GU50 Pair ry50 Power tubes BTB matched Paar

GU50 matched pair for powerful audio output stages

The GU50 is a rugged power pentode rated for up to 40 W of plate dissipation per valve. BTB's matched pair is particularly attractive for powerful single-ended, push-pull and parallel audio output stages.

The MTA principle published by Frank Blöhbaum in Linear Audio, including its later high-power treatment, may provide technical inspiration for an original design. This page does not claim that a GU50-specific circuit was published, tested, approved or recommended by him.

Suitable applications

• Powerful single-ended audio amplifiers
• Push-pull and symmetrical valve output stages
• Parallel power-valve circuits
• Original active-feedback and MTA-inspired designs

Technical specifications

Valve type

Indirectly heated power pentode

Offered configuration

Two GU50 valves assembled by BTB as a matched pair

Related valve types

Historically related to the LS50 and associated with the EL152 family, but not directly interchangeable

Socket

Dedicated 8-pin GU50 socket, different from the LS50 socket

Heater

12.6 V at typically 0.705 A, with a published tolerance of approximately ±0.065 A

Transconductance

Typically approximately 4 mA/V, specified as roughly 3 to 5.5 mA/V at the reference condition

Maximum plate dissipation

40 W

Maximum screen-grid voltage

250 V

Maximum screen-grid dissipation

5 W

Maximum cathode current

230 mA

Anode-voltage limit

Up to 1000 V under the specified RF conditions; an audio operating point must be calculated separately

Dimensions

Maximum diameter approximately 45.3 mm and maximum height approximately 93.5 mm

Connections – bottom view

Pin 1

Cathode

Pin 2

Control grid g1

Pin 3

Screen grid g2

Pins 4 and 8

Heater

Pin 5

Suppressor grid g3

Pin 6

Anode

Pin 7

Internal shield

High-power audio design notes

A GU50 output stage requires a substantial driver, a controlled screen-grid supply and an output transformer whose impedance, power rating and DC capability suit the selected operating point.

Matching is particularly useful in a push-pull circuit. Parallel valves should additionally have individually measurable currents and suitable cathode or balancing resistors.

Safety notice: GU50 amplifiers may use high voltages and substantial dissipation. Effective touch protection, fusing, discharge resistors and adequate cooling are essential.

Questions about GU50 audio amplifiers

Is the GU50 matched pair suitable for a powerful push-pull amplifier?

Yes, two closely performing GU50 valves provide a useful basis for a symmetrical push-pull output stage. The 40 W maximum plate-dissipation rating per valve offers substantial reserve, while the bias, screen voltage and output transformer must be calculated for the intended amplifier.

Can the GU50 be used in an MTA-inspired high-power amplifier?

Yes, the GU50 may serve as the power device in an independently developed MTA-inspired output stage. The published MTA principle is only the technical reference; no GU50-specific circuit, test, approval or recommendation by Frank Blöhbaum is claimed.

Is BTB matching useful when several GU50 valves operate in parallel?

Yes, suitable selection can help achieve more consistent current sharing. Safe parallel operation still requires individual current measurement, appropriate balancing resistors and adequately rated heater and anode supplies.