\Pro"logue\, n. [F., fr. L. prologus, fr. Gr. ?, fr. ? to say
beforehand; ? before + ? to say.]
I don't normally do this sort of thing, but I have been getting questions from people starting out with tube amps and high sensitivity speakers. As not everybody can get those old tubes I tend to use, or the anything but cheap components, I have placed this project on my site. It is fairly simple, the tubes can be found much easier and for only a few dollars. I've kept the power supply relatively simple as well. I think this is something you can use and enjoy the result for a long time to come.
The circuit uses the 5842 to get enough gain to be driven directly from a CD player or other source without the need for a preamp. You will need an output of about 1 to 1.5V from your source, something almost every commercial CD player should be able to manage. The output tube is the 1626, originally used in transmitters etc, this tube has been overlooked by many people. It is very cheap ($3-5 for a new tube) and uses the very common octal socket (like the EL34 etc). Driven as shown in the circuit below, it should be able to deliver around 1W into a 5K output transformer.
The 150 ohm resistors on the grid are to prevent the tubes (mainly the 5842) from oscillating. This tube is very sensitive due to it's high transconductance and gain. The cathode bypass capacitors were chosen to have a -3dB frequency of around 4-5 Hz. Resistors are labeled with a wattage rating, those not rated can be 0.25W, there is no dissipation worth mentioning on these. I have chosen for a 50K volume control to keep the resistance as low as possible on the grid of the 5842, while also offering a nice load for the CD etc. Using 10K is nicer for the 5842, but will put high demands on the output of the CD/source.
The voltage listed in the schematic for B+ are not so critical. You can stray a bit without any penalties, but try to stay above 18mA with this tube to keep the sound alive.
This amp is really the ideal amp to start out with when you have high sensitivity speakers. It puts out about 1 Watt, which is more than enough for anything over 96dB (Lowther, AER, Fostex, Phy, Supravox, compression drivers etc). The 5842 delivers more than enough gain to drive the 1626 into headache levels of sound pressure (at least for me). I've also tried running the 1626 at a lower voltage and a bit higher current (200V/25mA using 820R in the cathode) and this is also very nice on my system. If you can do with the lowered power level (0.6W) and can adjust the B+ supply (needs a B+ of only 220-230Vdc) this setting would get my preference. On 100dB+ systems this still produces enough sound pressure to put your fingers in your ears.
I've used standard 6V and 12V regulators for the filaments to keep it simple. You could also use LM317 regulators, or something similar, and set it up to match the exact voltages of the filaments (6.3V and 12.6V).