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/ECC88/6922/6DJ8/6N23P, 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, 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.

PCB ID Description Price each QTY in stock
300B_BOARDS Boards set for one Damn Good 300B SET Amp:
(1x) 300B Driver Board, Rev. 1.0
(1x) 300B Power Supply Board, Rev. 2.0
(3x) Universal Filament Regulator Board, Rev. 3.2
(1x) 21st Century Maida Regulator Board, Rev. 1.0
$199 > 10

Solution Graphics

 

The amplifier design is comprised of two main parts:

300B SET Driver Circuit

300B SET Power Supply

In addition, three Universal Filament Regulators are used. Two are configured for 5.0 V, 1.2~2.5 A and supplies the 300B tubes. The third is configured for 6.3 V, 1.2~2.5 A for powering the driver tubes. The amplifier is powered by a 21st Century Maida Regulator configured for 400 V, 225 mA (R3 = 10 Ω, 2 W; R9 = 200 kΩ, 3 W; C1A = 1.0 uF, 875 V, polypropylene). The wiring diagram for the complete amplifier is shown below. Click on the image for a larger view.

300B_Amp_TopLevel

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

300B_Amp_Glow_1000x1500

300B_Amp_Open_1000x1500

 

 

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!

Luke's 300B Amp

‘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.