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Figaro Tube Line Preamplifier

The beating heart of this project is the 12B4-A. This 5.5 W single indirectly heated triode an amplification factor μ of only 6.5, coupled with a plate resistance of a only 1kΩ , is the ideal candidate for a great linestage. Due to low μ I opted for a plate CCS ( Constant Current Source )

A CCS presents an ‘infinte’ AC load to the tube, maximizing gain. This high resistance load to the plate, make essentially a near horizzzontal load line. A CCS create a more linear stage with lower distortion and also improve Power Supply Rejection Ratio (PSRR).

There are many possible design for CCS plate load, some very simple ( DN2540 ) but in this case I have chosen for for a more sophisticated pentode tube CCS. I tested various pentodes in the end my choice fell in the russian 6J51P ( other possible candidates D3A, E810F… have been discarded because they are more rare and expensive ).

The stage operating point is: 12B4A anode voltage 150V, current 15mA. The resistance that fix this value on the 6J51P cathode must be 56 Ω ( 1%, 1 W ), and on the 12B4A cathode 1,2 KΩ ( Allen Bradley carbon composite 2 W ).

12B4A’s which tend to be microphonic need grid stopper carbon composite ½ W resistor ( 3,9-4,7 KΩ ). To avoid hum from power supply, each channel stage is provided of a Mosfet ( STP2NK90Z ) stabilized circuit with a common Zener ( two 1N5383B 150 V- 5W in series  ).

The ( custom ) power transformer is equipped with the following winding:

370 V CT at 100mA for HT anode supply, 5 V at 2 A for 5Y3GT filament, 7 V at 2 A for low filaments ( 12B4A ) and another 7 V at 2 A for high filament ( 6J51P ).

I used DC for preamp tube filament heating to eliminate or minimize hum, The 6,3 VDC for 12B4A filament must be 20 V circa raised off the ground and 6,3 VDC for 6J51P at 160 V circa from ground. The drop resistors R1, R2 will have to be chosen for otain 6,3 V to the tube filaments.

Few note about output capacitors used in this project.

The best one I test is Jupiter copper foil paper and wax: very very natural, rich harmonic texture, spatious, detail, smoothness everything you are looking for in a capacitor but very expensive.

The cheaper alternatives in my opinion are:

Miflex KPCU copper foil: does not achieve the Jupiter performance but this is also very interesting neutral and coherent but slightly midrange forward;

Intertechnik Audyn True Copper MKP : dynamic sound with rich natural harmonics and detail;

Russian K75-10 Hybrid paper and polyethylene in oil: cheap but a great capacitor.

HV Power supply capacitors must be polipropilene in oil ( like Electronicon Castor oil 600 V MKP ) while for the 12B4A cathode bypass capacitor Elna Silmic II 470 uF / 50 V is the best choice.


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