5-20 Amplifier 20W (1960 Design)

Mullard Wireless, London (see also Mullard Radio Valve)

Data
Forum
Pictures
Collections

  • Year
  • 1960 ?
  • Category
  • Kit (Parts plus instruction) or building instructions only
  • Radiomuseum.org ID
  • 367506

Click on the schematic thumbnail to request the schematic as a free document.

 Technical Specifications

  • Number of Tubes
  • 5
  • Valves / Tubes
  • Main principle
  • Audio-Amplification
  • Wave bands
  • - without
  • Power type and voltage
  • Alternating Current supply (AC)
  • Loudspeaker
  • - This model requires external speaker(s).
  • Material
  • Metal case
  • from Radiomuseum.org
  • Model: 5-20 Amplifier 20W - Mullard Wireless, London see
  • Shape
  • Chassis only or for «building in»
  • Notes

  • The Mullard 5‑20 is the revised 20‑watt high‑fidelity amplifier design published by Mullard Ltd (UK) in the 1959/1960 second edition of Mullard – Circuits for Audio Amplifiers. It represents the final and most widely adopted form of the 5‑20 concept, updating the original 1955 National Radio Show version with revised component values, improved stability networks, updated transformer specifications and a new mechanical layout. The circuit uses an EF86 low‑noise pentode, an ECC83 cathode‑coupled phase splitter and a push‑pull pair of EL34 output pentodes with distributed loading. A GZ34 rectifier supplies the amplifier and provides additional HT capacity for an associated pre‑amplifier or tuner. Overall feedback is 30 dB and the rated output is 20 W with distortion below 0.05 %.

    The 5‑20 evolved directly from Mullard’s earlier 5‑10 design of 1954, which introduced the EF86–ECC83 front end and the use of global negative feedback in a standardised, constructor‑friendly format. The 5‑10 established the topology and constructional approach that Mullard later extended to higher power. The 5‑20 retained the same input and phase‑splitting architecture but replaced the EL84 output pair with EL34 valves operating under distributed loading, giving higher power and lower distortion. The 1960 edition refined the original 5‑20 by improving high‑frequency stability, adjusting time constants, updating transformer recommendations and providing revised chassis drawings.

    This design was widely used by UK kit builders and commercial manufacturers in the late 1950s and early 1960s, and it became one of the most influential British high‑fidelity amplifier circuits of the valve era. Mullard did not manufacture the amplifier; it was issued as a design for constructors and industry. The same 1960 design was also printed in overseas editions, including an Australian printing with local component lists and transformer equivalents.

    1960 Mullard 5‑20 — Summary of Design Changes

    • Improved HF stability using a new CR phase‑advance (lead) network across the EF86 anode load.
    • Revised global feedback network with updated resistor values for better loop‑gain distribution.
    • Updated LF time constants (coupling and decoupling capacitors) to improve low‑frequency phase behaviour under 30 dB feedback.
    • Revised screen‑grid resistors on the EL34s to improve linearity above 15 W.
    • Tighter matching requirements for ECC83 anode‑load resistors to improve balance in the phase splitter.
    • Revised power‑supply design with explicit HT capacity for a pre‑amplifier and FM tuner (≈35 mA).
    • Added HT output sockets and configurable smoothing components (R22, C8) for ancillary units.
    • Updated chassis layout with new hole patterns, tagboard layouts, and wiring routes for improved HF balance.
    • Updated transformer recommendations (Colne, Parmeko, Partridge, etc.) reflecting late‑1950s component availability.mproved distortion performance, reducing THD to <0.05% at 20 W with feedback.
    • Refined construction guidance for stability, grounding, and wiring symmetry.
  • Mentioned in
  • - - Manufacturers Literature (Mullard – Circuits for Audio Amplifiers, 2nd Edition, 1960)
  • Author
  • Model page created by Gary Cowans. See "Data change" for further contributors.

 Collections | Museums | Literature

 Forum