Replacing the meter lamps in a vintage Pioneer SA-6700 amplifier might seem straightforward, but as discussed in an Audiokarma forum thread, it’s crucial to understand the circuit involved to ensure proper long-term operation. Simply swapping in any lamp can lead to issues, especially if the replacements are significantly out of specification.
The original lamps in the Pioneer SA-6700 (and similar models like the SA-606) are integral to a constant current source circuit. This circuit, detailed in a forum reply, utilizes a zener diode (D27, a 2.7v ZS-027), transistor (Q15), and resistor (R174) to stabilize the current flowing through the meter lamps. This clever design ensures consistent lamp brightness even when the main power supply voltage fluctuates, particularly under varying loads from the power amplifier.
Pioneer SA-6700 Constant Current Circuit Diagram
As explained in the forum discussion, transistor Q15 maintains a constant 2V voltage drop across resistor R174 (15Ω). This is achieved by leveraging the difference between the zener diode voltage (2.7V) and the base-emitter junction voltage of the transistor (approximately 0.7V). This results in a stable current of approximately 133mA (2V / 15Ω) flowing through the lamps. The three 62Ω 3W resistors act as ballasts, dissipating excess power and protecting the transistor from overheating.
Using lamps with different electrical characteristics than the originals can disrupt the delicate balance of this constant current circuit. The circuit is specifically designed to match the original lamp specifications. Deviations can lead to unpredictable behavior and potentially affect the longevity of the components.
However, modern LED technology offers an excellent alternative for Pioneer SA-6700 meter lamp replacement. LEDs are more efficient and generate less heat. Furthermore, by carefully adjusting the component values within the constant current regulator circuit – specifically the zener diode (D27) and/or resistor (R174) – the current can be precisely tailored to match the requirements of the chosen LEDs. This modification allows for a reliable and energy-efficient upgrade to the meter illumination in the Pioneer SA-6700, ensuring consistent brightness and extending the lifespan of the lighting system.
