Which reduces to Gain = mu / 2.
Thus, the gain can be set by using different input tubes, such as the 12AU7, 12AT7, 12AV7, 12AX7, 12BH7, 12FQ8, 5751, 5963, 5965, 6072, 6211, and the E80CC, without having to change the values of the cathode resistors. For octal fans, the only two real choices are the 6SN7 and the 6SL7.
The beauty of this input stage is that it must also halve the power supply noise, as it forms a ½ voltage divider. And halving the noise is what the output stage needs to see in order to ignore the noise at its output. The output tube can be any low mu, low rp triode. I like the 5687 because its linearity and its huge cathode, which allows for huge current draws. The downside to this triode it its 0.9 amp heater current draw. Other triodes that can be used are the 12AU7, 6XB7, 6BL7, 6922, 12B4, 12BH7, 12FQ8, E288CC, and the 6AC7 triode connected or 6V6 or 6BQ5 or 12BY7.
I plan to use this circuit as line stage amplifier / headphone amplifier. When its function is driving the power amplifiers, the output coupling capacitor will be only a 1µF InfiniCap. When its function is driving the headphones, the output coupling capacitor will consist of the InfiniCap in parallel with a 40µF Solen polypropylene capacitor. The power supply will be kept in a remote box and will use a choke based filter rather than a voltage regulator.
Subject: Concerning "5687 black plate tube"
Recently I came across a preamp schematic called "Pat's ultimate preamp." It talks about 5687 black plate tube. It also says: "I fine-tuned it in Tube CAD" Do you have any further details about this? I would like to build a tube preamp for my dry sounding transistor guitar amplifier
The 5687 is mentioned in the answer to the last e-mail. I own about 40 5687s, but I do not know how many, if any, are "black" plate, so I cannot comment on their sound. Tube CAD is a tube circuit analysis program that I wrote. It contains 52 circuit topologies and costs $39. I use it almost every day in my tube design work and although it is not perfect, I recommend it anyone who builds tube projects. The best place to read about it is the GlassWare web site at http:/www.glass-ware.com .