The Loading circuit is part of the fully differential (balanced) Class-A tube amplifier. It was designed to coerce an audio signal to hit the tube at different points on its response curve. As the operator increases the gain to the tube, the signal is padded down in equal measure. This allows very precise and repeatable loads to be applied to the tube. There is no audible level gain or loss when operating the Loading circuit. This ensures all you hear is the effect the tube is having on the audio signal without distraction from changes in level.
To faithfully reproduce the sound of the audio passing through the tubes and other circuits in the P331, we implemented a low distortion, fully balanced, solid state amplifier for the output gain section. Solid-state output can be cleaner and, above all, faithful to the source. We were not interested in coloring the signal at the output. In fact, in our testing and in our quest for precision and repeatability, a solid state output was the obvious choice. The output gain circuit includes 12dB of boost or cut in 1dB increments. However, for even finer control, the “Resolution” switch allows you to “zoom in” .25dB or .5dB to the current setting on the Gain switch.
TRUE HARD-WIRE BYPASS
Each section of the P331 (with the exception of the Loading circuit) can be fully hard-wire bypassed. There is also a global hard-wire bypass to remove the entire unit from your signal chain...although, we’re fairly confident you won’t want to use it much.
The P331 is designed to run fully transformerless in its default state. This provides the purest, cleanest and most unaffected audio path through the unit. However, what would an amplifier be without the option for a little iron? In keeping with the P331’s versatility, the operator can switch an output transformer into the circuit and select two optional settings that will affect the signal in different ways. The sound the transformers will impart can change depending on how much or how little the tubes are being loaded, coupled with internal impedances of the Output Gain circuitry.
Input signal passes through 100% unscathed.
Lifted frequencies are affected by tube loading harmonics.
The Lift circuit is a gentle filter that affects the highs, lows or both the highs and lows of the signal. Audio passes through the Lift circuit before the Loading circuit. With any of the lift filters engaged, the audio is passed unscathed through the fully balanced P331 circuitry and blended with harmonic content of the Lift and Loading circuits working in concert.
In 1939, just a few weeks after the start of WWII in Europe, RCA Radiotron introduced the 6SN7 octal (8-pin) double triode tube to the American market. This was the birth of the true common ancestor of all modern double triodes used in high fidelity amplifiers. The 6SN7 is still in production after 75 years.
6SN7 vacuum tubes of the 1940s vintage were chosen for this amplifier due to their lower distortion capabilities that exceed that of the standard and ubiquitous 12A*7 variety. The 12A*7 varieties were incarnated due to commercial needs such that they were “better, cheaper, lighter” at the expense of embodying more distortion. With the advent of feedback, tube amplifiers could be mass produced with these less expensive 12A*7 style tubes using more feedback to reduce distortion.
While feedback reduces distortion, it also lowers overall amplifier gain. It might have been considered heresy to reduce the gain through feedback (and reduce distortion) in earlier tube designs since minimal gain was still extremely expensive prior to the 1950s. Thusly, earlier tube designs were lower distortion ON PURPOSE; especially for their critical role in modulation based communication systems that had more stringent requirements than the average 1950s vintage guitar amplifier tube.
Notwithstanding, ultimate circuit topology can bring out more second harmonic characteristics than usual with any tube design. Hence the impetus for the variable circuit topologies within the P331 Tube Loading Amplifier to allow the user to do so as necessary or not at all.
Additionally, these earlier tube models provide more internal headroom and require voltages that can fry a basic 12A*7.
They are, therefore, not for the faint of heart.
For any amplifier to be truly great you have to start with good clean power. The P331’s linear power supply is the most critical “component” for audio integrity next to the actual gain structure and circuit topologies within this amplifier. High voltage plate/anode tube power supplies start at around 320V DC and are filtered (up to 6 times) and smoothed with active regulators. It only uses film capacitors and real inductors (chokes) to remove ripple and noise from the high voltage power supplies.
We’ve taken the best of vintage analog technology and married it to the best of modern computer-optimized circuit design. This allows a level of precision and clarity which was simply unimaginable when tubes of this vintage were originally developed.
Each parameter on the P331 is fully repeatable and can be recalled via front panel rotary and toggle switches commanding basic CMOS and relay-based combinatorial logic (no software or micro-controllers are used) to 84 sealed precision relays. No audio runs to the P331’s front panel and only the finest components are used throughout the audio path.