Amplifier repair

Amplifier repair, amp modification, amp service, complete amplifier rebuilds, all of these services are done right here at Plexi Palace!

Service is where we perform our best!
We service most tube amplifiers including: Marshall, Vox, Hiwatt, Fender, Orange, Gibson and Selmer. We can service your amplifier for a problem, custom modify it for you, or completely rebuild your amp from the ground up. We are experts at getting the "tone" you want!

What goes wrong with Vintage amps?

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.

That would be fine for today's appliances but in 1965 the line voltage was usually 110 volts. This poses a problem because most older Marshall JTM 45 or JMPs have power supply transformers designed for export to a 110-volt line voltage. The difference here is (120/110=1.09) or about 10%. If the normal B+ is 450 volts and you are feeding your amplifier from the wall socket, there will be a 10% increase of the line voltage into the power transformer. This translates into a 10% increase of your amps internal voltages. Instead of 450V B+, your amp is developing 495 B+ volts. What's the problem? The problem is parts (such as capacitors) that have a maximum rating of 450 volts are now getting over voltaged. This over voltage will quickly end the life of a new capacitor, not to mention an original capacitor that is pushing 35 years.

We have a solution!

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.

The output voltage is now adjustable and can be maintained at 110 volts keeping your amplifier at its intended B+ levels. This will not only improve the longevity of your amplifier's components but help prevent premature tube wear, especially when the amp is driven into overload which is commonly done to obtain the unique tone of the amp.

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.

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.

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 bydifferentground points in a high gain amplifier.

Our experienced, professional staff of technicians provide a complete and thorough technical service for your amplifiers. You can count on us to be there when you need us.