Repairing Tube Amps

Vintage Amp Repair  sdguitaramp@gmail.com

Check Points along the Signal Path 

You are responsible for your own safety. If you do not understand electronics and the dangers do not attempt to do any repairs.

This is a good illustration of checking the signal path of a tube amp. This can be very dangerous because of high voltage so use extreme caution. The amp must be on, connected to a speaker or dummy load, then ether use pop test by touching a test prob to these points or using an oscilloscope with a test tone in the input, usually about 1000 hz. Always start from the back and work your way to the front. 

Before you get started with this, always check the basics such as fuses, speaker hook-up, broken wires at the input and output jacks, you know the obvious. You might be surprised what you might find. And yes I have found blown fuses on customer's amps.

A schematic is very much needed if you can't find it through the obvious. Say you have a reverb problem, one can then check the schematic see the tube and connecting circuitry related to the reverb and possible get to the problem area right away. 12AX7 Tube Pinout

Get use to almost always replacing the tubes first thing. A tube only has so many hours in it and tubes work very hard. It's kind of like changing plugs in your car. Power supply circuits work very hard as well and should be checked early.

Having a schematic and possibly service info can give you the proper voltages that come from your power supply. These voltages will be AC and then DC after being rectified. There is usually a 6.3 AC voltage feeding all the heaters on your tubes, check these as well.

As you can see you will need a meter to check ac and dc voltage, resistance, and capacitance is useful as well. Capacitors are only good, much of the time, for about 10 or 15 years the they tend to change value or get leaky (difficult to check). In most cases its just easier and inexpensive to replace the caps than to check for leaks. Keep fresh capacitors and tubes in your amp. 

Rectifiers

rectification The conversion of alternating current into pulsating direct current by any means other than the use of a motor-generator.

rectifier Abbreviation, rect. An electronic or electro mechanical device that converts alternating current into pulsating direct current.

rectifier tube A two-element electron tube, once commonly used for converting alternating current into pulsating direct current in high-voltage, high-current power supplies.

full-wave rectifier A rectifier that delivers a half cycle of pulsating direct-current (dc) output voltage for each half-cycle of applied alternating current (ac) voltage. The successive output half-cycles have the same polarity.

diode rectifier 1. A diode device that converts alternating current (ac) to pulsating direct current (dc) in a power supply.

    Diode Type Rectifier   

Tube Type Rectifier