Crucible
Overview
The Crucible is a 1W push-pull, cathode biased tube power amplifier that uses a 12ax7 for the phase inverter tube and the two halves of a 12au7 as the push-pull output pair. Designed for use along with a guitar preamplifier, whether driving a speaker directly or when using the built in reactive dummy load, the Crucible adds power amplifier warmth, compression and overdrive, providing the missing link for the full tube amp experience.
In recent years both tube and solid state guitar preamplifiers have become more popular. Some of their uses include: providing a more authentic analog tone for recording, especially in today’s world of digital recording interfaces and DAWs; use as the heart of a direct rig for performance, where the traditional guitar amp is less than ideal because of size and/or volume constraints; as a means of fundamentally changing the tone of an amp by plugging into the amplifier’s effects loop return, thus by-passing the amp’s built in preamplifier; for use with a separate power amplifier and speaker cabinet (popular in the ‘80s and ‘90s but less so today).
In the first two uses above the guitar preamplifier is typically combined with a speaker simulator (and effects if desired) for a potentially versatile and great sounding simulation of a classic guitar amplifier of the user’s choice. While there are now very good preamplifiers and speaker simulators available, there is one crucial part of the signal chain of a classic guitar amplifier that is under-represented or more often than not, completely absent in direct rigs: the tube (or valve) power amplifier. The tube power amplifier is crucial for adding warmth, dynamics, texture and the all important reactive feel that we expect when plugging into a great guitar amp.
Tube power amplifiers are available as separate units of course (and used to be relatively popular with professional guitarists in the '80s and '90s) and while they can sound great, they are more generally designed for clean volume and headroom. In a typical scenario such a power amp is used with a preamplifier and speaker cab and it is the preamp that is largely responsable for fundametally generating the clean and overdriven sounds, with the tube power amp adding warmth, dynamics, volume and the all important tube feel and response. However, classic tube overdrive is also generated in the power amp...just think of so many of the iconic guitar tones of the 60's, 70's amd 80's - they were mostly recorded with classic heads, stacks and combos, providing a combination of both preamp and power amp overdrive. So while the Crucible forms part of a modular setup, unlike in most modular setups, the Crucible is also an integral part of the overdrive generating circuit itself. It allows the user to access warm clean tones with a moderate preamp output, or great sounding power amp overdrive when the preamp's output is cranked up. It's low output power and features make it much easier to harness the tonal attributes of a tube power amplifier, both for low volume applications and recording.
In designing the Crucible a push-pull design was chosen because the vast majority of great electric guitar tones were created with push-pull amps. While single ended amps have their own unique response and can sound great in their own right, a push-pull design is essential for nearly all of the classic overdriven tube power amp tones we have become accustomed to on our favorite rock and blues recordings.
A simple signal path includes a 12ax7 phase inverter tube feeding the two halves of a single 12au7. A custom output transformer then feeds a speaker output, or a built in reactive dummy load. A variable line out allows the signal to then be fed into cabsims and/or recording interfaces for direct rigs and recording, or into another amplifier or power amp for live performance.
Features
A post phase inverter Volume control allows the phase inverter to be overdriven while still having control of the final output volume. In this way overdrive can be generated even at extremely low volumes, or you can turn the Volume up and overdrive both the 12ax7 phase inverter and the 12au7 power tube.
A Presence control works in the classic way by reducing negative feedback on high frequencies, resulting in more top end.
The Tone control is like a “cut” control on a Vox amp, reducing top end after the phase inverter tube. Having both a Presence control and a Tone control is particularly useful as when negative feedback is not used, or when the post phase inverter Volume control is turned down, a traditional presence control will no longer work. A “cut” style tone control will however still function in both instances. When the Volume control is turned up and negative feedback is turned on, both the Presence and Tone controls will be active.
A three-way negative feedback switch provides two amounts of negative feedback for tighter and leaner responses, while the 3rd position removes the feedback completely for a fatter and slightly looser response.
A Line Out control captures the full tone of the amplifier and sends this attenuated signal out of the line out jack for further processing. Plug this output into effects such as delay and reverb and then on to cabsims, audio interfaces or into another amp or power amp. Adding effects from the Line Out places them after the power amp – particularly useful when driving the Crucible into overdrive.
A single 8ohm speaker jack connects to a speaker cab (4 and 16 ohm cabs can also be used).
A Speaker Mute switch mutes any speaker plugged into the speaker jack and connects the output of the Crucible to an internal reactive dummy load instead.
While the Crucible can be used with a speaker or with the built in reactive dummy load, the Line Out can be used in both cases. A great sounding wet/dry rig can be set up by using the Crucible as the “dry” amp pushing a speaker cab, then sending the Line Out signal to reverb and/or delay and on into a second amp or power amp and speaker.
A Ground Lift switch helps reduce potential ground loops when plugging the Line Out into other audio equipment.