Power Amp

With such a flexible preamplifier design, choosing a power amplifier design for the Revelation was a challenging task. The output power of the amp needed to be high enough to provide sufficient headroom to keep the modern high gain sounds tight at gigging levels, but not so high that the tones would be sterile and lifeless. By the same token, we also recognised the renewed interest in lower wattage, overdriven power amp designs, particularly for the more 'vintage' tones, where the overdrive duties are shared between the preamp and the power amp.

So after much experimenting with various designs, we settled on a switchable power amplifier design, utilising a pair of EL34s. The EL34's were chosen for their great focused rock tones when overdriven. When the power amp is not overdriven (which would be the case in most scenarios with this style of amplifier), the negative feedback in the amplifier helps to regulate the frequency response of the power amp, to produce a relatively 'controlled' sound where (dare we say it!) the inherent characteristics of the power tube are negated to a large extent.

Output Power and Tube Configuration

The first interesting feature of the Revelation's power amp section is the variable output power switch, located on the rear of the amplifier. By varying the internal high voltage applied to the tubes, it is possible to vary the output power of the amplifier between 100%, 60% and 30%. This reduction in the high voltage also applied to the preamp tubes, so these settings not only affect the power amp section, but also the preamp. The lower the output power, the quicker the preamp tubes clip, and the more compressed the tone.

For each output power mode, there is an independent bias controls, so your amp tech can optimise the bias for each power level.

A further reduction of the output power is possible using the Triode/Pentode switch. This not only affects the output power, but also the tone of the amplifier. The transfer characteristics of triodes is very different to pentodes, and the resulting tone is much 'softer'. The open loop gain of the power amplifier is also reduced in triode mode, making the power amp response much looser, and the presence and depth controls less effective. This is a great 'vintage' tone.

The Revelation ships with EL34s, but it is possible to replace the output tubes to almost any octal base type, even 6V6. However, For the 6V6, the 100% output power mode cannot be used as the internal high voltage of the amplifier is well beyond the maximum rating of this valve. Also, the speaker load should be doubled. i.e. if your speaker cab is 16 ohms, connect this to the 8 Ohm output

The table below provides a rough guide of the output power produced by the 3 most common types of octal output tubes used in guitar amps. Note that this is a guide only.

Presence, Depth and Feedback

The Revelation features Presence ad Depth controls to adjust the high and low frequency negative feedback of the power amplifier. In addition to this, there is a 3 position feedback switch which controls the amount of negative feedback

When the Feedback Level (F.B) is set to high, a large signal is fed back into the phase inverter. This has three effects. Firstly, it makes the response of the power amp very tight and controlled. Secondly, it makes presence and depth controls extremely active, so that their range is greatly increased. This is great if you intend on using the presence and depth controls for more radical EQing. Thirdly, it reduces the overall gain of the power amplifier, so you will hear a drop in volume. It then follows that when the F.B. is set to low, the response of the power amp will get a little bit looser, the presence and depth controls will have a reduced effect, and the output volume of the amplifier will increase slightly. This setting is the best setting for the vast majority of situations, and should be your first port of call.

Finally, with the Revelation, you have the ability to completely disable power amp feedback. This is done with the F.B. switch in the middle (off) position. In this setting, the power amplifier has no negative feedback, and so the presence and depth controls have no effects. In this mode, the inherent character of different types (and brands) of power tubes becomes very prominent, as well as the open-loop response of the output transformer. The sound is loud, bold and raw. It’s a great compliment to Channel 4’s Mid and High gain modes.

Return to "In Depth"