6550WE Sovtek Quad tubes Röhren BTB matched Quartett

Sovtek 6550WE – BTB-matched quad for four-valve push-pull output stages

This set contains four Sovtek 6550WE beam power tetrodes with eight-pin octal bases. It is intended for compatible hi-fi, guitar and bass output stages using a total of four 6550 valves and whose voltages, bias range, heater supply and mechanical clearance suit the exact valve.

BTB matches this quad at a practical plate voltage of 500 V. A complete four-valve set supports closely corresponding current distribution and loading in output stages using two valves per push-pull side or two valves per stereo channel.

Suitable applications

• Push-pull output stages using a total of four 6550 power valves
• Stereo power amplifiers using one matched pair per channel
• Guitar and bass amplifiers with a documented four-valve 6550 output stage
• Hi-fi power amplifiers with a compatible bias range and output transformer
• Complete replacement of an existing set of four output valves

BTB matching and package contents

Package contents

Four Sovtek 6550WE valves

Selection type

BTB-matched quad

Matching plate voltage

A practical 500 V unless the individual product states otherwise

Test system

Roetest V11, current as of 2026

Parameters considered

Plate current, transconductance, grid currents, plate resistance and acoustic checks

Technical benefit

Closely corresponding loading of all four valves in one output stage or two stereo channels

Difference from a sextet

Amplifiers using six 6550 valves together, including the SVT-CL, require a set selected as a matched sextet

Technical data

Valve type

Beam power tetrode

Base

Eight-pin octal K8A

Heater voltage

6.3 V nominal; the data sheet gives a permitted range of 5.7 to 6.9 V

Heater current

Not separately stated in the specific Sovtek 6550WE data sheet; verify the heater winding from the amplifier or service documentation

Typical Class A tetrode example

400 V plate, 225 V screen grid, -16.5 V grid bias, 87 mA plate current and 4 mA screen current

Plate load in the tetrode example

3 kΩ

Output in the tetrode example

20 W at 14% total distortion under the stated data-sheet conditions

Typical Class A triode-connected example

250 V plate and screen, -14 V grid bias, 140 mA plate current and 12 mA screen current

Plate load in the triode example

1.5 kΩ

Output in the triode example

12.5 W at 7% total distortion under the stated data-sheet conditions

Maximum plate voltage in tetrode operation

800 V

Maximum plate voltage in triode connection

440 V

Maximum screen-grid voltage

440 V

Maximum plate dissipation

42 W in tetrode operation or 44 W in triode connection

Maximum screen-grid dissipation

6.6 W

Maximum cathode current

230 mA in tetrode operation or 192.5 mA in triode connection

Maximum continuous plate current

190 mA

Maximum grid-leak resistance with fixed bias

0.051 MΩ

Maximum grid-leak resistance with cathode bias

0.24 MΩ

Maximum envelope temperature

250 °C

Manufacturer construction features

Thicker plate structure and improved heat dissipation

Approximate BTB dimensions

Approx. 101 mm high, 43 mm base diameter and 47 mm envelope diameter; production variations are possible

Country of origin

Russia according to the current BTB product page

Pinout

Pin 1

Marked S/NC in the Sovtek sheet; do not use it as a general-purpose tie point

Pins 2 and 7

Heater

Pin 3

Plate

Pin 4

Screen grid g2

Pin 5

Control grid g1

Pin 6

No internal electrode connection shown in the base diagram

Pin 8

Cathode and beam-forming plates

Comparison with KT88 and other 6550 sets

6550WE matched quad

For amplifiers using a total of four 6550 valves with a compatible common bias or adjustment arrangement

6550WE matched sextet

For amplifiers using six 6550 valves together, including compatible SVT versions

KT88

Related octal output valve, but not interchangeable in every amplifier without checking bias, ratings, heater load and mechanical construction

KT120

Larger high-power version with greater heater demand; not an unchecked replacement for a 6550WE

Selection and application notes

Matching does not replace bias verification. In a fixed-bias amplifier, set the quiescent currents after installation and check them again after warm-up. In a cathode-biased circuit, cathode voltage and dissipation must also remain within safe limits.

The 800 V, 42 W and 230 mA figures are limits. They must not be used simultaneously or without a complete load-line and dissipation calculation.

Check the heater supply, negative grid-bias range, screen-grid circuit, output transformer, valve retainers and ventilation before substitution. New valves do not automatically increase amplifier output power.

Power valves become very hot and operate with lethal voltages. Replace and bias them only with the equipment disconnected, discharged and sufficiently cool.

Questions about selection and use

Is the Sovtek 6550WE BTB-matched quad suitable for a push-pull output stage using four 6550 valves?

Yes, the Sovtek 6550WE BTB-matched quad is suitable for compatible push-pull output stages using a total of four 6550 valves. The heater supply, plate and screen-grid voltages, bias range, output transformer, valve retainers and mechanical clearance must suit the specific amplifier.

Can the Sovtek 6550WE quad replace an existing set of four 6550 valves?

Yes, the Sovtek 6550WE quad can replace an existing four-valve 6550 set when the amplifier is designed for the standard 6550 type and its electrical and mechanical requirements match. A KT88 or KT120 set must not be substituted without checking merely because the base is similar.

Should the bias be checked after installing the Sovtek 6550WE quad?

Yes, the bias or quiescent current must be checked after installation and correctly adjusted where the amplifier provides that facility. BTB matching supports closely corresponding valve values but does not replace adjustment for the actual plate voltage, screen-grid voltage and amplifier circuit.