| Most vintage tube amplifiers suffer from problems that are related with tubes and capacitors. Occasionally old resistors will drift from their original values causing things to get very unbalanced. One of the underlying problems that is widely over looked is the actual line voltage level out of the wall socket. The line voltage test in the picture to the right shows 120 volts AC sitting at the AC plug in our shop. |
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Make sure your amp is plugged into the correct line voltage. One method to accomplish this task is to use a Variable AC transformer or Variac for short. The variac is adjustable and can be set to a desired voltage lower then the voltage it is tied to. Variac transformers allow the user to change the line voltage feeding the amplifier. The variac is plugged into the wall socket and the amplifier is plugged into the variac. |
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A simpler way to regulate the voltage going into your amplifier is an Autotransformer. The autotransformer is set to provide a selected output voltage with a given input. The transformer to the left is a step up transformer. Again the input is designed for 110 volts and the output will be 220 volts. If 120 volts is fed into the transformer 240 volts comes out. The solution? Set the voltage selector on the amplifier to 240 instead of 220. Most old Marshalls have 240 taps. |
Certainly original parts are the
best! When working with vintage amps it is important to try and
preserve the original components and make use of the original parts. We
are committed to do the best job as well as provide the best quality
parts to service our customers amps. |
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A complete rebuild of a 1967
Plexi Marshall (pictured here) required tedious wiring of a NOS board
into the chassis. The technicians use correct colored wire to match
what was used originally. Selected Teflon wire is provided with all
service work. Teflon coated wire will not burn and is a better
replacement then plastic coated wire. |
When original parts are not
available, specialty companies such as Mercury Magnetics make an exact
clone of the Marshall Plexi mains and output transformers. For this
particular amplifier we fabricated and constructed the power supply
filter board, as was the original design of this year Plexi. A single
filter capacitor is found topside and is seen below the mains
transformer. |
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Severe corrosion is common place with older British amplifiers. The chassis to the left was completely stripped and cleaned of all corrosion. Taking all the grounds to one location in the amp rather then different points within the chassis provided an improved ground system. This will effectively eliminate ground loops and signal grounding problems. The ground bus that is located on the filter capacitor board will greatly reduce hum that is caused by different ground points in a high gain amplifier. |
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| plexi@vintageamps.com PO Box 1089, Apple Valley, CA 92307 760-961-8324 |
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