ECC88-Based Hampton Tube Preamplifier

Overview

The objective of building this circuit is to create a preamplifier based on the Hampton standard with the utilization of vacuum tubes for use with moving coil cartridge.

Terminology

• ECC88 – a VHF double triode with a 20 mm diameter thin glass tube envelope having separate cathodes designed for service in the front ends of VHF radio receivers where one triode acts as the local oscillator while the other as the frequency changer or mixer
• 6DJ8 – a pair of screened triodes with a 20 mm diameter thin glass tube envelope that is obtained from a Y amplifier of a Tektronix oscilloscope and has a ruggedly constructed design used for wideband amplification due to its low impedance and low voltage
• TL071 – a low noise JFET input operational amplifier with features such as common-mode input voltage range, high slew rate, operation without latch up, compensated internal frequency, high input impedance at the JFET input stage, low noise, low total harmonic distortion, protected from output short circuit, low input bias and offset currents, wide common-mode and differential voltage ranges, and low power consumption
• LM317 – an adjustable 3-terminal positive voltage regulator capable of supplying in excess of 1.5A over an output voltage range of 1.2V to 37V and requires only two external resistors to set the output voltage due to its internal current limiting, thermal shutdown and safe area compensation, making it essentially blow-out proof

Circuit Explanation

The design of a moving coil cartridge as a small electromagnetic generator contains the magnet and coils in reverse where the coils have been attached to the stylus while moving within the region of a permanent magnet. The coils were light, tiny, and made from very fine wire bur has an improved ability of tracking of the cartridge with greater fidelity and extended frequency response. They are preferred by some audiophiles even though they are generally expensive and extremely small precision instruments.

Since the moving coil cartridges are very small, the signals generated are also small in terms of its level. They are more prone to induced hum and noise because the signal is only a few hundred microvolts. This is why the above circuit was built to prepare the signal from the heads of moving coil for further processing or amplification. It should be of good quality circuit with extremely low noise inputs which is made possible using two ECC88 or 6DJ8 vacuum tubes in parallel to reduce any noise.

The circuit operates simultaneously with an existing phono circuit which is used during the manufacturing and playback used on vinyl records for frequency response and highest quality restoration of signals. The impedance of the circuit is adjusted by the use of switches S1 & S2 for the cost of the head. The supply of the circuit should be completely stable in which a step up transformer is preferred mostly.

Part List

Application

Some of today’s preamplifiers are still using the services of vacuum tubes because of their ability to deliver quality audio and unique tones when used with amplifiers. The vacuum tube preamplifier is being used together with amplifier for musical instruments with tone control circuits. In this circuit, it is intended for moving coil cartridge applications for the ability to amplify and equalize the signals with high quality.

Source:users.otenet.gr/~athsam/hampton_tube_preamplifier_for_moving_coil.htm