Valve Amplifier Single Ended Output Stage Explained

Tube Guitar Amp Circuit Diagram using 12AX7 and 6V6GT tubes.

The Valve Amplifier Single Ended Output Stage Is probably the simplest form of power amplifier. They are usually used for relatively small amplifiers, e.g., less than 15W, and are invariably cathode-biased. Higher powers are obtainable with either very large power valves, or by using multiple valves in parallel, but beyond about 15W it is usual to use the more efficient push-pull system. The circuit diagram below shows a single stage output using a 12AX7 pre-amp valve and a 6V6GT power valve.

The sound produced by a single-ended guitar amplifier is rather unique. Single ended designs can only operate cleanly in Class A, and whereas a push-pull output stage tends to cancel out even harmonic distortion, a single ended one won’t. Since for power amplification it is usual to use pentodes or beam-tetrodes, which produce predominantly odd harmonics, we can expect all possible harmonics and a particularly rich sound from a single ended guitar amplifier.

Importance of Valve Choice in Guitar Amplifier Design.

Here I have given a basic overview of the design process. More complex tutorials are available online.

The principle of operation for the cathode-biased single-ended output stage is somewhat simpler to understand than that of the push-pull amplifier, as a output stage is similar to a typical pentode stage used in pre-amplifiers. Its only real difference is in using a transformer for an anode load, rather than the resistor that you’d see in a pre-amplifier stage. The output power from a SE pentode stage will be roughly half the anode dissipation of the power valve chosen so, if designing an amplifier capable of 10W, a valve capable of 20W will be required. There is a wide range of valves that can be used but the most common for guitar use are the EL34 Tube (approx 25W), 6L6GC Tube (approx 30W), or a pair of parallel EL84 Tube (approx 24W).

Using big bottle valves such as the 6550, KT66, KT88 and KT90 would deliver the power however consume too much at the same time. If an amplifier which will distort at high volume is required a good choice is the 8w to 11w EL34.

The importance of the Speaker Choice

The type of speaker or speaker cabinet that used has an enormous impact on the ultimate sound of the valve guitar amplifier, and contributes to the dynamic response and compression for which valve guitar amplifiers are admired. It is also remarkable how much a highly sensitive speaker can increase the relative volume of a given amp when compared with an insensitive speaker.

Replacement Guitar Amplifier Valves.

We have a full range of replacement guitar amplifier valves and tubes including single valves, matched sets and complete guitar amplifier re-valve kits available for the most popular amplifier types. These can be purchased by using the following links:

Read our Guitar Amplifier Technology Pages

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