1605V4 — Cathode-Tapped Power Triode

The 1605V4 (also designated as EML1605-V4) is a high-power, indirectly-behaving directly heated triode designed for high-fidelity audio amplification. It is characterized by a medium-mu design, high plate dissipation, and a unique cathode-tapped filament structure that allows it to function as a true three-connection triode with an independent cathode connection. The tube is related in design and sonic character to the classic Marconi PX25, while offering significantly higher voltage capability, higher impedance, and substantially greater power output than standard 300B-type tubes.

Design Philosophy and Sonic Character

The 1605V4 is engineered to address the limitations of traditional large power triodes, which can sound sterile at low power levels. To preserve a sweet, silky sound character even at low output levels, the tube features wide, box-shaped anodes similar to those of the Marconi PX25. Sonically, the 1605V4 is positioned between the 300B and the PX25, but it is capable of operating at much higher power levels.

To achieve higher output power without excessively increasing anode current, the 1605V4 is designed for higher anode voltage operation. This approach increases plate dissipation while keeping the DC current at a moderate level. This offers a significant advantage for single-ended (SE) output transformers, which do not have to operate under excessive DC current. Consequently, the transformer is easier to construct, and its primary inductance can be higher.

Furthermore, the 1605V4 features a lower bias voltage relative to its operating voltage, which improves power supply efficiency by reducing the voltage dropped and wasted across the cathode bias resistor. Although this lower bias voltage limits the maximum grid signal swing, the tube's higher gain (approximately twice that of a 300B) compensates for this, requiring a lower total input signal and simplifying the design of the driver stage.

Construction and Mechanical Features

The 1605V4 incorporates several advanced mechanical and materials technologies to ensure stability, longevity, and low microphonics:

• Hard-Metal Construction: The tube utilizes hard-metal construction rather than classic nickel. This ensures a highly precise and stable anode distance, prevents damage during shipment, and provides reproducible electrical parameters and long-term stability.
• Wolfram Grids: The grid wires are made of wolfram (tungsten), an extremely hard metal that allows for highly precise grid wire spacing. This precise geometry ensures excellent uniformity and linearity in the tube's characteristic curves.
• Gold-Plated Grid: The gold-plated grid structure provides increased bias stability, protection against accidental overload, and improved curve linearity.
• Anode and Cooling Design: The tube features 0.5 mm thick anodes equipped with 12 coolers (6 on each side of the anode) to manage heat dissipation.
• Suspension and Envelope: It features a double mica top, a soft rubber suspended base, and an anti-microphonic anode and grid suspension system. The envelope is a hand-blown glass bulb.
• Getters: The vacuum is maintained using a three-getter system consisting of two side getters and one top getter.
• Physical Dimensions: The tube has a physical size of 200 mm in height (including the socket, but excluding the pins) and a diameter of 68 mm. The weight of a single tube is 190 grams.
• Aesthetics and Pins: The tube features gold-plated pins and is printed with real gold metal, with red lettering burned directly into the glass. It is designed to be used with high-quality sockets, such as Yamamoto tube sockets, and is supplied with a black ceramic socket featuring a white bottom.

Heater and Cathode-Tapped (V4) Technology

The defining feature of the 1605V4 is its Cathode-Tapped heater (V4-Version), which distinguishes it from the standard V3-Version (which uses a conventional heater).

In a standard directly heated triode (DHT) like the 300B, there is no physical cathode terminal. Biasing requires an artificial center tap created via an external resistor network. This configuration forces the entire AC signal current, along with the DC bias current, to pass through these centering resistors, introducing a compromise in signal purity.

The 1605V4 solves this by serializing the internal heater wires and tapping them precisely at the electrical center. This center tap is brought out to a dedicated pin, acting as a true physical cathode (K). This configuration eliminates the need for external centering resistors or hum-balancing networks. The heater itself is powered by a floating 5.0 V supply connected across the H1 and H2 terminals. To minimize contact resistance, the H1 and H2 heater connections are each routed through two parallel contacts on the base.

For flexibility, the 1605V4 can also be operated in a classic DHT configuration. By disconnecting the cathode tap, the remaining four wires can be wired in the traditional manner of a standard directly heated tube (such as a 300B or 520B-V3).

Base and Pin Connections

Due to the five-wire electrical interface (Anode, Grid, Cathode Tap, and two dual-contact Heater connections), the 1605V4 cannot use a standard 4-pin UX4 base. Instead, it is fitted with an Octal base.

The pin assignments, viewed from the bottom of the octal socket, are as follows:

Note: The physical orientation of the internal plates is parallel to the side of the base where the logo is printed.

Electrical Ratings and Operating Points

The 1605V4 is rated for high-power Class A operation. Its electrical characteristics, filament requirements, and maximum limits are detailed below.

Filament Ratings

The filament must be operated at its nominal voltage to ensure optimal emission and tube life. Operating the tube at a lower filament voltage in an attempt to extend its lifetime is explicitly discouraged.

Maximum Conditions

These maximum ratings represent the absolute limits of safe operation. They must not be applied simultaneously.

Typical Recommended Operating Point

The typical operating point is optimized for Class A service using either auto bias or fixed bias:

Recommended Transformer Configuration

For optimal performance at the typical operating point, a Lundahl LL1623-120mA transformer (or equivalent) is recommended under the following circuit conditions:

• Transformer Primary Impedance: 3000 Ω
• Transformer Secondary Options: 4 Ω (Alt.B), 8 Ω (Alt.C), or 16 Ω (Alt.D)
• Auto-Bias Cathode Resistor: 800 Ω
• Anode Current: 110 mA
• Anode Voltage to Ground: 668 V

Technical Summary

The 1605V4 is a premium, octal-based, cathode-tapped power triode designed for uncompromising Class A audio amplifiers. Combining the sonic refinement of the classic PX25 with modern high-voltage capability, it delivers up to 22 W of Class A output power at its recommended operating point of 580 V anode voltage and 110 mA anode current.

Its unique cathode-tapped filament design provides a true physical cathode connection, eliminating the need for external centering resistors and allowing for a cleaner, more direct signal path. With an amplification factor of 7.5, a transconductance of 8.8 mA/V, and a low anode impedance of 861 Ω, the 1605V4 simplifies driver stage requirements and optimizes output transformer performance, making it an exceptional choice for high-end single-ended triode designs.