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Class A2 amplifiers need a more elaborated driver than the usual A1.
Technical approach
To source the E140 grid properly it imposes a low impedance driver stage. It is the heart of this project. I could have used a simple cathode follower directly coupled to the final triode, but to have a steady E140 operating point I had to use a well regulated high voltage supply, which I did not want. A medium power triode used as voltage amplifier loaded by a step down transformer was the Shishido way. I bought two Tamura A-8713 (20K/600) line out trans. Not too expensive they accept 20mA unbalance primary current with very decent bandwidth.
Though providing a quite low impedance path to the E140, the IT voltage ratio (1,73 10-1 or -15,3dB) commands a tube capable of a very large anode swing with low distortion. Not so many candidates. Keeping in mind the necessity of a current not exceeding 10mA trough IT (it roughly corresponds to twice the E140 grid current at maximum positive swing) the 12B4A is the tube to go. At 20K load its linearity is excellent and is able to deliver 350Vpp with low distortion, giving a driving voltage behind IT up to 60Vpp.
I do not need that much.
Back to SFR E140 triode
after a careful study of the E140 characteristics the best operating point for full power is
Vak 240V, Ia 40mA, Vg +10V, Rload 5K.
It permits an anode swing of 330 Vpp with a grid swing of 40 Vpp.
The output transformer have a 4 10-2/-28dB voltage ratio, thus 330 Vpp or 117 Vrms will give 4,68 Vrms/8 ohm or 2,7W. With a dummy load the amp puts out 3,8W before clipping.
Enough for any sensitive speaker.
12B4A point of view
To get 40V pp on E140 grid the 12B4A triode must have an anode swing of
40 x 5,78 (ITvoltage ratio in that way) = 230 Vpp
The operating point will be set at
Ia 10/11mA with Rload 20K, Rk 3.3K, Vak 225/235V, V+ 260/270V
On the characteristics below we can see that this tube is up to the task.
First stage
this was the trickiest choice I had to make for this amplifier.
I needed a high voltage capability tube mainly because most of the A2 circuits I studied used some NFB and that I kept in mind the use of a local feedback loop between power stage and driver to:
1- get a better damping factor.
2- cancel amplitude distortion when tubes reach their extremes.
3- keep a good overall bandwidth.
To make a long story short I finally choose the E80CC among half a dozen contenders
( 5687, E182CC, 12BH7A, 6CG7...).
The E80CC is renowned for its sonic qualities and very low distortion. Many professional audio devices used this tube primarily intended for computer use.
The 12B4A needs 40 Vpp (14,1 Vrms) on grid to deliver 230 Vpp. Even with a 6dB feedback loop between the last stages the E80CC will swing the 12B4A with low distortion. Below a 150K loaded tube with a 32 Vrms (90 Vpp) output @ 1.4 Vrms (4 Vpp) input shows the high gain capabilities.
In facts DC load differs from AC. AC's one takes the next stage grid resistor Rg2 in account. In that case if I make Rg2 = 220K the AC load is about 90K. As seen below it does not change a lot the gain capability but slightly increases distortion (Philips data sheets give an output voltage of 20 Vrms (56 Vpp) @ 3,4% distortion under 250V/100K, we should be very close).
The main problem with high Rg will be Miller's effect. I will point out the incidence on 12B4A bandwidth in the next article.
Just two makers for this very fine audio tube, Philips and Tungsram.
Next episode: complete schematic, power supply and more...
