set the correct bias.

This amplifier, like the my other designs, should be connected directly to a CD Player or to a DAC without use pre-ampl. and the sensibility should be enougth in any combination.




Power supply






Components

The interstage capacitor should be a 0.33uF 630V Jensen Copper film paper in oil or a 0.33uF 450V Auricap from Audience

All the audio transformers used in this project are Lundahl products:



  • LL1627/185mA + PCB_B           Output transformers
  • LL1669A                                     Power supply transformer 
  • LL1638/8H                                  Power supply inductance

Lundahl is one of the world´s leading manufacturer of audio transformers used in many professional studio recording and radio stations so it has great experience and all these products use a custom audio C-core.

The input stage is a 12AX7 / ECC83 Mullard or Jan Philips The driver stage is a 5814 / 12AU7 / ECC82 Mullard or Jan Philips The output tubes used are the Svetlana El34.


About passive components types,  I don't leave many choices:  Caddock resistances on cathode, ELNA Cerafine capacitors on cathode.

A valid alternative to very expensive Caddock is to use a parallel of two normal 1% metallic oside resistor with inverted phase to compensate some of their inductance.

Here is show the connection of two resitors to create a parallel.

Actually the ELNA Cerafine capacitors are not easy to find because are no more produced but these could replace with the new Jensen Electrolytic that I will test soon.

I am using a DACT stepper attenuator instead of the normal ALPS or NOBLE solution, because the the sound is clearer.

About wire; forget the teflon ! Use only stranded tinned copper wire with pvc insulator, and for loudspeakers use the same.

A very good quality stranded tinned copper wire could be bought directly from E-Z-HOOK that carries an extensive line of fine stranded and extra flexible wire.

 

Simulation

The performances of this design has been optimized with electronic simulation using the WinSpice .3.1 software for Windows.
The WinSpice 3.1 has been develop by Mike Smith and it is a beatiful port of original spice3f4 develop by Berkeley University of California.
It can generate waveform plots to individual floating windows and contains a powerful scripting language

download the circuit

this show a distortion about 4% at 20W on 8ohm very near to measurement value.


Distortion measurement

Follows some test made with Clio system by Audiomatica about the distortion spectrum with 8 ohm resistive load



A good 2.2% at Pout = (Vrms * Vrms) / Rload = 8 * 8 / 8 = 8 watt



A good 3.1% at Pout = (Vrms * Vrms) / Rload = 10 * 10 / 8 = 12.5 watt



An acceptable 4.3% at Pout = (Vrms * Vrms) / Rload = 13 * 13 / 8 = 21 watt



Frequency response measurement

Follows some test made with Clio system by Audiomatica about the frequency response with sinusoidal  sweep and MLS impulse.



The graphic show a low cut-off of 10 Hz at -3db and 22 KHz at -0.25db that is perfect.





Listening test




Details


The layout of the EL34 of only one channel and  the input stage  per both channels.