Before launching the OX project we gained a wealth of experience in the design, modification and repair of a wide range of equipment. Time and time again we asked ourselves the question "Wouldn't this design be better if....?" To which the answer was most often "Yes, but it would be too expensive."

Right from the start with the OX amplifier concept we decided to completely set aside this approach and put two years into designing and building an amplifier without constraints. We wanted to incorporate all our best ideas and build an amplifier we could be really passionate about. We didn't know what the final cost would be, we just knew it would truly be the ultimate amplifier. The amplifiers we build today are the culmination of what turned out to be quite a bit longer than the original two years development we'd planned. Along the way we never compromised though. We remained absolutely true to our original objective.

THE PREAMP SECTION

Three channels
The clean channel is not just clean! It is full of rich valve induced overtones. The middle or crunch channel offers traditional master volume valve preamp sounds and the high gain channel has a more modern, high gain sound. Bags of thick sound and fantastic sustain. The preamp uses ECC83 valves in a very efficient configuration.

Definition control
The definition switch mixes a small amount of clean sound into the high gain channel. This improves the definition of each note or chord whilst still sounding incredibly overdriven. This means you can play huge power chords without the sound just going mushy and indistinguishable.

Alesis reverb
To avoid many of the problems associated with spring line units in very high power amplifiers, such as microphonics, induced feedback and hum, our amplifiers are fitted with a well respected Alesis solid state reverb. The reverb sound is mixed into the preamp so that only a small portion of the audio signal passes through the processor (unlike rack effects where the whole signal does).

Active tone controls
After a great deal of research and testing we opted for active tone controls. We have created a circuit which complements the natural spectrum of a guitar pickup but with the added benefit of a much broader sweep for each control than conventional passive designs. Each channel has its own Bass, Mid, Treble and Presence controls.

Channel mixing capability
Not only is it possible to change between the three channels from the footswitch, but channels can be combined, any two at once or all three. This means it is possible to have a huge range of sounds available, all controlled from the footswitch.

Reduced microphonics
Vibration transmitted from the speaker cabinet to the components in the amplifier can have a major detrimental effect on the sound. This is particularly true in the early stages of amplification. Our amplifiers are very rigid and heavy and are fitted with special anti-vibration feet. All valves are mounted in gold plated ceramic chassis sockets and all the components in the early stages, including the control potentiometers are encapsulated in a dense epoxy resin. This virtually eliminates flexing of the components and thus prevents unwanted microphonics. It also aids heat dissipation and drastically increases the life of the circuitry.

Crackle free controls
The carbon track potentiometers used in most amplifiers degrade with time. This results in crackle and even in some cases produces dead spots. All our potentiometers have high quality conductive plastic or cermet tracks which, whilst a lot more expensive, do not suffer from these problems.

THE POWER AMP SECTION

Unique two stage cascaded power amp
The first stage comprises a pair of EL84s in a self biasing class A push-pull configuration. This stage generates fantastic tone whether the amp is run clean, bluesy or flat out.The second stage uses two (G120 model) or six (G400 model) KT88s which deliver masses of power over a very wide frequency spectrum.

Individual bias controls and current meters
On the back panel there are six separate bias controls and six digital meters (two on a G120), one for each power amp valve. When you buy a matched set of valves they are selected to have similar plate dissipation characteristics, however, these change throughout the life of the valve. Within a couple of weeks the valves are no longer matched. With an OX amp the valves do not need to be matched. They can be matched in the amp and then readjusted at regular intervals throughout their lives.

Harmonic switch
Changes the character of the power amp by increasing the ratio of even to odd harmonics giving a smoother, less abrasive sound.

Triode switch
The power amp valves can be switched between pentode and triode mode. This reduces the power, and again gives a uniquely different character to the sound which is more mellow with less headroom.

Bass boost switch
This is not just a switch to alter the character of the preamp tone controls. It is an integral part of the power amp and drops the lower cutoff frequency by about 15Hz.

Massive transformers
Both the power transformer and the output transformer are huge. Most valve amplifier transformers limit the bass frequencies because they just don't have enough iron in them. This means that the flux induced tends to saturate and limit the power which makes it all the way from valve plates to the speakers. We use M6 grain oriented steel in thin laminations which increases this low frequency efficiency very significantly.

Output transformer protection
The most common cause of major damage to valve amplifiers is running them either without, or with an incorrect load. This can be because a cable has become damaged or even in some cases because a speaker fails. All our power amps monitor the speaker load and will revert to standby mode if a problem occurs. This protects the output transformer and valves before they are damaged.

Point to point wiring and hi-spec components
We only use high grade audio components which are rated well beyond the use to which we put them. These components are point to point wired by hand using traditional tag strips mounted on a unique star connected grounding frame.

SPEAKER CABINETS

Unique construction
Our 4 x 12 cabinets have a unique ultra-rigid design. The more rigid a cabinet is, the better it sounds. In order to control what the cone is doing you have to be able to control what the cabinet is doing. To every action, there is an equal and opposite reaction. This means that as the speaker cone moves forward, the baffle tries to move backwards with an equal amount of force and the pressure reduction in the cabinet causes the back and sides to be sucked in and vice versa. That's why a birch plywood cabinet sounds louder than a chipboard one, and with much better controlled bass frequencies. Taking this argument to it's natural conclusion, and because our amps are very powerful, with an extended bass response, we build metal cabinets.

Unique driver configuration
Cabinets are normally fitted with two pairs of different drivers matched in the diagonally opposing positions. This allows us to use one pair which distort at a lower level and one pair which can be pushed a lot harder before they break up, with an extended low frequency response. The combined result sounds great in any situation.

Built in power soak
Our 4 x 12 cabinets also feature a built in switchable power soak just in case you want to run the amp flat out without making your ears bleed.

Portability
Due to their construction, our cabinets are very heavy. They are however fitted with large wheels at the back. They are not castors which swivel about all over the place, just tilt the cabinet back and wheel it using the handle provided. It's like having a built in heavy duty sack cart.

Crimson Guitars and Ox Amps  -  Redefining Tone