Product Overview
The Music Man HD series amps were among the earliest hybrid amps, combining
a solid state preamp with a tube power amp. These were produced from 1974 until
1984 when Music Man was sold in an asset sale to Ernie Ball, who discontinued their
production. These amps were commercially unsuccessful, but have since developed a
cult following for their high headroom cleans and uniquely trashy overdrive. While
few famous guitarists are known for using this amp, I have seen them countless
times at small clubs and DIY shows. As far as dollars per watt goes, the HD130 and
its siblings are one of the best deals on the vintage amp market.
This pedal is a simplified take on Channel 1 of the original amp, which is the channel
that does not have reverb or tremolo. The preamp resembles a classic Fender circuit,
but with op amps used in the place of tubes. The op amps in the original amp
are operated at ±16V. In this adaptation we designed around a ±9V bipolar power
supply, using a charge pump to derive the negative rail. The gain can be rescaled or
you can use the reduced headroom to achieve extra distortion relative to the original.
To get into some specifics, there two versions of the HD130 bearing the 2100-130
chassis designator. The earlier version has an all tube power amp consisting of four
6CA7 power tubes, with one half of a 12AX7 serving as a driver and the other as
a cathodyne phase splitter1.
This version is considered by some to be superior due
to this extra tube, but the phase inverter has so much negative feedback that the
power amp stays clean until the amp is turned up to ear-splitting levels.
The later version is a partial redesign of the power amp, with the driver tube replaced by an op amp phase splitter (not unlike a balanced line driver) and an interesting (read: over-engineered and unreliable) cascode configuration using power
BJTs driving the 6CA7 tubes from their cathodes. This is a very linear configuration
with similar high headroom to the previous version. However, the preamp gain was
also scaled up significantly, which is the source of the trashy overdrive that makes
this amp special. Because this overdrive originates entirely in the preamp, we felt
it reasonable to omit the power amp.
