6M5

The 6M5 is an audio power pentode with a nine-pin Noval base, described in the Miniwatt booklet as Power Pentode. Its indirectly heated cathode uses a 6.3 V, 0.71 A heater. The documentation covers one valve in class A, two in class AB and triode connection: comparisons must retain the circuits’ loads, bias and distortion alongside output power.

For one 6M5 in class A the plate load is 7 kΩ in all three conditions. With plate and screen at 150 V, 160 Ω cathode resistance, 20 mA plate and 2.7 mA screen current, output is 1.3 W with 2.22 V RMS input. At 200 V, 140 Ω, currents 30 and 4.1 mA: 2.75 W with 3 V RMS. At 250 V, 170 Ω, currents 36 and 5.2 mA: 3.9 W with 3.8 V RMS. All these outputs are specified at 10% total harmonic distortion.

At the 250 V point Miniwatt gives 10 mA/V transconductance, 40 kΩ plate resistance and factor 22 explicitly referring to screen grid relative to control grid. These are not complete-amplifier gain. It also gives 0.32 V RMS input for 50 mW output and, separately, 4.8 W where grid 1 current reaches +0.3 µA. The latter is not presented as the same distortion condition as the 3.9 W result.

With two 6M5s in class AB, plate and screen 250 V, stated cathode resistance 85 Ω and 7 kΩ plate-to-plate load, output is 9.4 W at 4.6% total harmonic distortion with 5.6 V RMS input. Plate currents rise from 2 × 36 mA at rest to 2 × 39.5 mA at maximum signal; screen currents from 2 × 5.2 to 2 × 8 mA. The table thus describes changing stage consumption, not output alone.

Triode connection joins grid 2 to plate. At 250 V, 250 Ω cathode resistance, 33 mA plate current, 3.5 kΩ load and 6 V RMS input, Miniwatt gives 1.55 W at 8% total distortion. This is a different electrode connection with its own operating point, not a generic sonic promise. Five curve pages add current characteristics, power and distortion for one valve and a pair, and dependence on plate load.

Ratings are Design-Centre values: 300 V plate and screen in operation, distinct from 550 V without current. Maximum plate dissipation 9 W; screen 1.4 W without signal, 3.3 W at maximum signal. Maximum cathode current 55 mA, heater-cathode voltage 100 V, external grid 1 resistance 1 MΩ and heater-cathode resistance 20 kΩ. Capacitances: input 10 pF, output 6.2 pF, grid 1-to-plate maximum 1 pF, grid 1-to-heater maximum 0.15 pF. Rating type and conditions remain part of the data.

The small-button Noval base assigns 1 screen, 2 control grid, 3 cathode and third grid, 4 and 5 heater, 7 plate. Pins 6, 8 and 9 are internal connections, not spare contacts. Any mounting position is allowed. Maximum diameter is 7/8 inch, about 22.23 mm, body height including exhaust tip but excluding pins 2 13/16 inches, about 71.44 mm, overall length 3 1/16 inches, about 77.79 mm. These dimensions belong to the Miniwatt construction documented.

Documentation consulted: Miniwatt, 6M5, Power Pentode, June 1, 1960 booklet, two data pages and five curve plates A–E. All seven pages, drawings and available descriptive data checked. The booklet date is not an introduction date; working designations in the graphs are not treated as universal tube equivalences.