Remolon

For the 2sc1815, why not use ksc1815?, I made a Dynalo using ksa1015 and ksc1815, even in place of the jFet, and it works and sounds very well.

I will make the first trial not with the 2A3 but with the 5687. I have not found a reliable Spice model for 5687, so I am working on a graphical solution. With a voltage supply of 400VDC, a plate resistor of 10K and a grid bias of -8 volts, I have calculated a idle current of 20mA and a maximun output of 220Vpp. I will test a fixed bias from the 12.6 volts filaments winding and a cathode resistor of 402R. I have ordered the custom-made power transformer, and have finished the PSU, the input differencial amplifier and the stepped attenuator. Here is a picture (the differencial amplifier PCB without sockets and valves). Depending on the results, I will test a different tube looking for more volts, more current or do nothing.

R45 is useless but Rk4 and Rk5 facilitate the use of not matched 2A3 tubes, adjusting the bias individually. I changed all the input tubes. I will start with 6DJ8, and the following values for the other components Rload, Rf 10K R1 27K4 Rk1, Rp1 825R Rg1, Rg2, Rg3, Rb4, Rb5 221K Rk2, Rk3 100R Rp2, Rp3 47K5 Rg4, Rg5 1K Rk4, Rk5 10R Rp4, Rp5 2K5 (5W) C2, C3, Cp2, Cp3 1uF B+ 300VDC and Bias -20VDC

The discussion has been very helpful. Thanks. I will make the initial trial with the only change of the bias, it will be -20V

Kevin, you are right. Better go back to the past and work with the plate characteristics curves. If I do so: With B+ = 300VDC and Rp = 2K5, for a good response the bias should be -20V, and the input consequently limited. Then Vpp would be 130v (99dB for the SR-007?)

When you say that 2K5 for the plate resistor is too low, is because of the requirements of the amplifier or the headphones?. Even, I am considering to use 1K5 because the RCA 2A3 datasheet specifies for a push pull amplifier 3K plate to plate, and I am curious if the valve makes a distinction between the 3K image trough the output transformer of the loudspeakers impedance and the 1K5 resistor between each plate and ground. In normal operation, each 2K5 resistor would waste less than 2wats. With B++= B+=300VDC and Rp4= Rp5= 1K5 Spice calculates a current of 38.6mA for each valve, and a output of Vpp= 266v with a source of 2v(CD player), and, if is true, I hope that it is enough to move the SR-007 at an useful level. Facts will say. I have already used the input stage to modify, some time ago, an old Dynaco ST70 that works properly.

I am working on an amplifier with 2A3 triodes for the output. This is the schematic: V1 12AU7; V2, V3 12AX7; V4, V5 2A3 The values used for the simulation are: Rload 10K R1 4K75 Rk1 1K74 Rg1, Rg2, Rg3, Rb4, Rb5 274K Rp1 475R Rk2, Rk3 100R Rp2, Rp3 100K Rg4, Rg5 1K Rk4, Rk5, R45 10R Rp4, Rp5 2K49 C2, C3, Cp2, Cp3 1uF Cs4, Cs5 0.1uF B+ 300VDC and Bias -50VDC It is my first aproach to the Stax system, and any comment is welcome.

The voltage gain of the 6SN7 varies from 13 to 16. If the source provides 2V (a CD player), the second stage grid, U3 (U4), could receive 26V. With R8 of 300K, and R7 of 1K, 400V for B+ will provide an adequate bias of U3?