DG300B SET Amp

2019/03/24: The DG300B has had a solid run since its inception in 2012, but it is time for it to retire. Thus, it has been discontinued. Thanks to everybody who supported this project, in particular to those who bought circuit boards.

The amplifier design below deals with high voltages — LETHAL voltages. If you are not comfortable or qualified to deal with these potentially deadly voltages, please do not attempt to build this circuit. Proceed at your own risk, expense, and responsibility. 

My quest for an amplifier circuit that allows the legendary 300B triode to really shine has finally been fulfilled. It’s been quite a journey and has spun off both my Universal Filament Regulator and 21st Century Maida Regulator as side projects.

The present amplifier circuit supports the use of the Russian 6N6P tube as well as the E88CC/ECC88/PCC88/6922/6DJ8/6N23P, JJ ECC99, and 12BH7A. The type of tube is selected by wire link options. With some creative wiring, it is possible to use the d3A tube as well.

This amp is intended as a power amp for driving speakers. It provides approx. 10 W into 8 ohm (or 4 ohm if you use a 5k:4ohm OPT).

One builder of this amp is using it to drive his Audeze LCD2 magnetostatic headphones. He presented the amp at the Los Angeles Head-Fi Meet in July 2013 and at the San Diego Meet in November 2013. An image of his amp as well as some of the listening impressions are shown below.

Unpopulated circuit boards for building this amplifier are available for purchase as a set below. If you prefer to purchase the boards individually, please see their respective pages.

Damn Good 300B SET Amp

The sound quality of this amp is nothing short of phenomenal. I’m very impressed by the level of detail in the highs and the firm but slightly warm bass. The midrange tone and vocal detail is very nice as well. I’m finding new detail in recordings I thought I knew inside out. It’s fun listening to the detail that comes from the guitars of Mark Knopfler, Eric Clapton, Joe Satriani, John Mayer, and the like. What impresses me most, though, is the sound stage. The depth and breadth of the sound stage is unlike anything I’ve ever heard. Talk about holographic imaging!

The unpopulated circuit boards used in this design are available for purchase as a set below. If you wish to purchase the boards individually, they are available for sale on their respective pages.


The specifications for the DG300B are tabulated below.

Output Power10 W@ 3 % THD, 50 Hz
Gain7.3 dB
Input Voltage for 1 W Output1.24 V RMS
Input Sensitivity3.90 V RMS
THD+N @ 1 W, 1 kHz0.25 %
THD+N @ 10 W, 1 kHz1.5 %
THD+N @ 10 W, 50 Hz3.0 %Electra-Print OPTs
Bandwidth36 kHz-3 dB ref. 1 kHz @ 1 W

Due to the relatively low gain of the DG300B, some builders may wish to use a preamp with a bit of gain. The Neurochrome THAT Driver circuit, configured for a gain of 15 – 20 dB, is ideally suited for this application.

The THD vs frequency at various output powers is shown below.

DG300B THD+N vs Power, Freq

The frequency response is +0/-1 dB within the audible spectrum (20 Hz – 20 kHz).

DG300B_ Amplitude Response @ 1 W, 8 ohm (ref. 1 kHz)

The amplifier design is comprised of two main parts:

DG300B SET Driver Circuit

DG300B SET Power Supply

In addition, three Universal Filament Regulators are used. The amplifier is powered by a 21st Century Maida Regulator — Rev. 2.0.

My implementation of a stereo amplifier measures approx. 30 × 40 × 25 cm (W × D × H) and weighs in at 17.3 kg.




Build Budget

As with any tube amplifier, and 300B amplifier in particular, the build budget is dominated by the cost of the tubes, transformers, and chassis. A reasonable budget build using Edcor CXSE-series output transformers starts at about $1100, not including the chassis. A higher end build using Electra-Print output transformers will set you back about $1500 + chassis. The budget breaks down as follows:

DG300B Board Set199
Resistors, capacitors, semiconductors, connectors, wire, etc.225
Drive tubes (JJ E88CC)20
Output tubes (JJ 300B)250
Input transformers140
Output transformers140 (Edcor)
500 (Electra-Print)
Power transformer125
TOTAL1099 – 1459

The input transformers are recommended but can be omitted if you don’t need the balanced input or want to shave off $140 from the build budget. They can always be added later should you change your mind.

The chassis cost is variable depending on your artistic flair and desires. It can be as simple as a wooden frame with a cover of sheet metal. For a pre-made metal chassis, I recommend looking at Landfall Systems and ModuShop.

Builder Feedback

Head-Fi.org member ‘Sceleratus’ used my DG 300B Board Set to build a headphone amplifier. His implementation is shown below. Polished bronze on burley maple with walnut corners. Output transformers from Electra-Print. Bravely done, Sir!

'Scleratus' DG300B Implementation‘Sceleratus’ presented the amp at the Los Angeles Head-Fi Meet in July of 2013. Here are some of the listening impressions from the meet:

Frank Cooter: Highlight of the meet for me was Sceleratus’ DIY 300B amp. Anyone who followed his build thread in the DIY forum knows a bit of the hell he went through to pull this off. The results were well worth the effort. All the time, pain,frustration, and money paid off.

This is a wonderful sounding amp that looks great. Sceleratus’ amp has excellent smooth and extended highs. A major accomplishment  for a single-ended DHT amp. No problem there whatsoever.

Jeffrey Franz: Tom Christiansen Design: This was a 300B, DIY build by a really nice guy whose name, of course, I have forgotten. He based it on a circuit by the bolded name provided here. A truly fine sound and, until the BHSE/SR-009, as open-sounding as anything I heard, and that’s important to me.

See his build thread on Head-Fi.org: The Christiansen “DG” 300B Amplifier Build Thread. Another build thread is running on DIY Audio: Building the Damn Good 300B. You can also join the discussion here: DG300B Vendor Thread on DIY Audio.