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H-Box and Spin Doctor.

The prototype valve Spin Doctor and H-Box have completed their development. Tests and demonstrations have been positive. Suggestions and ideas have been taken into account in finalising the designs. Both are '2U' format, suitable for rack mounting, placing on another instrument or table top use etc.
The H-Box, a valve pre-amplifier with differences.

1: Switchable inputs, valve or FET: The valve input stage provides active mixing of two inputs, if required & can be cascaded to increase gain. This facilitates input distortion effects for electric guitar use.

The first triode can be switched out, replaced by a current mode Field Effect Transistor (FET) input stage, using the same power supply. Lower noise, lower distortion, more top end (less Miller effect), greater Voltage headroom, without the progressive overdrive compression typical of valves. The subsequent valve stages will still compress if required, giving a range of optional overdrive characteristics.

2: Switchable tone controls: Two sets of active tone controls are available, 6 dB per octave and 12 dB per octave. Cut or boost of bass, middle and treble or a switched flat setting. Tone controls in use, if any, are colour coded & illuminated.

The 6dB per octave set are based on a passive design from a hybrid stage amplifier I built in the '70s. Suitable for Hammond organs and other keyboards. The 12 dB per octave set may suit guitarists more, this is a matter of taste.

3: Versatile output: The single ended 'class A' output stage is designed to drive 2 145/7 Leslies, plus 2 122 Leslies or Hammond tone cabinets and almost all amplifiers, or the Spin Doctor. The output valve is switchable between pentode and triode modes. A Leslie speed control pedal is included, with panel indication.

Low noise optical signal switching is used where possible. A moving coil V.U. Meter is switchable between main and filter outputs. An Earth Lift switch can isolate the electronics, to solve problem hum loops, whilst leaving the case earthed for safety.

Ron Lebar. XHTML Edited: 15-2-20. Loaded:
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