300B, 2A3, 45 or 845/211: how to choose the output triode

Choosing a single-ended amplifier usually starts with the output tube, and the question asked most often is simple: 300B or 845? Our guide to power triodes for single-ended amplifiers describes each type and how it is commonly heard. This guide is its practical companion. It compares the 45, 2A3 and 300B with the 845, 211 and 805 on the points that decide what you will live with: output power, drive requirements, supply voltage, filament heating, loudspeaker sensitivity, room size, heat, weight and safety.

300B or 845: the short answer

• Choose a small triode (45, 2A3 or 300B) if your loudspeakers are sensitive, about 93 dB and above, your room is small to medium, and you prefer an amplifier that works at a few hundred volts.
• Choose a large transmitting triode (845, 211 or 805) if your loudspeakers are around 88–92 dB, your room is large or you listen loud, and you accept supply voltages around 1,000 V, more heat and a more demanding driver stage.
• Compare complete amplifiers, not tubes. The output transformer, the driver stage and the power supply shape the result as much as the tube type does.

The output triodes at a glance

Filament ratings are standard datasheet values. Plate supply and drive are typical single-ended operating points, not limits; in Class A1 the peak drive a stage needs is roughly equal to its grid bias. Real designs vary, so read the table as orders of magnitude: the small triodes live at a few hundred volts, the large ones around a kilovolt.

Power: what 2, 5, 15 or 20 watts mean in a room

Every doubling of power adds 3 dB, and ten times the power adds 10 dB. The maximum level at 1 m is roughly the loudspeaker’s sensitivity plus 10 × log10 of the amplifier’s power in watts. That brings very different systems to the same place:

• Solaria or Eurybia (2 W) with 100 dB loudspeakers: about 103 dB at 1 m.
• Atlas (5 W) with 95 dB loudspeakers: about 102 dB at 1 m.
• Endymion with 845 tubes (20 W) with 90 dB loudspeakers: about 103 dB at 1 m.

So the useful question is not which tube is louder, but which loudspeakers and which room it has to work with. The level falls as you move away from the loudspeakers. A large room, a long listening distance or a taste for loud orchestral and rock peaks calls for more headroom: more sensitive loudspeakers, more power, or both. A small room with the chair close to the loudspeakers needs the least. Our guide to loudspeakers for single-ended amplifiers goes further into sensitivity and impedance.

300B or 2A3?

In our range the difference is 5 W against 2 W, which gives the 300B about 4 dB more headroom. That is audible on loud peaks with loudspeakers in the low 90s, and much less important with loudspeakers of 97 dB and above. Both tubes are easy to live with: the 2A3 runs from about 250–300 V and needs about 45 V of drive, the 300B from about 350–450 V and roughly 60–90 V. The 45 sits beside the 2A3, with similar voltages and power. On sound, Eurybia (2A3) is voiced for tonal weight and a dense, full bass, Solaria (45) for speed and a harder-hitting bass, and Atlas (300B) for colour, warmth and a sweet midrange.

845 or 211?

The 845 and the 211 have the same 10 V filament and the same large four-pin base, and run from similar supplies. The difference is gain. The amplification factor of the 845 is about 5 and that of the 211 about 12, so the 845 needs a far larger drive voltage. In Endymion the 845 gives 20 W and the 211 15 W per channel, a difference of just over 1 dB. The 805 is the third option: a high-gain triode run with a positive grid, which gives Endymion 25 W but asks the driver stage to supply grid current. The tube is chosen when you order, and the amplifier is built around it.

Drive requirements: why big triodes need a big driver

A triode’s grid has to be swung by a signal roughly as large as its bias, and the lower the tube’s amplification factor, the larger that swing. A 300B needs some 60–90 V peak, which an ordinary small-signal tube can deliver cleanly. An 845 at around 1,000 V needs well over 100 V peak, and that voltage must arrive with low distortion, because the output tube amplifies whatever the driver produces. The driver becomes a substantial amplifier of its own, with its own high-voltage supply.

That is why an 845 amplifier is a bigger engineering task than a 300B amplifier of the same standard, and why its driver stage accounts for a large part of its sound. Higher-gain tubes move the problem rather than remove it: the 805, like the 811, works with positive grid voltage (Class A2) and needs a driver that can deliver current as well as voltage. Our guide to transmitting tubes in single-ended amplifiers explains A2 drive in more detail.

Supply voltages and safety: the 400 V class and the 1,000 V class

The small triodes work from plate supplies of a few hundred volts: about 250–300 V for the 45 and 2A3, about 350–450 V for the 300B. That is already dangerous, and nobody should work inside a tube amplifier without training. The 845, 211 and 805 work around 1,000 V, more than twice as much, and every part of the supply, the wiring and the output transformer has to be built and insulated for it.

• Filter capacitors can hold a lethal charge long after switch-off. Work inside the chassis belongs to a qualified technician.
• Keep loudspeakers or a suitable load connected whenever the amplifier is on; an unloaded output transformer can develop high voltages.
• Keep children and pets away from hot tubes and from the space around the amplifier.

Every Demograf Audio amplifier in this guide has an external power supply chassis with kenotron (tube) rectification, galvanic isolation and chokes, so the high-voltage supply sits apart from the audio circuit. Each unit is made by Dmitriy Gubchenko and comes with its own measurement report.

Filaments, heat and weight

In a directly heated triode the filament is also the cathode, and the datasheets show how far apart the two classes are. The filament of a 45 draws about 3.75 W, a 2A3 about 6.25 W and a 300B 6 W; each 845, 211 or 805 draws 32.5 W. That is before any plate dissipation. In Class A the output tubes dissipate their standing power continuously, so a pair of large triodes with their driver and supply turns a good deal of mains power into heat whether or not music is playing.

The thoriated tungsten filaments of the 845, 211 and 805 glow brightly; the oxide-coated filaments of the 300B, 2A3 and 45 glow dimly. Give any of these amplifiers free air above and around the chassis, keep them out of closed cabinets and, in hot climates, place them in an air-conditioned room. Plan the support as well: every model in this guide weighs from 45 kg with its power supply.

Matching the triode to your loudspeakers and room

The table is a rule of thumb, not a limit. Impedance matters too: a single-ended amplifier has a relatively high output impedance, so a flat impedance curve at 8 Ω or above is the easiest load. Our amplifiers are wound for 8 Ω as standard, and a 4 Ω or 16 Ω winding can be specified when you order.

Does the output tube change the sound?

It does, though often less than forum debates suggest. The 300B is commonly described as warm and generous in the midrange, the 845 as weighty in the bass and lower midrange, the 211 as more open in the treble, and the 2A3 and 45 as direct and quick. The voicing of the complete amplifier often matters more: Endymion, for example, is voiced to sound linear and transparent rather than vintage-coloured, whichever triode it carries. Where you can, listen to complete amplifiers with your own loudspeakers.

Which Demograf Audio piece fits

• Solaria (45, 2 W) or Eurybia (2A3, 2 W): high-sensitivity loudspeakers such as the three-way Orpheus at 103 dB, and other loudspeakers of 98 dB and above.
• Atlas (300B, 5 W): loudspeakers from about 93 dB, such as Cassandra MkII at 95 dB, in small and medium rooms.
• Endymion (845 at 20 W, 211 at 15 W or 805 at 25 W per channel): loudspeakers from about 88 dB, large rooms and higher levels.

All four are integrated amplifiers with headphone outputs as well as loudspeaker terminals; see our amplifiers for loudspeakers. Every tube amplifier from Demograf Audio is built single-ended as stock, and a push-pull build is available to order; our guide to single-ended and push-pull explains the difference. Each is made to order by Dmitriy Gubchenko and delivered with its own measurement report. Tell us your loudspeakers and room size, and we will suggest the triode that fits; how to order explains the next steps.