Whitestone Audio Instruments, USA
Whitestone line amplifier, tube loading amplifier, 6sn7 tube loading amplifier.

The Whitestone Audio Instruments P331 Tube Loading Amplifier was developed to bring professional mix and mastering engineers versatile, elegant, precise and repeatable audio circuit variation. There’s nothing like running a signal through a well-designed piece of analog gear, at “unity” gain without any “processing” applied at all. Simply exploiting the sound of some of these analog input and output gain circuits with processing bypassed is often-times just the thing to bring subtle life or organic size and depth to recorded audio. That’s what the fully balanced P331 is all about; unprecedented control of how an audio signal passes from input to output and how it is affected throughout its journey.

Soul (Loading Circuit)

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.

Whitestone Audio Instruments, USA