How to get the correct voltage for your vintage amplifier:
The voltage at your wall outlet has increased gradually over the years. Thomas Alva Edison chose 110 Volts as the standard because that voltage gave a bright output from his light bulb without burning out the filament. After WWII, the voltage standard increased to 115 – 117 VAC. In the late 1970s the standard was closer to 120 VAC. The voltage at my house today stays at about 122 VAC.
When electrical engineers design an electronic circuit, they start by assuming a Line Voltage. In the mid-sixties, they would usually choose 117 VAC. This Line Voltage established the design voltage and current for the circuit and all its components.
Unfortunately, a Fender Deluxe Reverb of 1966 was designed for 117 VAC, but today the amp is seeing a line voltage of closer to 125 VAC. The modern voltage is 7% higher than specified. Which means that all of the voltages in the amp’s circuit are too high – and since Leo Fender tended to specify components that were right at the limit of the 1960 voltage, running an old amplifier at modern voltages can lead to component failure.
Particularly with 5E3 and later Fender Deluxe amps, modern wall voltages drive the 6V6 plates at well over 380 Volts – this leads to short tube life and fried filter caps.
A Solution:
I have made a number of “Vintage Voltage” devices that are basically a center-tapped 12 VAC bucking transformer and some filtering. I included alarms which sound if the ground is missing, or the wall polarity is reversed, both dangerous situations. The box is easy to put together by someone with the necessary skill.
This allows the user to select ~110VAC, ~117VAC, or line voltage (~125VAC). It can be used as a “Brown Sound” box, although that is not its intent. The intent is to supply a vintage amplifier with its design voltage; in the 40s – 50s it was usually 110 VAC, and in the 60s-70s it was more often 115 – 117VAC. Modern line voltage is 120 – 125 VAC.
The simplest way to reduce your line voltage to something more compatible for your vintage amp is to use a bucking transformer. This is a small transformer that subtracts a set amount of Volts from whatever your line voltage is. For example, if your line voltage is 122 VAC, a 6-Volt bucking transformer will provide your amp with about 116 VAC – this is just right for blackface era amps.
Filament transformers are readily available (6.3 VAC) and are small enough to fit into a medium-sized enclosure. A simple bucking circuit is shown below:
Employing a center-tapped 12 VAC transformer gives the ability to have two voltage-dropping levels – 6 and 12 volts, as in the following circuit. This design incorporates a voltage meter so you can know your true output voltage.
This design features a 4-position switch. The user can select an output voltage of 0 (off), 113 VAC, 119 VAC, or Line voltage (125 VAC, in this example).
It is fairly straightforward to add valuable options to this design. The multi-function Vintage Voltage device (schematic below) incorporates surge protection, filtering, and an audible and visual alarm in the event of a missing ground or reversed polarity. Both missing ground and reversed polarity are not uncommonly found, especially at old venues which might have DIY or not-to-code wiring.
Reversed polarity is particularly dangerous. It can lead to serious injury or death in certain circumstances. Reversed polarity has been the cause of death in a number of cases with musicians. For this reason, the Vintage Voltage Device does not allow you to turn off the alarm if it detects an unsafe condition.
Due to current present on the neutral leg of the power entry, the Vintage Voltage Device cannot differentiate between a missing ground and reversed polarity; therefore any alarm will be one or the other of these conditions. If the Vintage Voltage Device is behind an isolation transformer, then the following design can discriminate between a missing ground and reversed polarity:
PARTS LIST AND NOTES:
