As Jeff says, ir is a DC coupled Common Cathode>Cathode Follower.
Bottlehead used 12AU7, Frank deGrove has an (old thread) on a 6SN7 version, i have a version with 6DJ8/6922/6C23∏ (can use 6N1P but it needs twice the heater current).
And for the 6DJ8
Given it is a linestage i don’t think the PS is all that much overkill… here is the one Frank did.
Do note that adding a CCS to the to of the first stage and the bottom of the second stage makes for significant improvements. Particualrily on the follower.
dave
Bottlehead used 12AU7, Frank deGrove has an (old thread) on a 6SN7 version, i have a version with 6DJ8/6922/6C23∏ (can use 6N1P but it needs twice the heater current).
And for the 6DJ8
Given it is a linestage i don’t think the PS is all that much overkill… here is the one Frank did.
Do note that adding a CCS to the to of the first stage and the bottom of the second stage makes for significant improvements. Particualrily on the follower.
dave
Referring to the schematic of Post # 1:
1. The output voltage of a circuit of 1uF and 470k Ohms will go to 0.37 x 50V = 18.5V in 0.47 second (1 RC time constant).
Better use a Very Slow Start B+, to keep large voltages out of the preamp output (going to the power amplifier).
2. A 6SN7 has 2600 to 3000 uMhos. A 12AU7 has 2200 to 3100 uMhos. The cathode impedance varies from 323 Ohms to 455 Ohms.
Using a 27k Ohm cathode resistor, or using a CCS (constant current sink) in the cathode follower will not significantly change the output impedance of the preamp.
Changing from a 27k Ohm resistor to a CCS:
Using a CCS Will reduce the distortion a little bit if you only load the preamp with a 1 Meg Ohm scope probe.
Using a CCS will Not reduce the distortion significantly if you load the preamp with a 10k Ohm input impedance of some solid state power amplifiers.
Using a CCS in the cathode follower, and a power amplifier with an input impedance of 20k, 47k, 100k . . .
Your Mileage May Vary.
Two 6SN7 tubes (stereo), have 1.2A of filament current.
With 10,000 uF filter capacitor, and 120Hz full wave rectification, the ripple will be 159mV peak to peak (0.159V peak to peak).
And, it will not be very sinusoidal, it will have lots of mid and high frequency harmonics.
Be sure to keep the filament ground loop currents out of the rest of the amplifier circuitry.
Or, just use AC powered filaments and get real good 6SN7 tubes,
Or use a very clean DC supply.
1. The output voltage of a circuit of 1uF and 470k Ohms will go to 0.37 x 50V = 18.5V in 0.47 second (1 RC time constant).
Better use a Very Slow Start B+, to keep large voltages out of the preamp output (going to the power amplifier).
2. A 6SN7 has 2600 to 3000 uMhos. A 12AU7 has 2200 to 3100 uMhos. The cathode impedance varies from 323 Ohms to 455 Ohms.
Using a 27k Ohm cathode resistor, or using a CCS (constant current sink) in the cathode follower will not significantly change the output impedance of the preamp.
Changing from a 27k Ohm resistor to a CCS:
Using a CCS Will reduce the distortion a little bit if you only load the preamp with a 1 Meg Ohm scope probe.
Using a CCS will Not reduce the distortion significantly if you load the preamp with a 10k Ohm input impedance of some solid state power amplifiers.
Using a CCS in the cathode follower, and a power amplifier with an input impedance of 20k, 47k, 100k . . .
Your Mileage May Vary.
Two 6SN7 tubes (stereo), have 1.2A of filament current.
With 10,000 uF filter capacitor, and 120Hz full wave rectification, the ripple will be 159mV peak to peak (0.159V peak to peak).
And, it will not be very sinusoidal, it will have lots of mid and high frequency harmonics.
Be sure to keep the filament ground loop currents out of the rest of the amplifier circuitry.
Or, just use AC powered filaments and get real good 6SN7 tubes,
Or use a very clean DC supply.
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The original Foreplay design was by George Wright, some time before the 1998 release. There were a lot of variations, both official and by owner/experimenters, before it was discontinued. The circuit in Post #1 incorporates many of them. I'm pretty sure the basic topology goes much further back!
