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EL34 PSE (Parallel Single Ended)
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:
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|
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
|The input stage is a 12AX7 / ECC83 Mullard or Jan Philips|
|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.
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.
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
Photo of details