This page is presently being constructed.
Historical Background of guitar amps
I hope to complete this page before the end of 2006. When completed it will contain design and service information for Electric guitar amps and also a special section for Acoustic guitars.
Rock music evolved from Blues, the music of the streets. Most musos' of the 50s and 60s were poor and guitar amps made to a budget. Some but not all technical principles of amp designs were well thought out.
Fender and Marshall were the dominant and most copied brands. The powerful amps had 4 output valves in parallel push-pull and gave approx 60 - 100Watts.
The Myth
Most amp manufacturers were conservative from a previous generation with a background of country, jazz and religious music. They were horrified by the anti-religious, drug driven, sex crazed rock musos of the 60s' diving their amps at full power into hard distortion for sustain. When these amps were first designed in the 50s, it was inconceivable they would be used in this way.
The Reaction
Many manufacturers reacted by making warranties void if amps were driven at full power, some threatened to cut off retailers who sold their amps to bands that played 'music of the devil'.
The parody of this historical contradiction has been rewritten, to fit mythical beliefs that brilliant designers created these amps for what 'rock musos' wanted.
The technical limitations of these amps, the use of cheap speakers, characterised their sound, enabling them to distort easily, exactly what rock musos' of the 60s wanted.
The cheap speakers had low weight, high resonant cones, with small voice coils and were very efficient. The speakers responded well to transients, maximum efficiency approx 2KHz, enabling the electric guitar to sound bright and alive. The disadvantage was the high inter-modulation distortion, cluttered chords, the notes dirty, uneven and inconsistent between low and high power.
No allowance was made in the majority of amps for the output valves to be calibrated and balanced. Amps rarely performed at their full capacity, which inadvertently protected the speakers. Also the limited circuitry could not drive the output valves symmetrically.
These problems caused excessive 2nd harmonic distortion. Justified claims were made that this distortion benefited the sound of electric guitars.
The major design problem with most valve guitar amps was the screen grid of the output valves being over-voltaged. This over-voltage is not a problem if the amps are not driven to full power. When the amp is driven at full power, the screen grid draws excessive current, modulating the gain of the tubes.
Sustain becomes erratic and difficult to control. Valve life is shortened to approx 10%.
In theory the screen grid should be supplied from a separate regulated voltage no greater than 400Volts. The more powerful amps had a HT supply voltage of 450-550Volts. To reduce costs the screen grid was connected to the main HT supply through a small amount of unregulated filtering.
The HT electrolytic capacitors were smaller than they should be to maintain a smooth Voltage supply. The sawtooth ripple of the power supply also modulates the gain, imposing dirtiness within the notes. The reason this problem has not been rectified is due to limited technical understanding of the dynamics of guitar amps used at high power, and cost.
A skilled electronic service technician can modify and improve the design and performance of most high powered guitar amps in 2 steps.
Technical Design Modifications
(1) Balance output valves
The mechanical equivalent to balanced pistons in an engine.
The current through each parallel pair must be identical for the amp to deliver full power. Current imbalance through the output transformer magnetizes the core, reducing its performance.
The quiescent current through each valve has to be monitored for calibration. Valve amps responsibly designed should have a separate Bias control for each valve, and meters or LEDs for calibration.
Place 10 ohm resistors between each (pin 8) cathode and ground. Resistors must be 5Watt wire wound, so they cannot vaporise, depositing carbon throughout the amp, if an output valve shorts.
400milli-volts across 10 ohms is 40milli-amperes.
Depending on the HT supply voltage (Eg 500V) this will be 20-Watts quiescent dissipation per tube.
From eight new output valves you will be lucky to match 4.
Be wary of companies marketing expensive relabelled 'matched output valves'.
From eight new output valves you will be lucky to match 4.
If it is not possible to match 4 valves, then select valves so each parallel pair combined, is equal in current, to the other parallel pair. Most amps have an overall negative Bias control. Adjust this control so 400 milli-volts appears across each of the 10-ohm resistors (40 milli-amperes each). Or each parallel pair combined equals 80 milli-amperes.
Measure the amp at full power with a dummy load. Check that the full power current is balanced. If it is not, check the driver valve and the coupling capacitors for leakage. If the amp is very old, check the output transformer for shorted turns with an inductance meter.
(2) Regulation of screen grids. (Pin 4)
Construct a regulator circuit using a Power FET as a source follower. Choose a large 600V 100Watt iso-pac TO3P or similar. Bolt it firmly to the chassis. Double insulate with an extra mica washer (for safety). Double-check the insulation. If the chassis is thin steel, add a flat piece of aluminium between the FET and the chassis. This will help distribute the heat keeping the FET cool.
At full power the FET can dissipate 10 - 20Watts. The drain is connected to the filter (choke) side of the HT supply. The source is connected to the screen grids. Depending on amp design, the screen grids may be connected via the standby switch.
A regulated reference voltage is applied to the gate. There are many ways to achieve this; the simplest is to use two 200V zener diodes in series fed from a bleed resistor. Use 5Watt zeners for reliability. Insure the gate has a 12Volt zener connected to the source for protection. A small 2K2 resistor is put in series with the gate for parasitic stability.
Each screen grid must have its own 2-5 Watt. 200-500 ohm resistor for patristic stability. (Pin 4)
Each control grid must have its own separate 2K2 - 10K ohm resistor for patristic stability. (Pin 6)
Enjoy driving your guitar amp at full power sustain with confidence.
--- to be continued Graphics to be added ----