A Start-Up Delay For Vintage Amplifiers

By Alexander Rubli K.

Many articles have been devoted here on a HV delay for an amplifier. Applying a 400-volt Jolt to all components when they are cold will only serve to shorten their life span.

I have read dozens of articles and reviewed recommendations on adding a retard circuit via a relay-controlled timer, adding a rectifier tube, or even adding a "standby" switch. All these solutions can work. But what does one do when you own a vintage amp? There simply is no space to add a transformer, a tube socket, or relay. Drilling a hole into the chassis so as to add a standby switch is not an option. Most collectors would not purchase an original amplifier if major modifications has taken place.

My proposed circuit is a small PCB with only two terminals that go in series to the HV.

Attaching the PCB to the amplifier can return to its original state by connecting the cable you unsoldered back where it was. There are no connections to ground, no relay, no switch, and no runs of cables.

The circuit contains a SCR that acts as the switch, a 555 IC timer, and other associated components. Once the SCR switches On, all components on the circuit vanish from the electrical path.

Drop voltage of the SCR is less than 1 volt. The voltage of the associated components is taken from a shunting cascade of zener diodes.

How to install:

CAUTION!!! This project involves the risk of electrocution!!! Due to high voltage, electric shock is dangerous and can be fatal!!! Always unplug the unit from the AC socket. Fully discharge the electrolytic capacitors with a 10 Kohm resistor to the ground. Wait a few seconds before trying to handle the amplifier. Always check the voltage before soldering or un-soldering any cable.

How to connect:

Locate the positive lead of the main rectifier and the first electrolytic capacitor. You will notice several other connections that lead to the amp; one to each OPT, and a resistor that goes to the next HV stage. The connections to the PCB go from one side to the junction of the positive sides of the rectifier and the first electrolytic Capacitor; the other connection goes from the remaining cables that were connected to that point, observe fig 2, notice the direction of the arrow on the PCB.

Calibrating The Voltage Reduction Circuit:

The circuit is intended for use with an HV of 350- to 400-volt. If you have an amplifier with lower HV, try bypassing ZD2. Voltage across R1 should be 45 volts or more.

The LED:

When the amplifier is powered on, the LED will turn on until the timer fires up and takes the SCR to the "on" state. After this occurs, the LED will be off.

The timing circuit will fire up the SCR in about 45 seconds if the tubes are cold. If the tubes are hot, the time is reduced about 10 seconds.

Note: The necessary current for this circuit originates primarily from the idle current in the output tubes. Should you be troubleshooting the amp, and the tubes are removed, the SCR will never activate.

PCB: Measures 3" x 1 1/8", shown here at 200%.
NOTE, the revision "B" board which is the one available, measures
63 x 31 mm

See article in PDF formatThe completely assembled PCB is available from the author, Alexander Rubli at

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