This set of rack-mount monoblocks served also as the breadboard for the finalization of design specs
(and yes, we tried almost every topology including SRPP+ styles, and modifications on the taps, before settling on the Mu-Follower configuration, because of sound quality, the only object in this build)
The Redline Pro Rack-mount Series, serial #002L
We chose rack-mount to integrate these power amps into the studio environment.
This entailed a separately housed powersupply, and heavy-duty cabling to protect sound techs.
The case allowed a quick-hinge to access tubes and do testing.
The amps were fully functional with the cases closed,
but the power tubes (sockets) must be oriented such that grid-sag cannot occur
when running full-tilt. Later versions of the amp reverted to upright positions,
because questions were raised regarding future reliability and standardization of available tubes.
most of the wiring-harnesses below in the prototype were taken in and out multiple times,
so we weren't too concerned about final layout, but only easy point-testing etc.
The configuration shown (6550 / 6L6GC / 12AX7A) was an implementation
which delivered the right amount of power to compete with the MacKintosh's,
which at the time were the hands-down industry standard for mixdown.
Our amp blew the Mackintosh away in both power and clarity, quite a feat at the time,
although we weren't sure just how good our design was, until we A/B'ed it against the Bryston SS power amps, considered the cleanest high-power amps available commercially in the 90s.
here in the side-view, you can see the simple layout for the tube circuits,
consisting of perf-board units mounted close to the tubes.
It is important to realise that the two 12AX7s here are each running at different heater-voltage offsets, about 300 volts apart, and so each partial mini-board is a different set of components (i.e., amplifier and CCS top).
Another view of the simple layout for the power-stage is shown below:
The reason you see some resistors end to end (besides for connecting),
is that standard resistors are only rated for about 300 volts, but in this circuit,
there could be transients 5 times that high. In key positions, after voltage-testing,
we sometimes made up a value by series, doubling the voltage rating, and power handling.
The signal caps were gooped to prevent vibrations in the signal-path.
The inside back panel held both the OT and the input circuitry, as well as a DC cooling fan.
This was added because in the studio environment, the amps were in a separate sealed room,
mounted alongside other devices (amps, board PSs for the Neves etc.).
The noise of the fan was small, and did not intrude into the circuit which was nearly impervious to hum or noise pickup.
The use of the Jensen input transformer allowed a choice of balanced and unbalanced inputs,
allowing flexibility in application in studio situations.
The back of the panel shows the triple-insulated PS cabling, also protected by heat-shrink and cut-resistant plastic fibre tubing, which provides safety in the event of potential cable damage from door slamming, pulling, sharp edges in the rack etc.
All in all, these amps performed well above the Mackintoshes that they replaced,
creating a stunning staging clarity which was very useful for both recording and mixdown sessions.
Most of the engineers were shocked by the imaging,
which however was not always easy to reproduce on cheaper commercial units
which consumers were expected to be playing back the mixes on.
This forced us to use a variety of mixdown tests, so that we were not misled by the great sound,
as to what a mixdown was going to sound like in a real-world home or club environment.
For those purposes, mixdowns were also tested on Cerwin-Vega club gear, and typical home stereo units.
The idea of a 'near-field' standard was not yet solidified in the big studios at that time.
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