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- Triode (12AX7) - This configuration incorporates this high-mu dual triode into a typical RC coupled class A audio pre-amplifier configuration. This tube was chosen, because it had been and still is the industry standard pre-amplifier valve. It has a relatively flat linear operating region in the middle of its dynamic operating range, producing relatively lower levels of distortion compared to some of the other devices offered in the VVA. But, by moving the Operating Point to either the saturation of cutoff extreme, more “tube-warmth” effect can be produced by this device. This is the same device as the European type ECC83.
- Triode (12AT7) - This amplifier configuration utilizes the same type of RC coupled pre-amplifier circuit described above, but using a 12AT7 high-mu dual triode. The primary difference is that the 12AT7 was designed primarily for RF mixing applications. As a result, it has a large degree of non-linearity throughout its entire dynamic operating range, including the middle. As a result, you will be able to obtain a higher level of “evens” (even order) harmonic distortion (the most pleasing harmonic distortion) in which to add back into the signal path of the VVA. It also produces some odd order distortion products. This is the same device as the European type ECC81.
- Triode (12AU7) - This amplifier configuration is simulating the driver / phase inverter stage of a push-pull power amplifier. It utilizes the 12AU7 medium-mu dual triode, and, like the previously described circuits, is biased class A and is RC coupled. This device also has a significant non-linearity in the middle of its dynamic operating curve. (In power amplifiers, some of this non-linearity is removed via the use of negative feedback, and decreasing the mix control level on the VVA simulates this phenomenon.)
- Pentode (6EJ7) - This single stage, high-gain microphone amplifier configuration utilizes a sharp-cutoff pentode. It can produce a very pleasant “tube-warmth” effect when the operating point is properly set. This device is the same as the European type EF183.
- 2 Stage Class A - This is an 8 Watt class A power amplifier, consisting of a 12AU7 medium-mu triode driving a single 6L6GC beam power pentode audio output valve. Its effects are distinctive due to the convolution of the non-linearity of the triode interacting with those of the pentode, with both devices operating in class-A mode. The 6L6GC is similar in performance to the industrial type 5881, and also the European tetrode, type KT-66 (KT = Kink-less Tetrode).
- 2 Stage Class AB - This is a 25 Watt class AB power amplifier, consisting of a 12AU7 phase inverter / driver, pushing a pair of 6L6GC beam power pentodes. Because the circuit is push pull, the output devices produce a more symmetrical and reduced even-order distortion characteristic distribution. The more dominant distortion products are the odds with this circuit configuration. The operating point is fixed at the factory, and cannot be adjusted for this amplifier configuration.
- 2A3 Push-Pull - The 2A3 is what some people refer to as a “retro – triode”. It was invented in the 1930’s, had a directly heated cathode, and produced a high power output at its time of development. It was often found used in theatrical applications and public address systems. The “Push-Pull 2A3” VVA setting uses the 2A3 triode implemented in a “push-pull” class AB1 power amplifier circuit designed to produce 15 Watts of output power. This configuration exhibits a more linear output transfer characteristic compared to its Pentode push-pull counterpart. We have included the 2A3 tube in this particular configuration in the VVA because a musician friend of ours (Les Paul) recommended that we do so because of its unique sonic characteristics. He explained to us that he used a push-pull pair of these devices as the power amplifier to “cut” all of the records that he released from his own home studios. The reason that he used these was the extremely clean sound that they produced. The particular devices that we used to create the 2A3 VVA models were of the “dual – plate” variety. The devices used in the characterization process for the 2A3 based VVA were taken from new (unused) but old stock (NOS) and were manufactured for the military by RCA Victor in 1953.
- 2A3 Single-Ended - This is a single ended class A power amplifier implemented using the 2A3 power triode. It exhibits reasonably good linearity and about 4 Watts of audio in a “single-ended” class-A configuration. Its dominant distortion products are “evens.” This is the only power triode in the VVA suite of tubes.
- Exciter - This check box enables the Harmonic Exciter feature of the DCArt10/DC Forensics10 VVA. The exciter uses a simulation of a vacuum tube rectifier (6X4) to produce harmonics. Asymmetry between the positive and negative going transfer function establishes the relationship between the degree of even and odd harmonics produced. For more details on its performance, please refer to the Harmonic Exciter description under the Operating Point Control description.
- Transformer Class AB - This check-box enables a push-pull, transformer coupled, 6L6GC based, class AB, 20 Watt power amplifier having a 12AU7 based driver / phase inverter stage. It produces a distortion dominated by odd order components since most even order products cancel out in push-pull circuits.
K. 6267 / EF 86 Pentode - The 6267 / EF86 pentode was suited well for use in low-level preamplifier service where low noise and minimal microphonics were important. It was often found used as the first-stage amplifier in tape decks. Its high-gain characteristic provide it with an interesting family of operating curves that provide useful harmonic distortion and signal compression in the VVA.
L. 2SK-175 MOSFET - This device is not a tube, but rather it is an N-Channel Audio Power MOSFET (the P-Channel compliment of which is the 2SJ-55). It was commonly found in high power, high quality audio power amplifiers and is included here because it has a set of operating curves which differ in shape somewhat from the various VVA electron tubes. It will provide you with a different distribution of distortion harmonics which you may find pleasing in some circumstances.
Just like the other filters and effects, the VVA is equipped with a set of descriptive presets. This is always a good place to start from when using the VVA. After you have found the preset, which most closely resembles the sound you are looking to achieve, you can go back and “tweak” the controls more precisely. After you have established a group of settings that you would like to keep, use the “Save” settings feature to give your preset a name so that you can recall it in the future.
Note: Voltage Amplification stages usually produce a phase inversion of the applied signal (180 degrees). All of the VVA devices and/or systems are phase corrected to zero degrees for user convenience.