Standby Switches

Misunderstood and Hazardous

The design of the earliest guitar amps was not so much "beginning with a clean slate" as it was an application of hifi circuits for use with pickup-equipped guitars and basses. Portable hifi gear was not new - all that was novel was the intended use. Clean sound reinforcement was still the mandate.

Tube manufacturers "of old" provided application circuits for their products, and included advice for designing within the safe limits of the tube. It is generally advised that the cathode of the tube be at its operating temperature prior to application of the plate voltage, to prevent "cathode stripping". Cathode stripping shortens the life of the tube and reduces the amount of power that the tube can control. For very expensive and high-powered tubes operating with extremely high voltages >1kV, separate switching for the heater and the plate supply is recommended. The plate supply switch is called a "standby" switch, as the tubes are standing by with warmed cathodes, waiting for B+ to appear.

Typical Standby SwitchMany tubes have controlled warm-up times for their heaters, minimizing heater surge currents and allowing plate voltage to be present before the cathode emits electrons. This appears to contradict the concern about cathode stripping - but here is where the application makes a difference. Plate voltages for tubes equipped with controlled heater warm-up are always <1kV, and most often <500V. The danger of cathode stripping is insignificant, so the need for delayed turn-on of the plate supply is unnecessary.

Guitar amp power tubes are inexpensive compared to the cost of, say, a transmitting tube for a radio station. Guitar amps operate at 500V or less - most at under 100W. Low cost, low voltage, and low power: three reasons that guitar amps should not require standby switches. So why do many have them?

Answer: Fender essentially misinterpreted the requirements, and everyone else copied Fender. Leo tended not to put anything into the circuit that he felt was unnecessary - but he came from a repair background where a standby switch is a service convenience. There was no other logical reason for its presence: proper use of the standby is lost on most players. Many modern safety agencies even insist that the standby be removed, since the standby's removal of the plate voltage reduces the idle power dissipation and quiets the amp, and in such a state, it is easy for the user to forget that the amp is still on, which produces a fire hazard. Fender's rationale about the service convenience is also flawed: a B+ fuse works just as well, as does any easily unsoldered connection in the plate wiring. Although cathode life is theoretically extended - if so, by an infinitesmal amount - heater life is not extended.

Apart from misinderstanding when to use a standby switch, Fender also chose a poor implementation. Their switch is in the DC path, between the main filter caps and the feed to the output transformer and the rest of the plate supply. This leaves the main filters charged during standby, producing no useful advantage, especially for servicing, as this leaves high DC voltage present in the circuit. Mechanical switches arc when DC is present, so switch life is shortened in this application. Despite the fixed bias voltage also being present at all times, there is often a surge of current through the tube upon transition to the operating mode. The situation is improved quite a lot if the standby is used only to switch the screen circuit of the output tubes. Tube dissipation can still go to zero except for the heater being powered, but the current through the switch is reduced by 10-100 times. Other manufacturers switch the plate winding AC, which does lengthen the switch life, as AC switching is what most switches are designed for.

A few older amp models had standbys that simply muted the audio signal. There are several ways to go about this, and it does not alter the idle heat of the amp. Such circuits allow the use of a physically small switch that is easier to fit into the front panel. However, the quieted amp still presents the same fire hazard described above.

How to Use a Standby If You Have One

The best way to use a standby switch is to leave it on all the time - that is, leave it in the operating position. If you feel compelled to manipulate it since it's there, flip it when you flip the power switch.

Never leave the amp in standby between sets. Just turn down the volume control.

Rewire the standby to control just the screen circuit. Switch stress will be reduced by 10-100 times, and the output tubes will run at zero plate current. Or, rewire the switch as an audio mute.

In a Power Scaled amp, the standby should be rewired into the screen circuit after the Power Scale regulator. This is especially important in amps with a ground-lift standby in the centertap of the plate winding.

There are alternate preferred wirings for standby switches, as shown in The Ultimate Tone (TUT) series: all are safely implemented using a subminiature switch such as those in the C&K 7000-series.

The Safest Standby Switch

 Bypass the standby switch internally so that it does nothing.


~by Kevin O'Connor

About London Power

London Power was founded in 1990 by Kevin O'Connor, after two decades of research into innovative audio amplification techniques. Kevin is an audio designer, author and speaker, known for his proprietary methodology called Power Scaling, and for his eleven books on audio subjects. Whether you seek amps, kits or information, you'll find useful answers here on not just the "how" but the "why" of audio amplifier design. Please enjoy the information on this site, and don't hesitate to contact us with questions.

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