Stamou Tasos, hailing from Greece, finished this DIY 300B triode amplifier project. In appreciation for the 300B amplifier schematics, Stamou has provided his own version of the 300B single-ended (SE) tube amplifier schematic by J.C. Morrison. I helped with translating, formatting, and showcasing his DIY 300B triode amplifier project. – Giovanni Militano – Editor
I discovered the circuit for my 300B triode amplifier on a website, which is the 300B Single-Ended (SE) tube amplifier schematic with direct coupled 6SN7 input stage that I chose to use. Please refer to the SE 300B schematic for details on Mr. J.C. Morrison’s SE 300B tube amplifier circuit, which will be called the “original schematic”. It was emphasized in the circuit notes to use high-quality output transformers and components for constructing the 300B amplifier. Premium components were utilized in the construction of this SE 300B triode tube amplifier, including Lundahl audio output transformers and a homemade chassis. This SET amplifier circuit, utilizing a 6SN7 driver stage, operates with a 300B Single-Ended-Triode. The SE 300B triode is used in the output stage with a maximum power output of approximately 8 Watts per channel. This 300B SET amplifier will need a set of loudspeakers with moderate sensitivity, around 91 dB or higher.
Figure 1 displays detailed information on the specific components and brands used in constructing my SE 300B triode amplifier. It is anticipated that slight adjustments to the values of R7 and R8 will be necessary to reach the desired voltages indicated in the diagram. The diagram in Figure 1 is designed for just one channel, except for the power transformer (T2) which is used by both amplifier channels. Remember this when compiling a parts list for the 300B SET amplifier project. The voltages observed in Figure 1 are readings taken from my 300B triode amplifier construction and closely match the voltages indicated on the original SE 300B diagram. The voltage at these test points may vary slightly with different tube sets, so do not anticipate matching the displayed voltages precisely. Keep in mind that the highest anode voltage allowable for the 300B triode tube is 450V.
I didn’t stray too far from the original SE 300B schematic I was using. I added a resistor (R8) to the power supply to modify the B+ voltage. To enhance efficiency, an anode choke (L2) was included in every channel. The value of R7 was changed from 27k to 15k in order to achieve the desired voltages. In order to enhance the performance of this 300B triode amplifier even more, the power supply filtering sections have been replicated for each channel. Even though the power transformer (T2) is used for both channels, each audio channel has its own rectifier, chokes, and filter capacitors. Employing an individual tube rectifier (5U4-GB) for every channel leads to a reduced voltage drop across the tube rectifier. To learn more about tube rectified power supplies, refer to these notes on designing vacuum tube power supplies. The anodes of the 300B tube are operational at a voltage of 430V. Decreasing R8 can enhance B+, however, keep in mind that the 300B triode tube’s maximum anode voltage is 450V.
I utilized Lundahl LL1623 Transformers (90mA) for the audio output transformers (T1) – PDF (52kB). From what I’ve seen, SE tube amplifier circuits need a top-notch audio output transformer for the best performance. Lundahl is a top producer of audio transformers and the Lundahl LL1623 transformers utilize a unique audio C-core design. The power transformer and chokes are made by a domestic manufacturer in Greece and tailored to specific requirements. I designated 200H, 40mA (L1) for the small anode chokes and 6H, 225mA (L2) for the tube power supply chokes. The power transformer (T2) includes multiple taps: 400-0-400 @ 250mA, 6.3V @ 2A (x3), 5V @ 3A (x2). The SET amplifier shares the power transformer (T2) between its two audio channels.
In the original SE 300B schematic a solid state filtered DC power supply is used for the 300B filaments. To improve on this idea I decided to use 5V integrated circuit (IC) voltage regulators for the 300B filament supply. In my initial attempt I tried two 7805 voltage regulators in parallel for each channel but the current draw from the 300B heater filaments was too large for the 7805 IC. In an email exchange with Gio he recommended that I try the LT1085 voltage regulator circuit shown in Figure 2 for the 300B filaments. This 5V regulated power supply for the 300B filaments was posted by Suncalc and details about the regulated circuit are in the 300B SET Design Project forum thread. With the LT1085 regulator circuit in place the 5V filament supply was stable and the 300B SET amplifier was whisper quiet. I used a separate LT1085 regulated power supply circuit for each audio channel. Note that I did not use the Hammond transformers shown in the Figure 2 Schematic. The LT1085 regulated power supply circuit is connected to the 6.3V taps from the power transformer (T2).
Building the 300B Single-Ended-Triode Amplifier
The 300B SET amplifier was built using point-to-point construction. The premium capacitors are very large in physical size so you will need to use a good sized enclosure for the amplifier. I spent a long time planning the layout of the 300B SET amplifier as you need to be careful so not to create ground loops in your amplifier build. For more information about grounding, see this helpful article about grounding and shielding for your DIY audio projects. Photograph 2 shows the 7805 regulators I initially tried for the 300B filament circuit. The long shafts on the left and right edges of Photograph 2 are from the 300B hum potentiometers (P2). For the internal input cable wiring I used Mogami W2330 shielded interconnect wire.
Caution: This 300B Single-Ended Triode amplifier project operates with voltages exceeding 400 Volts. Contacting voltages of this size can result in severe harm or potentially lead to death. If you are unsure about constructing projects with these abilities or are uncomfortable with projects that involve high voltages, it is highly advised that you avoid building this vacuum tube amplifier. It is necessary to adhere to your local electrical regulations for all grounding, wiring, and connections when building the 300B Single-Ended-Triode Amplifier – Inside and Back.
You will observe that I have drilled several ventilation holes in the bottom plate (Photograph 4). This is the point of entry for cool air into the amplifier. The upward movement of warm air will cause it to exit through the ventilation openings on the top panel (Photograph 5). Make sure to take ventilation into account when planning your construction of a 300B triode amplifier.
The finished amplifier is shown in Photograph 5. I made wooden MDF covers for the Lundahl audio output transformers. The MDF wooden frame and transformer covers were primed and painted with spray cans. In the end I was very pleased with how my chassis turned out for the 300B SET amplifier.
Sound – 300B Single-Ended-Triode Amplifier
I was so eager to hear this 300B triode amplifier that I just had to have a listen before it was even complete. Photograph 6 below shows my first listening session with the 300B SET amplifier. At this point, the amplifier had not yet been finished still requiring the transformer covers and ventilation holes in the top plate. The tube compliment was Electro Harmonix 300B Gold Grid, Electro Harmonix 5U4GB and Tung-Sol 6SN7GTB. For the 6SN7 driver tube I also tried NOS Sylvana and RCA tube brands, and I found that I like the sound from Tung-Sol brand tubes the best. The speakers shown in Photograph 6 are DIY double horn enclosures with the Fostex FE207E fullrange driver.
It’s a shame that you can’t personally experience the amazing sound of this DIY 300B SET amplifier. We conducted a head-to-head comparison with the Sofia Electric SET 300B utilizing JJ 300B triode tubes. We conducted trials with both triode amplifiers using Lowther DX3 horn speakers and Spicom Planar F3 (Dipole floorstanding speaker with 25″planar driver). After testing numerous tube and speaker combinations, we consistently favored the sound produced by my homemade 300B SET amplifier over the Sofia Electric SET 300B. This 300B triode amplifier sounds amazing and will be highly satisfying for DIYers.
Photograph 7 above displays the completed amplifier. I am very happy with the final outcome of the 300B SET amplifier build, both in terms of looks and sound quality. I want to express my gratitude to the website for offering the blueprints to create an amazing-sounding 300B triode amplifier.
Source: DIY 300B Single-Ended-Triode (SET) Hi-Fi Amplifier
