As you have probably noticed, my knowledge base articles are free of advertising. Instead of distracting you with annoying ads, I kindly request your donation. If you find the contents of this page to be useful, please consider making a donation by clicking the Donate button below.

Darn Good 300B: DG300B

The DG300B was discontinued in early 2019 and all remaining DG300B circuit boards have sold. My decision to discontinue the DG300B was due to the dwindling sales of vacuum tube circuits and due to the enormous support cost associated with these circuits. Instead I chose to refocus Neurochrome as a manufacturer and vendor of state-of-the-art solid-state circuits. I have nothing against vacuum tubes. I had a lot of fun designing vacuum tube circuits. But I consistently find the low-distortion solid-state circuits to perform better – both on the lab bench and in the listening room – so I have chosen to direct my focus there.

You can find the design documentation, including the schematics, for the DG300B below. This information is provided as-is, where-is and is for personal, non-commercial use only. I am more than happy to discuss licensing of the circuit should you wish to produce the circuit commercially. 

I will continue to support those who have bought my DG300B circuit boards, but I will not provide support for DIY builds of the DG300B. Should you need help with your DIY build, I suggest that you ask for help in the Tubes/Valves section of DIY Audio.

*** WARNING ***

The DG300B operates at lethal voltage. At some points in the circuit you will find potentials of up to 750 V. This is plenty to kill you! If you are not qualified and comfortable working with such high voltages, please do NOT attempt to build this circuit. Should you choose to build this circuit, you assume all risk and expense associated with this circuit.

Darn Good 300B Driver Board & Power Supply: DG300B schematics DG300B Design Documentation

My recommendations for output transformers are given on Page 13 of the Design Documentation.

The DG300B is powered by a Classic Tone power transformer (P/N: 40-18069). To the best of my knowledge this is the only transformer which is able to provide all the voltages needed and also has an international primary. Should you choose to get a transformer custom made for your DG300B build, follow the specs below:

Primary voltage: Should match your mains voltage.

Secondary voltage taps: 

  • 360 VAC @ 400 mA DC (B+ supply)
  • 60 VAC @ 120 mA DC (bias supply)
  • 2 x 12 VAC @ 2 A DC (filament supplies)
  • 6~6.3 VAC @ 100 mA DC (housekeeping supply)

All secondary currents are given as DC currents assuming full-wave rectification. Observant readers will note that I've dialled back the B+ supply a little by specifying 360 VAC rather than the 380 VAC provided by the Classic-Tone transformer. The slightly lower voltage results in lower power dissipation in the 21st Maida Regulator, thus, improves the reliability the circuit.

The DG300B Driver Board is fed by a 21st Century Maida Regulator configured to provide 400 V DC at 225 mA average, 350 mA peak (R3 = 10 Ω; R9 = 200 kΩ). Note that a heat sink with a thermal resistance of 2 K/W or lower is required for the Maida Regulator. A 2" long chunk of HeatsinkUSA 4.600" profile would be appropriate.

The filament and heater supplies are provided by three Universal Filament Regulators: Two configured for 5.0 V (for the 300Bs) and one configured for 6.3 V (for the two driver tubes). The design documentation for the Universal Filament Regulator is provided here: Universal Filament Regulator Design Documentation & Schematics Universal Filament Regulator Design Documentation

Key Specifications

The key specs for the DG300B are tabulated below. These values assume the use of the E88CC driver tube. The gain will be 3.5 dB higher for the ECC99 and 6N6P tube types.

Parameter Value Notes
Output Power 10 W @ 3 % THD, 50 Hz
Gain 7.3 dB
Input Voltage for 1 W Output 1.24 V RMS
Input Sensitivity 3.90 V RMS
THD+N @ 1 W, 1 kHz 0.25 %
THD+N @ 10 W, 1 kHz 1.5 %
THD+N @ 10 W, 50 Hz 3.0 % Electra-Print OPTs
Bandwidth 36 kHz -3 dB ref. 1 kHz @ 1 W
Dimensions & Weight 30 × 40 × 25 cm (W × D × H)
Weight 17.3 kg


I built two stereo DG300B amps as shown below.

DG300B amplifiers

Note the large heat sinks for the 21st Century Maida Regulator on the rear panel.

DG300B amplifiers, rear view

The inside is pretty tightly packed. The three Universal Filament Regulator modules are stacked in the centre. Note that this build used an earlier revision of the Filament Regulator than the one linked to above, hence, has a two-pin connector for the output voltage.

DG300B Inside View


Please Donate!

Did you find this content useful? If so, please consider making a donation by clicking the Donate button below.