DESCRIPTION OF THE UNIT
The signal is applied to the 1475 INPUT TRANSFORMER, a high quality/ transparency/wide bandwidth type which steps it up by 6dB and it then enters the INPUT AMPLIFIER. This is a high dynamic range mu-follower circuit using parallel connected 6SN7 and 12AU7 tubes.
The input impedance of the unit is 10k, most preamps, sound processors (like synths) will be able to drive it.
The INPUT LEVEL CONTROL adjusts the amount of signal that enters the INTERSTAGE section.
The interstage section is the heart of the unit, a modified mu-follower design using the 6BA6 tube as a voltage amplifier and a parallel connected 12AU7 (top) tube as an active load and output buffer.
The operating modes and conditions of this circuit can be manually adjusted through the BIAS CONTROL, TRIODE/ PENTODE and PLATE OUT/SCREEN OUT switches.
The BIAS control in low settings makes the circuit produce even harmonic products and in high settings less even and more odd harmonic products depending on whether you operate in triode or pentode mode. Setting the bias control too high can result in grid current distortion in high level signals; this distortion even if it is controlled and in low levels, will add harshness to the coloration especially in signals of multiple frequencies (intermodulation distortion).
The BIAS control affects amplitude as well as shape of the output signal, the higher the bias setting the higher the amplitude will be. This has an effect more on the PLATE OUT/PENTODE operation where the output signal level difference between the minimum and maximum bias settings can be as high as 20dB's.
The interstage section in most settings is a very non-linear circuit due to the 6BA6 tube characteristics and it is the main source of sound coloration which can reach very high levels. In addition, (again due to the 6BA6 tube characteristics) this stage also adds a certain amount of compression to the signal chain.
The TRIODE/PENTODE switch when set in 'pentode' simultaneously reduces the output of this stage when the signal is taken from the plate 6BA6 plate, due to the much higher gain that this circuit produces in plate out/ pentode mode.
Triode mode produces even low order (mainly second) harmonic coloration and in pentode both even and odd harmonics are generated, depending on how you set the bias and drive controls. Pentode mode/plate output signal amplitude can reach high values at high bias settings.
As with all my other units the operating high voltage supply B+(HT) must be disconnected from this stage during the triode/pentode switchover by using the stand-by switch.
In pentode mode you can select between two outputs: SCREEN OUT and PLATE OUT. The SCREEN OUT signal is rich in third order harmonics, compressed, smooth and clips very softly introducing a unique form of coloration. It is a new design idea that came by accident while I was experimenting with the 6BA6 tube in the unit.
The PLATE OUT is the normal pentode mode similar to the pentode mode sound in my other designs.
The signal is then passed into the parallel connected ECC99 tube follower that drives the 8424 interstage single ended transformer. This is a very clean audio transformer that drives the output stage. It is very difficult to drive this transformer into saturation without exceeding its maximum current rating due to its high quality and size
The primary winding of this transformer is driven in single ended mode and the two secondary windings work the push pull mode; they drive the output stage, two parallel connected 12BH7 tubes that drive a 1161 push-pull output transformer.
Push-pull output stages (at least in theory) only contain odd harmonic distortion products when overdriven.
The INTERSTAGE LEVEL control adjusts the amount of signal that enters the output stage. If it is set high enough the 12BH7 grids are driven positive creating a 'crunchy' grid current distortion/coloration effect.
The harder both interstage and output stages are driven the higher the order of the harmonic products will be.
Finally the OUTPUT LEVEL CONTROL adjusts the amount of signal coming out of the output of the unit.
The output dynamic range of the unit is extremely high due to the diversity of different types of coloration/distortion that it can produce. The type of coloration generated depends on how high or low either the interstage and/or output stage are driven, the bias settings and the operation mode.
(1) As it is the case with my other units, when you overdrive the input or the output stages or you switch into a mode that produces more coloration, it is not just the shape of the signal (and thus the sound) that changes but also the amplitude of this signal.
For high coloration both INPUT and OUTPUT DRIVE CONTROLS must be set high causing the output stage to produce a very high output signal especially in pentode mode. As I already mentioned the amplitude also increases at high BIAS settings, especially in pentode mode.
At the output there is a hard clipping circuit that abruptly limits the signal's amplitude to a safe level. When this hard limiting takes place an LED lights up, and when that occurs the OUTPUT level control must be turned down.
(2) If the desired tone is mild coloration then both INPUT LEVEL and OUTPUT DRIVE CONTROL must be set very low while the INTER-STAGE operates in triode mode. Under these conditions the OUTPUT LEVEL CONTROL must be set high otherwise the output signal will be very low.
For these reasons (1)&(2) I added an extra three step rotary (0,-6dB,-12dB) OUTPUT ATTENUATION SWITCH in order to enable fine adjustment of the high level range of the differently shaped output signals.
The maximum output impedance is approximately 1.8k so it is suitable to drive mixers/amps/processors with an input impedance of 10k.
There are two large toggle by-pass switches, one per channel, and a LED per channel to indicate the status (by=pass or not).