Power triodes for single-ended amplifiers: 45, 2A3, 300B, 845, 211 and 805

The output tube decides much of a single-ended amplifier’s character, and it decides how much power you have to work with. This guide compares the power triodes used in single-ended (SET) designs, from the 2-watt 45 to the 25-watt GM-70, describes how each is commonly heard, and shows how to match an amplifier’s power to a loudspeaker’s sensitivity.

Why directly heated triodes for single-ended amplifiers?

In a single-ended stage one tube (or several in parallel) amplifies the whole waveform, in Class A. The distortion it produces is mostly of low order, with the second harmonic dominant, and it changes gradually as the level rises. A triode has a low internal resistance compared with a pentode, so it needs little or no negative feedback to control the loudspeaker. In a directly heated triode the filament itself is the cathode, which keeps the tube simple.

The trade-off is efficiency. A single-ended stage turns most of its supply power into heat, and its output transformer must carry the full standing current without saturating, which makes it large and heavy for the power it delivers. Our article on how tube amplifiers work covers the basics of single-ended and push-pull design.

The power triodes compared

Typical power depends on the operating point, the supply voltage and the output transformer, so treat the third column as a range seen across many designs. The sensitivity column assumes a medium room of 20–30 m² and normal to lively listening levels; the arithmetic is explained further down.

The small triodes: 45, 2A3 and 300B

45

A receiving-era triode with a 2.5 V filament and the lowest power of the classic types. It is commonly described as transparent, quick and intimate. In Solaria it gives 2 W per channel, voiced for a dense, saturated tone with a textured midrange and a faster, harder-hitting bass than the 300B.

2A3

Introduced in the early 1930s, in single-plate and dual-plate versions, with a 2.5 V filament. Descriptions vary more than for any other type: some listeners hear it as articulate and lean, others as dense and full, which says as much about the circuits around it as about the tube. In Eurybia it gives 2 W per channel and is voiced for tonal weight: a full bass and a heavy, saturated midrange.

300B

Designed by Western Electric in 1938, with a 5 V filament. It is the reference point for most comparisons and is commonly described as warm, sweet and generous in the midrange. In Atlas it gives 5 W per channel.

300B or 2A3?

This is the comparison searchers ask about most. The 300B gives roughly twice the power of the 2A3, which widens the choice of loudspeakers. On sound, the 2A3 is often described as more direct and the 300B as richer and more forgiving. In practice the output transformer, the driver stage and the power supply often make a larger difference than the tube type, so compare complete amplifiers rather than tubes in isolation.

The transmitting triodes: 3C24, 811, 211, 845, 805 and GM-70

3C24

A small transmitting triode with a tantalum anode and a thoriated tungsten filament, also known as the 24G. Tantal runs a pair in Class A at 3 W per channel and adds an electrostatic headphone output; it is voiced for real timbre on voices and precise placement in space.

811 and G-811

A high-gain transmitting triode designed for zero-bias Class B service. In a single-ended amplifier it is driven into positive grid voltage (Class A2), so the driver stage must be able to supply grid current. The G-811 is its transmitting-tube counterpart. Helios MkII uses a pair with a 6P3S driver for 15 W per channel. The G-811 and the original RCA 811 are different tubes from modern budget derivatives such as the SV811-10.

845, 211 and 805

The large transmitting triodes, with 10 V thoriated tungsten filaments and anode supplies around 1,000 V. The 845 has a low amplification factor and needs a very large drive voltage; the 211 has a higher one and is easier to drive; the 805 is a high-gain tube run with positive grid drive, which gives the most power of the three.

845 or 211?

Both are common in SET amplifiers of 15–20 W. The 845 is commonly described as fuller and weightier in the bass and lower midrange, the 211 as more open and extended in the treble. Endymion is built around any of the three: 845 at 20 W, 211 at 15 W or 805 at 25 W per channel, with a linear, transparent presentation rather than a vintage colour.

GM-70

A transmitting triode with a 20 V filament and a large anode, run at around 1,000 V or more. In a single-ended amplifier it gives 20–25 W. It is commonly described as dynamic and controlled, with the scale of the big triodes; compared with the 845, owners often mention a firmer bass. Ether is built on GM-70 Triode / GK-71 Pentode transmitting tubes, a pair in pure Class A at 25 W per channel, with an external power supply of about 100 kg; our guide to transmitting tubes in single-ended amplifiers covers the GM-70, the 813 and the 811 in more detail.

A pentode in a triode family: the 4E27

Not every single-ended amplifier uses a triode. The 4E27 is a transmitting pentode, and a pentode delivers more power from the same supply. Tantal Senior uses vintage 4E27 tubes with tantalum anodes for 30 W per channel in Class A, with no negative feedback and a transformer volume control. The rectifiers in our power supplies are tubes too: the 6D22S, a diode used as a kenotron for a darker tone, or the 5C3S for a brighter one.

How much power do you need? Speaker sensitivity and SET power

Loudspeaker sensitivity is the sound level measured at 1 m with 1 W of input. Each doubling of power adds only 3 dB: 2 W adds 3 dB, 4 W adds 6 dB, 8 W adds 9 dB, 16 W adds 12 dB and 25 W adds about 14 dB. A 100 dB loudspeaker on a 2 W amplifier can therefore reach about 103 dB at 1 m, while an 88 dB loudspeaker on the same amplifier stops at about 91 dB.

At a typical listening distance of about 3 m the level in a room falls by several decibels, depending on the room. Orchestral and rock peaks at the listening seat can reach 100 dB and more, and a single-ended amplifier pushed past its rated power distorts gradually rather than clipping hard, but audibly. That is where the rule of thumb in the table comes from.

Is 2 watts enough?

For loudspeakers of 98 dB and above in a medium room, usually yes. For loudspeakers around 90 dB, no: the amplifier runs out of headroom on the first loud passage.

Which loudspeakers make life hard for a SET amplifier?

• Low sensitivity. Below about 88 dB even a 25 W single-ended amplifier has little headroom.
• Low or wildly varying impedance. A SET amplifier has a relatively high output impedance, so its frequency response follows the loudspeaker’s impedance curve. Flat curves at 8 Ω or more are the easiest load; output transformer taps for 4 Ω or 16 Ω help match the load.
• Complex passive crossovers. Steep multi-way filters absorb power and create impedance dips.
• Electrostatic panels. Their impedance falls towards high frequencies; our two electrostatic models drop to about 1 Ω at 20 kHz, which is why active versions exist.

Practical points: high voltage, heat and weight

The transmitting triodes run at anode voltages around 1,000 V, which stay in the filter capacitors after switch-off. Service belongs to a qualified technician. All of these amplifiers produce real heat: give them free air above and around the chassis, keep them off closed shelves, and in hot climates place them in an air-conditioned room. Expect the output tubes to need replacing after some years; a reserve matched pair can be ordered with each amplifier. And plan for weight: Ether with its power supply weighs about 130 kg in total.

Which Demograf Audio piece fits

• Loudspeakers of 98 dB and above, such as our three-way Orpheus at 103 dB: Solaria or Eurybia, and Tantal from about 96 dB.
• Loudspeakers of about 93–97 dB, such as Cassandra MkII at 95 dB: Atlas, or Helios MkII for more headroom.
• Loudspeakers of about 90–93 dB: Helios MkII.
• Loudspeakers from about 88 dB, or large rooms: Endymion, Ether or Tantal Senior.

Most of our loudspeakers are designed with these amplifiers in mind. Every amplifier is made by Dmitriy Gubchenko; each is also available as a push-pull version with twice the power, except Tantal Senior. Tell us your loudspeakers and room size, and we will suggest the tube that fits.