Transmitting tubes in single-ended amplifiers: GM-70, 813 and 811

Some of the most powerful single-ended amplifiers are built on tubes that were never designed for music: transmitting tubes, made for radio transmitters. This guide explains what a transmitting tube is and looks at three types that appear in single-ended (SET) amplifiers: the GM-70, the 813 and the 811 family. It covers how each is used, what it asks of the driver stage, what living with supplies around 1 kV means, and how Demograf Audio uses these tubes. For the 845, 211 and 805 and the small triodes, see our guides to power triodes for single-ended amplifiers and choosing between the 300B and the 845.

What is a transmitting tube?

A transmitting tube is a power tube designed for radio transmitters: radio-frequency amplifiers, oscillators and the audio modulators that impress the programme onto the carrier. Compared with receiving tubes such as the 45, 2A3 and 300B, transmitting tubes are built for much higher plate voltages and dissipation. They have large anodes, usually thoriated tungsten filaments, and on many types the anode is brought out to a cap on top of the bulb. Their datasheets often include audio operating points for modulator service, which is how several of them found their way into hi-fi.

In a single-ended amplifier a transmitting tube offers more power per tube: typically 15–25 W per channel, against 2–8 W for the small triodes. The trade-off is high voltage, a lot of heat and, in most cases, a demanding driver stage.

The GM-70: a large directly heated triode

The GM-70 is a directly heated transmitting triode with a 20 V filament and a large anode. In single-ended amplifiers it is commonly run from anode supplies around 1,000 V or more and typically gives 20–25 W per channel. Like the 845, it needs a large drive voltage, so a GM-70 amplifier is as much about its driver stage and power supply as about the output tube. Its sound is commonly described as dynamic and controlled, with the scale of the large triodes; owners who compare it with the 845 often mention a firmer bass.

Searches for a “GM-70 SET amplifier” or a “GM-70 integrated amplifier” usually mean exactly this: a stereo single-ended amplifier with one GM-70 per channel, with or without its own volume control.

The GK-71 pentode

The GK-71 is a large transmitting pentode. A pentode adds a screen grid and a suppressor grid between the control grid and the anode. Compared with a triode it has higher gain and higher internal resistance, so it needs less drive voltage for the same power, while the output stage behaves differently into the loudspeaker. That is why the choice between a triode and a pentode output stage is a choice of character as well as of circuit.

The 813: a beam tetrode, often triode-connected

The 813 is a beam power tetrode with a 10 V thoriated tungsten filament and a top-cap anode, designed for radio-frequency and modulator service. In single-ended amplifiers it is often triode-connected (triode-strapped): the screen grid is tied to the anode, usually through a small resistor, so the tube behaves as a triode with a lower internal resistance than in tetrode mode. It can also be run as a tetrode for more power. Most searches for an “813 amplifier” or an “813 tube amplifier” refer to such single-ended designs. No Demograf Audio model uses the 813 today.

The 811, 811A and G-811: high-gain triodes driven into grid current

The 811 is a directly heated transmitting triode with a 6.3 V thoriated tungsten filament, a top-cap anode and a high amplification factor of about 160. It was designed for zero-bias Class B service: in a push-pull modulator it needs no negative grid bias at all. The 811A is the version with higher plate dissipation. The G-811 is a vintage transmitting triode of the same type.

Used single-ended, such a high-gain tube sits close to zero bias, so on signal peaks its grid swings positive and draws current. This is Class A2. The voltage swing it needs is modest, but the driver must supply that grid current without distortion, which calls for a driver with a low output impedance: a power tube, a cathode follower or a driver transformer. In Helios MkII a pair of G-811 triodes is driven by the 6P3S, a beam tetrode of the 6L6 family, after a 6Zh4 input stage, 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.

Transmitting tubes at a glance

Driving transmitting tubes

Transmitting tubes are demanding to drive in one of two ways. Low-gain triodes such as the GM-70 and the 845 need a large voltage swing, which means a driver running from a high supply of its own and producing that swing with low distortion. High-gain triodes such as the 811 and the 805 need a modest voltage but real current once the grid goes positive. Either way the driver is a substantial amplifier in its own right, and it shapes the sound as much as the output tube does. This is also why two amplifiers on the same transmitting tube can sound very different.

Living with about 1 kV: safety and placement

• High voltage. Anode supplies around 1 kV can be lethal, and filter capacitors can hold their charge long after switch-off. Only a qualified technician should open the chassis.
• Top-cap anodes. On tubes such as the 811A and the 813 the anode cap carries the full plate voltage. Never touch tubes or caps while the amplifier is on or soon after.
• Load. Keep loudspeakers or a suitable load connected whenever the amplifier is on.
• Heat. Large filaments and anodes in Class A produce a lot of heat. Give the amplifier free air above and around it, keep it out of closed cabinets and, in hot climates, use an air-conditioned room.
• Weight. Ether weighs about 130 kg in total: about 30 kg for the amplifier and about 100 kg for its power supply. Helios MkII weighs from 45 kg with its power supply.

Both amplifiers keep the high-voltage supply in a separate chassis, with kenotron (tube) rectification, galvanic isolation and chokes, and both are set at order time for 100, 120 or 230 V mains with the matching plug.

Tube availability and replacement

The GM-70, the GK-71 and the G-811 are found today mainly as new old stock (NOS) and vintage stock. Quality varies between batches, so tubes are tested and matched in pairs before use. There is no fixed replacement interval: tube life depends on the operating point, the hours of use and how the amplifier is treated, and a tube is replaced when measurement shows lost emission or when it becomes noisy. Our guides to tube life and care and NOS tubes cover this in more detail.

When delivering to countries that have imposed sanctions, we do not install sanctioned components; we fit vintage and modern equivalents instead.

GM-70 or 845

The GM-70 and the 845 are both large directly heated triodes from transmitting service, and in a single-ended amplifier they ask for the same things: an anode supply around 1,000 V, a driver stage that delivers a large voltage swing with low distortion, and an output transformer built for the tube. The differences that matter when you choose are the filament supply, the power each gives in our amplifiers, and the size of the amplifier and its power supply.

• Filament. The GM-70 takes 20 V, the 845 10 V (datasheet values). Either way, the filament supply is a sizeable part of the power supply.
• One tube or a family. Endymion takes the 845, or the 211 or 805 to order, and the tube sets its power. Ether is built for the GM-70 triode, with the GK-71 pentode as the alternative.
• Size. Ether’s power supply alone weighs about 100 kg; Endymion weighs from 45 kg. Plan the room and the route into it before you order.

Neither tube is a reason in itself to choose one amplifier over the other. Compare the power with your loudspeakers and room, using our guide to loudspeakers for single-ended amplifiers, and the size and weight with your space. The 845, 211 and 805 are covered in more detail in our guide to choosing the output triode.

Which Demograf Audio piece fits

• Ether: GM-70 Triode / GK-71 Pentode, pure Class A, 25 W per channel, with an AL4 input stage and an external power supply with 6D22S rectifiers. For loudspeakers from about 88 dB and large rooms; it also has headphone outputs and its own volume control.
• Helios MkII: G-811 triodes, pure Class A, 15 W per channel, voiced for a detailed, holographic presentation with a punchy, easy bass. For loudspeakers from about 90 dB and for hard-to-drive headphones such as the HiFiMan HE-6 or Audeze LCD-3.
• Endymion: if you prefer the 845, 211 or 805, the transmitting triodes covered in our guide to choosing the output triode.

Every tube amplifier from Demograf Audio is made by Dmitriy Gubchenko, built single-ended as stock, and delivered with its own measurement report. Our amplifiers for loudspeakers page shows the full range.