2A3 Tube amp: "a tribute to simplicity"
I do not know why but I prefer simple, low power triode designs. May be it's the warm sound produced by these tubes or because I like horn speakers and I know that this relation is erotic. It's a kind of magic, a sweet sound triggers up your feelings, that can reproduced only from a combination of a DHT tube amp and a pair of horn speakers.
So last year I decided to construct a low power SET based on 2A3 tube. I had no previous experience on this specific tube, and this is something that excites me.
While I was looking for the simplest SET design of a top notch Valve amplifier that is based on a 2A3 DHT (Direct Heated Triode ) vacuum power tube, I came upon the "Loftin-White" circuit.
This circuit has its roots back on 1929, when published for the first time in the American magazine Radio News, by E. H. Loftin and S. Y. White.
The main characteristic of this amplifier, is the use of a direct coupled circuit.
That means there is no capacitor or coupling transformer between driver and power stages.
For the history the prototype "Loftin-White" amplifier used the "224" tetrode in the driver stage and the "250" triode in the power (output) stage.
The direct coupling circuit It seems easy to implement but It is not. These circuits allows DC and AC signals passing to the grid of the power gain stage, because there is no coupling capacitor. That's the good point affects to sound quality. From the other hand, sometimes the existence of some oscillations (hum and RF) that pass to the final gain stage, affects the sound of the amp, and produces noise or a kind of blear and harsh sound.
To eliminate these problems I had to work with exactly the same parameters in both channels ( matched pairs of driving and output tubes , accurate and exactly the same adjustments, short cabling, etc) and top quality materials only.
Applying this method the final result is amazing. The amplifier reveals an amazing sound stage with warm sound and enjoyable harmonics.
In this specific design the 2A3 power output tube, running at 60mA plate current. Plate voltage is 408V. The driver stage uses the high gain 12AX7/ECC83 dual triode. Input impedance is 100K ohms.
I chose a matched pair of Sovteks 2A3 power tubes and a matched pair of Electro-Harmonix 12AX7.
If you try to build this amp, It is also recommended the "Emission Labs" EML 2A3 (matched pair). All tube sockets are porcelain high quality.
The rectifier bridge circuit in the power supply section uses the GZ34 / 5AR4 full wave rectifier.
The rectifier circuit uses a single section "pi" filter. Combined with JJ or Mundorf capacitors obtained excellent listening results.
The most critical component in the sound path is the output transformer. Therefore there is no compromise in the quality of the output transformers used in all my amps. For this amplifier I chose, the “ETUDE 1” series, a high quality single ended output transformer, by Monolith Magnetics.
The heart of any Monolith transformer is a high grade grain oriented SiFe dual C-core. The resonance free frequency range is typically better than 7Hz - 75 kHz (-3 dB).
Because of their excellent quality, the result is fast transients, live voices, and exceptional low, mid and high frequencies.
Finishing & Construction
The wood casing (could be oak or cherry) is individually crafted by hand.
Underneath the CNC machined chassis all weldings are made one by one by hand. (point to point wiring)
For inquiries and more Information please contact me via the contact page HERE
I would like to extend my sincere thanks to Dr Ward De Ceuninck and Yves Pexsters the guys behind Monolith Magnetics for their effort.
An interesting video about the 2A3 tube amp :