Power Supply Circuit for EICO AF-61 Audio Pre-Amplifier

OddMix.com - Technical Note - Power - TN091115 - Karl Nagy

The EICO HF-61 Pre-Amp Amplifier is as fine equipment today as when it was designed in the late sixties. The 12AX7/ECC83 miniature all glass noval dual triodes vacuum tubes were the new and stylish thing at that time, but in spite of their age, they still hold up properly.

The one recurring criticism - that is valid - was the lack of an internal power supply. EICO and many other companies had the philosophy at the time that a pre-amp is seldom used alone. Indeed pre-amps with a very few exceptions are almost always attached to a large power output amplifier that is capable of driving speakers totaling hundreds of watts. Since all of that power is there and available, it stands to reason to tap into the main amplifier's power supply to provide the few miliwatts needed to operate the pre-amp.

Figure 1. EICO AF-61 Pre-Amplifier Power Supply Schematics [6 KB]
Figure 1. EICO AF-61 Pre-Amplifier Power Supply Schematics

The problem is that few of the tube type power amplifiers survived to supply the power for these exceptionally able pre-amplifiers. There is not too much problem to put together the necessary circuit. See the schematics of the power supply Figure 1.

A small transformer with a 12.6 Volt for the tubes filaments, and another secondary winding for the anode voltage that is required. The filament current is 150 mA for each of the three ECC83 dual triodes therefore any transformer capable to provide 5-600 mili-amperes will do fine here. For the anode voltage the maximum catalog values for the ECC83 tube is 250 Volt at around 0.7 mA per triode for a class A amplifier. If all six triodes would be fully loaded with this maximum current the resultant current would still be less than 10 mAmps. Operating the amplifiers with a reduced anode voltage their anode current is even less. A good operating voltage with a one way rectifier as in the schematics is around 180 Volt.

Adding it all up for the power transformer, the total load is about 6 Watts for the heaters and not more than another six for the anode. For the audio purist by using a full wave bridge rectifier assembly the anode voltage would be even smoother at the expense of a few more parts. If a nice old choke can be found, the circuit would be nearly ideal. Since there is plenty of room inside the case of the instrument, all of the power supply circuit can be housed internally. As the photos depict a piece of vector board makes a good possible arrangement for the power supply circuit.

                     Parts list for the EICO AF-61 Pre-Amplifier Power Supply:

                       T1 - Power Transformer 1 - 120 VAC to 12.6 VAc and 120 Vac
                       C1 - Electrolytic Capacitor - 4-10 uF, 250 VDC
                       D1 - Diode Rectifier - 1N4004
                       Octal tube socket - Optional for external circuitry

Because finding a high voltage small transformer is no longer an easy affair, the high voltage can be generated using a single transistor oscillator DC-to-DC converter. Or the line voltage can be used with a proper isolation transformer. Two 12 Volt transformer, or two bell transformer connected back to back can also be used with equal ease. The two transformer method provide for the necessary circuit isolation and the bell or control transformers are readily available as all home heating systems use those control transformers. Unfortunately two transformer cost more than one, so it is worth while to look for one with the required windings.

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