EA ELECTRONICS - Amplifier Article #1

Click free Vibrato Switching on Deluxe Reverb

Warning! Tube amps operate with lethal voltages. If you are not absolutely certain about what you are doing leave this kind work to qualified technicians.

My 1968 Deluxe Reverb had a problem when the Vibrato was switched on, a loud click noise came through the speaker. I spent a lot of time analyzing the problem and comparing the schematics for the Deluxe with the schematics for my Vibrolux re-issue which does not have the problem. 

One of the things I discovered is the click only happens when the reverb was on. This led me to investigate the power supply. If the oscillator causes a voltage transient in the supply, this can be amplified by the high gain pre-amp stages. The Vibrolux power supply has some extra filtering for the oscillator. Rather than add the extra filtering to the deluxe, I took another approach. I grounded the the point in the oscillator where the vibrato switch is normally connected (this means the oscillator is running at all times).  The vibrato can also be switched on and off by adding a switch between the light dependant resistor (LDR) and ground, which is what I did.

But that wasn't enough to fix the problem. I seems that the wiring from the output of the reverb tank to the foot switch is acting as an antenna and picking up a transient when the vibrato is switched on. On the Vibrolux the reverb switch is not connected to the output of the tank, it is connected to the cathode side of V3B near the reverb control.

Rather than redesign these parts of the Deluxe I designed the circuit below which could be added with minimum modification to the existing circuitry. This circuit uses 2 H11F1 opto-isolators to switch on and off the reverb and vibrato. There are only 6 components in the circuit and it all fits nicely on a printed circuit board (PCB) about 1 square inch in size.

Here is the PCB I made mounted at the end of the chassis near V1. Note the 2 bright green wires going to the V1 tube socket, these wires tap into the 6 volt filament wiring to supply power to the PCB.

Location "A" below is the vibrato modulator which has a neon bulb and a light dependant resistor inside a piece of black heat-shrink tubing. The dotted line just to the left of "A" marks the location of the wire used to connect the LDR to ground. I removed this wire. The white wire above the A runs over to one of the optos on the small PCB I added. 

Location "B" is part of the vibrato oscillator. I removed the wire which used to go to the footswitch connector on the rear panel and simply shorted location "B" to ground as shown by the diagonal white line.

Disconnect the wire running from the reverb out to the reverb footswitch and connect the shielded cable from the reverb opto to the reverb out connector as shown below. This is the most noise sensitive part of the mod so I am using shielded cable. 

The final step is to connect the wires running from the LEDs in the optos to the footswitch connectors. These lines are immune to noise so wire routing is not important. 

Warning! Tube amps operate with lethal voltages. If you are not absolutely certain about what you are doing leave this kind work to qualified technicians.

EA Electronics

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