Technical Discussion of Old and New Features

Tone Cut control

As used in AC30s since 1959 and AC15s before that, this control cuts high frequencies. It is placed in the power amp section after the phase inverter and before the output tubes. It works like this: The preamp signal goes into the phase inverter. The output of the phase inverter has two signals one of which is 180 degrees out of phase with the other. When signals that are out of phase are combined equally, they cancel each other out. The Tone Cut control works by combining only high frequency signals from one side of the phase inverter with the other. The components included to achieve this are a potentiometer (pot) and a capacitor. The pot controls the amount of signal while the capacitor value sets the cutoff frequency as capacitors allow high frequencies through and block low frequencies. The larger the cap, the lower the frequency. The value of 0.0047 has been used in most AC30 guitar amps (as opposed to AC30 bass amps – yes they did exist!) since 1959 and is most appropriate. You can think of the Tone Cut control as a high-pass filter or low-cut filter.

Master Volume control

There are many different Master Volume circuits. The one used in the AC30CC is the same as used in the modern AC15 and the AC30HW. It is post phase inverter. The advantage of a post phase inverter MV is that it is part of the power amp. A pre phase inverter MV is after the preamp but not the power amp.

MVs are usually used to get distortion at low volumes. Distortion in traditional tube amps (non MV) is created in all sections of the amp including the preamp, power amp (phase inverter and power tubes), transformer and speaker. Using a post phase inverter MV which is after the phase inverter tube, but before the power amp tubes, give you more authentic traditional tube amp distortion as compared to pre phase inverter MVs.

It works by combining the signal from one side of the phase inverter with the out-of-phase other side of the phase inverter.

Channel linking

Guess what? Not only can you combine the AC30CCs two channels, it can be turned into a channel switching amp! How do you achieve this? All you need is an external A/B box. The benefit is that the user can switch between two different AC30 tonalities. The Link switch has to be set to the up (non linked) position. An A/B/Y box will allow you to select and combine channels. 

Reverb

Steve G. designed a completely new circuit which, as you have heard, sounds amazing. Here is a short version of an explanation from a web article written by our own Nick B.

In the analog domain there are basically two types of spring reverb drive and recovery circuitry used in guitar amps – valve and solid-state. Both have their pros and cons. A valve circuit has an inherent soft-clipping characteristic that’s rich in even-order harmonics and is pleasing in terms of both sound and feel. As valve circuits involve a transformer it is relatively expensive. Other drawbacks of a valve driven circuit are that the valve creates heat and eventually wears out with use. Also, particularly with high-gain circuits, there’s a lot of radiation from the wiring and transformer which can cause instability and noise. The solid-state option is cheaper but has usually less headroom and a harsh clipping characteristic in the drive circuit. Because of these reasons, solid-state reverb drive and recovery circuits were chosen and Steve decided to develop a brand-new circuit to get over the drawbacks normally associated with solid state designs.

“Although some of the solid state reverb circuits already out there are pretty good, it has always been my impression that no-one, and I include myself here, has taken them far enough in getting a really good sound or maximum benefit out of the reverb. I’ve managed to find a special MOSFET that works in the same mode that a valve does and, even though its gain characteristics are slightly different, you could unplug a valve from an amp’s preamp circuit, solder this MOSFET in its place and the amp would work without you having to change or add any other components. And, being a MOSFET, it has the even order harmonics similar to those you get from a valve. The MOSFET is being used in both the drive and recovery stages of the Custom Classic’s reverb to emulate the soft clipping that naturally occurs in a valve circuit.

In addition, I’ve used five special op-amps in parallel and as they are 60 volt devices as opposed to the regular 30 volt ones they give a lot of headroom and also provide a great deal of current/drive capability without saturation. They are wired in a circuit configuration that emulates the behavior and characteristics of a transformer.”

Dwell switch

The dwell switch allows you to select Low or High reverb drive settings. In effect, it is a “post fader effects send level switch” (in this case the effect is reverb). When using spring reverb you need to be driving the input as hard as possible in order to maximize the signal-to-noise ratio while getting good reverb depth. If the circuit is not driving the springs hard enough then you’ve got to add more gain to the recovery stage which will add noise and possibly cause instability and feedback.

Most guitar amp reverb units (especially those guitar amps capable of high gain settings) have drawbacks because you have to compromise how much drive you put it in verses how clean or overdriven the amp has to perform. Obviously, if the preamp’s running clean it is not going to be driving a lot of signal into the reverb. Conversely, if the preamp is turned up then there’s  going to be a much higher signal which then starts saturating the reverb and you get an undesirable ‘crashiness’.

If the AC30CC is set to a clean sound and you want a lot of reverb, you can increase the level of drive into the reverb by selecting High. If you want to run the preamp hotter, lower the reverb drive level by selecting Low.

Normal channel Brilliance switch

The Normal channel gives you two distinct, classic AC30 sounds. With the brilliance switch “off” this channel is the classic Normal AC30 sound. This is the channel that Brian May uses along with his booster pedal. With the Brilliance switch engaged, this channel becomes almost identical to a vintage “Treble” AC30’s Normal channel. This is the sound of the early Beatles. Both channels are eminently usable signature AC30 sounds.

Custom/Standard Tone switch

This mini-toggle switch offers you two passive EQ options on the Top Boost channel. Standard gives you the classic, highly interactive Vox AC30TB tone stack while Custom reconfigures the components into something closer to the more common guitar amp tone stack as used by most designers. FYI, the Custom tone stack option is what is used on the Top Boost channels of both the AC30HW and the modern AC15. When both tone controls are set at 12 o’clock the difference between the two EQ settings are minimal. Once either or both controls are turned up the difference becomes more obvious. As the Bass control is turned clockwise, there is a mid scoop (cut) in the Standard EQ setting that is far more pronounced than in the Custom setting. In Custom, adjusting either control has less affect on the other. This difference is achieved simply by switching the bottom leg of the bass to ground. The Custom setting is recommended for higher gain sounds because of the consistent mid response.

Cathode bias resistor value switch (Output Bias)

The cathode resistor value sets the bias of the EL84 power tubes. The lower the resistor value, the lower the bias voltage and the higher the current draw. Conversely, the higher the resistor value, the higher the bias voltage and the lower the current draw. The Warm setting is the 82 ohms, the same setting as the original 4 input AC30s from 1959-1960. The Hot setting is 50 ohms, a similar value to AC30s from 1960-2004. At 50 ohms, the amp will produce about 33 watts clean and at 82 ohms the amp will produce about 22 watts clean and the tubes will last longer.

Smoothing capacitor value switch

The power supply works by converting the 120 volt AC wall supply into various the AC and DC voltages that make the amp operate. The power transformer converts the 120 volts AC to various AC voltages some of which are then converted to DC which is need to make the pre and power amp tubes operate properly. Rectification is needed to convert AC to DC. Vintage and modern AC30s use a high quality full-wave tube rectifier with an input choke pi-filter and 16uf smoothing capacitors both before and after the choke. This low value of 16uf was enough to get rid of most of the hum and keep ghost notes to an acceptable level while keeping the feel nice and loose (sag). Because of the nature of tube amp manufacturing, in some AC30s hum and ghosting was higher than others and a somewhat objectionable. In the AC30HW we used 50uf capacitors before and after the choke. The potential hum and ghosting problems were eliminated but the feel became a little tighter (less sag). Not bad, but not the same as a vintage AC30.

In the AC30CC we give the user both options. The low value (Vintage) is 22uf which is slightly higher but operationally virtually identical to a vintage AC30 or AC30TB. The high value (Modern) is 44uf.

The 22uf capacitors are always in the circuit. To achieve 44uf, additional 22uf capacitors are switched in both before and after the choke through 10k resistors which eliminate any potential popping when switched.