EL84 SE – The Small One

The Little One single ended amplifier was my first tube powered amplifier ever made. Its construction took a little time and effort, while satisfaction was immediate! So I’ll be brief with explanations.
The idea to taste the sound of a tube amplifier for the first time encouraged me much. I’ we recovered the final stage of an old radio stereo receiver with tubes -  Gerufon Ultrastereo – and  make it functional with the help of my good old friend Niki. I was so impressed with the way it sounded, that I could not make up my mind to dismantle it,  in order to rebuild it in a proper manner :]. So my first test was running the original audio output of  Gerufon stereo receiver, to which the entry was an ECC83 tube, the tube EL84 was used in pentode connection  – producing 4.5 watts per channel – and the amplifier was using global negative feedback.
The power supply was used for the primary test was the original Gerufon Radio Ultra Stereo 62W one, with rectifier elements made from selenium. The level of the output signal in pentode mode was big enough to excite the 15” coaxial drivers of my 97 dB loudspeakers. During the evaluative listening sessions it happed to pass my first record rolling night without ear fatigue, just pure pleasure!
Taking this test as the base, I’ve began experiments on the new amplifier, with the following remarks:
- I used as the input valve the E188CC SQ; it can attack EL84′s grid satisfactory, and the produces a pleasant natural and delicate sound, with a soundstage so large, difficult to obtain with transistor topologies in the same budget ;D
- EL84 triode was strapped in triode mode, thus reducing the power output to a little 1.9 Watt but decreases the high order distortions too (total distorsion figures decreases from 10 to 8.2% :))) ).
Class A single ended triode connection
Plate voltage…………………. 250 Volts
Plate current(zero signal) ……..  35 mA
Cathode resistor………………. 279 Ohms
Plate load …………………… 3-5 kOhms
Power output…………………..  1.9 Watts
rms input voltage………………  6.7 volts
distortion(2-nd and 3-rd harmonics.  8.2%
- global negative feedback was not used anymore, the amplifier being quite stable without it
- PSU was achieved using an inexpensive silicon bridge rectifier with snubber capacitors across each diode, and passive filters CRC with cheap electrolytic capacitors . For all filaments     alternative voltage was used, without having even the slightest problem with humm.
- the signal capacitor used in the initial version was a 470nF WIMA / 630 V MKP, then being used an Auricap 0.33uF/400 V – the most expensive component from the entire assembly!

- the output transformers are some old Unitra Fonica 5kOhm: 8ohm, which have quality core and sounds better that those scrapped from Gerufon receiver- Chassis was made entirely by hand, consisting in a walnut wooden frame, over which a 3mm thick aluminum plate was attached .The holes in the plate, as well as the plate brushing finish was done totally by hand- three aluminum foots were used as chassis support, those fixing the aluminum mesh that protects the underneath- for the psu, a toroidal transformer made by romtoroid was used, with separate windings for high voltages and filaments .

As simple and easy to build is this single ended amplifier, the greater the pleasure when enjoying its warm and involving sound!
It’s easy to build and tune up, facile for assembling in a fancy chassis, and could easily serve as ‘moon base’ for future tube projects. You shall take care just to build some decent high sensitivity loudspeakers in order to afford the low watt tube luxury!

I have tested several vacuum tubes as driver – first stage – 6N1P, ECC83, ECC82, ECC88, but the sheer pleasure was attained with E188CC-SQ gold pins, AZDAM labeled. I also did a little tube rolling for the final tube with various ‘communist’ contenders made in Serbia, Hungary, Russia and Czech Republic. So far, those EL84 made by TESLA factory were making me happy. If you are a metal fan , the Serbian made EI 6BQ5 sounds a bit more powerful, with ‘balls’ for rock music.

Here is a small gallery on the Small One :       

EL84_amp_corner_view_03

Picture 8 of 8

Happy building and listening!

© attitube 2011

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