Tech's Files: More Vacuum Tube Tales

Sep 1, 2009 12:00 PM, By Eddie Ciletti

MYSTERIES OF TWO VOX AC30S

Polls


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However, designers have not been consistent about how standby switches are wired. On this recent AC30, the switch is between the HV center-tap and ground. Other versions relocate the switch after the rectifier and filter capacitors. If the rectifier tube is already warmed up, a switch at the center tap sees the initial stress of charging a capacitor quickly. The post-rectifier standby switch lets the filter capacitors charge up to a higher-than-normal voltage, because without the power amp, there is no load. In this application, the voltage must be within 70 percent of the cap's voltage rating or the overcharging could potentially shorten the cap's life. When in standby, one solution is to simulate the power amp load with resistors. A third option is rewiring the standby switch in series with the rectifier's filament so that the HV slowly turns on (or off). This is kinder to the tubes, transformer and capacitors.

One Last Check

The AC30 is hardly technician-friendly: It's not easy to audition tubes or gain access to the electronics without pulling the entire chassis out of the cab. So before my assistant reloaded the chassis into the cabinet, we did one last test, only to hear a strange buzz from the power transformer, as if it were intermittently shorted.

After the amp sat idle for a week or two, the lack of exercise and self-cleaning increased the switch's contact resistance. A new switch fixed the final problem and I disassembled the old ones to see why they were failing. On the inside, both switches were Double-Pole/Double-Throw (DPDT) types with six contacts, but on the outside, only four contacts are “available,” making this strictly an on/off switch rather than the optional on/on. Inside the switch are two brass “paddles,” one for each pole (circuit). Riveted to the brass are two silver-ish “buttons” that mate with either of two internal contacts (one unused).

In this case, the “common” (wiper) connection at the pivot point was a poor-quality brass (an amalgam of copper, zinc and a healthy dose of impurities). The lack of quality materials and an environmental seal caused the brass to tarnish. This added resistance at the pivot point, which worsened each time the switch was thrown with the power on. If you've ever accidentally plugged something in with the power switch on, the resulting spark is visual proof that circuitry can be “reactive,” the reflected inertia of all the stuff being driven. Inside a switch, sparks from high voltages or currents can become a self-oscillating arc, degrading the switch contacts by depositing carbon until there is more resistance than conductance.

Normally, spring-loaded contacts mate with enough force/impact to make a solid, reliable connection. And if the internal contact pressure/impact or wiping action is sufficient, the switch self-cleans its contacts. By design, switch contacts should have low resistance, which means using (or plating with) high-quality materials (nickel), precious metals (silver) and gold for audio applications.


Eddie thanks Christian Groves and Tom Morrongiello for their Voxes. Visit Eddie at www.tangible-technology.com.

AUDIO SCIENCE

The most basic passive electrical components, switches, literally make or break a circuit. Mechanical switches come in many forms — including rotary, toggle, slide, lever or pushbutton types — but all are either continuous (like a light switch) or momentary (like a doorbell) styles. These also come in “normally on” or “normally off” versions, depending on whether engaging the switch makes or interrupts the circuit flow. The simplest, most common switch is a SPST (Single-Pole/Single-Throw) type, with one set of contacts that are connected (“on”) or disconnected (“off”). Other variants include the SPDT (Single-Pole/Double-Throw) with a single contact that can connect to two different contacts; and DPDT (Double-Pole/Double-Throw), where two isolated contacts can be routed in two different ways. Some rotary switches are ganged, providing for complex switching — such as SP8T, 4P12T, etc. — from several stacked “wafers” (layers) of contacts, all moved from a single knob.






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