back to
www.audiodesignguide.com

To get more information contact me at: webmaster@audiodesignguide.com


EL34 PSE (Parallel Single Ended) ver. 2

Introduction:

I started this project in 2004 but only now (Nov. 2009) I have been able to bring it to an end. 
Back in 2004, I asked a friend of mine to test the first prototype with his four Quad ESLs.
He's a professional musician, a composer and a serious audiophile.
I liked the idea to have him testing this project over a longer period of a few days. 
Everything went just fine, but there was just one problem: he fell so much in love with it, 
that he didn't think I needed to finish it until now! :-)

here the previous article

The project has some interesting features:


Schematic

For the output transformers I chose the Lundahl LL1627/185mA with secondaries connected in B configuration
to have a primary load impedence of 2.3K on 8ohm load. 

In this way 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 equal 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 easily ask Lundahl for a custom LL1627/185mA or a 220mA to increase the anodic current to 55mA
per each EL34, but I don't really see any advantage in this.

I have all gate of output tubes at the same bias voltage 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 switching the amp off, insert the EL34 and then switch it on again to set the correct bias.

This amplifier, like the my other designs, can be connected directly to a CD Player or to a DAC without using a pre-amp.

The power supply fix the filaments reference at +80v instead of 0v so to consider the max cathode to heater voltage.

In the datasheet is mention this limit:

tube model used max cathode to heater voltage (max Vkf) circuit cathode voltage (Vk) heater voltage (Vf) Vk - Vf
ECC83 / 12AX7 180v 1.3V 80V 79V
ECC82 / 5814 / 12AU7 180v 220V 80V 140V
EL34 100v 1V 80V 79V

The circuit cathode voltage (Vk) is calculated with:

Total EL34 current 185mA
Each EL34 current = 185 / 4 = 46mA
Resistance on cathode = 47 // 47 = 23.5ohm
Cathode voltage = 23.5ohm * 46mA = 1V

ECC83 current 1mA
Resistance on cathode = 1300ohm
Cathode voltage = 1300ohm * 1mA = 1.3V

ECC82 current 10mA
Resistance on cathode = 22K // 22K + 22K // 22K = 22Kohm
Cathode voltage = 22Kohm * 10mA = 220V

ECC83 and ECC82 sockets, bottom view.

EL34 Socket, bottom view

 

Power supply






Components

The interstage capacitor should be a 0.22 400v or 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.

For the wiring
, 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.

The cabinet has been created using an aluminum plate with a thickness of 3mm and a size 35 x 55 cm and a wood frame picture with a section 2 x 6 cm.



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
 


Distortion measurement

Follows some test made with Clio system by
Audiomatica.

Distortion spectrum of driver stage (ECC83 & ECC82) on a 220Kohm load at 1KHz.



Distortion spectrum of driver stage (ECC83 & ECC82) on a 220Kohm load at 100Hz.


Distortion spectrum of the complete amplifier on a 8ohm load at 1KHz and an input voltage of only 200mVrms

Distortion spectrum of the comple amplifier on a 8ohm load at 100Hz and an input voltage of only 200mVrms



Frequency response measurement


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



 

Listening test



Photo of details