Tube acoustic amplifier 20-35W (RiK 1974/09)

Radioamator i Krótkofalowiec Polski
Year 24, September 1974, No. 9

We receive many letters requesting the identification or sending of a schematic for a 20÷30W tube amplifier. To meet our readers' requests, we are publishing below a description and schematic of such an amplifier, taken from technical literature from about 10 years ago, when it was still a fully modern design.

Editorial team

The schematic of a 35W tube amplifier is shown in the figure. A distinctive feature of the amplifier is its use of four popular EL84 tubes in the output stage. Describing the amplifier's operation is unnecessary; it can be successfully built by more advanced radio amateurs, for whom the schematic itself explains almost everything.

However, let us pay attention to what seems less obvious or is not apparent from the amplifier's schematic diagram.

The use of four EL84 tubes provides the following benefits:

• the anode voltage is relatively low (300V), which facilitates the selection of the mains transformer and electrolytic filter capacitors,
• when operating in class AB1, the control voltage is low and the entire output stage can be driven from a conventional phase-inverting stage.,
• the best operating resistance is low (4000Ω from anode to anode), which simplifies the design of the output transformer and the use of deep negative feedback (up to 25dB). It should be remembered that class AB1 is particularly advantageous because it does not cause distortion problems at low and medium amplitudes, which are sometimes so troublesome in underdeveloped class B amplifiers.

The negative voltage for the output stage tubes is obtained from a separate rectifier circuit; this allows for setting the initial operating point of the tubes (potentiometer R₁₂) and symmetrically adjusting the operation of the push-pull circuit arms (variable potentiometer R₁₃). The power supply can be of any type. With an anode voltage of 250V, output power of up to 20W can be achieved with a maximum current consumption of 180mA. However, it is advisable to use a good 300V, 250mA power supply with low internal resistance to ensure the voltage does not decrease with current changes from approximately 160mA to 230mA. The filter can, of course, be expanded with a choke and an additional electrolytic capacitor. Instead of semiconductor diodes, selenium rectifier circuits can be used. The negative voltage is obtained either from a separate 25V mains transformer winding (5mA load) or by using an additional transformer (e.g., a rewind bell transformer).

Schematic diagram of a 35W tube acoustic amplifier
(the diagram can be enlarged by clicking on it)

The performance of an amplifier depends primarily on the output transformer. This should be a perfectly constructed (sectionally, symmetrically wound, with a sufficiently large core) low-frequency transformer with significant primary inductance and low leakage inductance.

The approximate technical data of the transformer in case of need to meet Hi-Fi requirements are as follows:

• core cross-section: 20÷25cm²,
• inductance of the primary winding: 70÷90H,
• primary winding resistance: not more than 2×100Ω,
• leakage inductance (entire primary to secondary winding): of the order of 10mH,
• primary impedance (anode to anode): 3600÷4000Ω,
• winding ratio adjusted to the expected load.

The resistance of resistor R₆ in the negative feedback loop is given for the secondary winding intended for connection to 8Ω; when making a transformer designed for a 4Ω load only, the value of resistor R₆ should be reduced by 30%.

For smaller requirements, a "simplified" transformer can be used, limiting the frequency response band and reducing the depth of the negative feedback used.

The technical specifications of a well-made Hi-Fi amplifier are as follows:

• output power: 20W at h≤0,4% (40Hz and 1000Hz),
• output power: 35W at h≤3,0% (1000Hz), the power for music (momentary) can reach 40W,
• transmitted frequency band: 20÷50000Hz,
• input sensitivity: about 1.0V (depends on the depth of negative feedback).

Optimal working conditions of the tubes; anode voltage 300V, quiescent value of anode current 35mA (for one tube), maximum value of anode current at the highest continuously output power 46mA, maximum value of the screening grid current at the drive 11mA.

If full power is not used, the anode current value can be slightly reduced to 30÷25mA (one electron tube).

The amplifier schematic and data are from the book: E. Rodenhuis - Hi-Fi Amplifier Circuits, Philips Technical Library, Eindhoven. The rectifier circuit has been modified.

A.W.