Shiny Eyes Radu’s 300B SET—a Simple Pleasure Amplifier

I designed this amp for whoever needs a single amplification device. You can use it with a passive preamp, AVC/TVC recommended, but not absolutely necessary. A good stepped attenuator ( remote or not) will perform excellent too.

This amp is a delight for tube rollers. However, my intent was to offer many choices of appropriate drivers for a better sounding 300B amp. The amplifier is a 3 stage one, having the 300B as power stage, loaded on a Hashimoto H-30-5S. The driver stage has a 4 pin socket to use with the 45 tube, or an 5 pin to 4 pin adapter to use with either 46 or 47 in triode mode. The driver is loaded on a LL2753/30mA and has a switch to toggle between the 45 and the 46/47 drivers. The first stage is a gain stage and has an octal socket for use with the 6J5, or a 5pin to octal adapter for using the 76 super triode. As one can see, there are many combinations possible with the hope that one will find their preferred sweet spot. Sure enough certain combinations will not provide full gain to use with a passive volume control, and a preamp with gain will be required.

This amp was designed to give as low distortions as possible and keep the 300B plate dissipation low enough so the tubes will last along time.

The amp is providing clean 6W. The input sensitivity, worst gain case scenario (76-45-300B) is 1.7Vrms, and best gain case scenario (6J5-47-300B) is 0.34Vrms. The output transformers have 4-8-16 ohms taps so any options can be wired. Having a 5Kohms primary, one can always use 4 ohms speakers on the 8 ohms tap or 8 ohms speakers on the 16 ohms tap and get almost double the power for a bit higher level of distortions. I tested the amp on 4 ohms Buchard S400 MKII used on the 8 ohms tap and is sounds as good as it can be. Sure enough more sensitive full range speakers will be a better match to a low power DHT amp.

The amplifier has 2 enclosures and 2 umbilical cords between the power supply and the main amp. Each enclosure has two nice looking Simpson panel meters. The power supply meters show the high voltage on each channel. The main amp meters show the plate current of the 300B on each channel. These meters are coming in handy to show the proper operation of the amp. Any problems will reflect on these meters indication.

The power supply is dual for the high voltage ( see two toroidal 200VA transformers and two sets of 6CJ3 rectifiers) having a CLCLC filtering using 4 pole Mundorf HV+ capacitors. There are 5 independent low voltage regulated supplies to provide power to all filaments. Therefore the umbilical cords are carrying DC only. The way is designed this amp is super quiet.

This amp is built with lots of TLC and provide an exquisite sound when using the appropriate speakers.