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Test Bench - Marshall Lead 100 Amplifier


Test Report on: Marshall Lead 100 Amplifier
Date: February 1975

It's taken eight years for Marshall to come up with a solid state amp. Years after many other manufacturers have been gamboling in the sun with their transistor circuits, Marshall finally drew up for battle. The reason it's taken so long is that Jim Marshall kept refusing the amps his designers offered him until they finally managed to capture the Marshall sound in a transistor circuit. More than anything else, it sounds like a Marshall.

Construction


The cabinet housing has been built from marine plywood for ruggedness and finished professionally in a black vinyl material. Large functional A.B.S. (heavy duty plastic) corner caps have been attached to each corner of the cabinet with strong gilt steel studs. Underneath the cabinet there are four large practical A.B.S. feet designed by Marshall, which we considered to be first class.

The amplifier chassis is formed in strong 16s.w.g. aluminium attached to heavy gauge extruded aluminium panels on the front and rear of the unit. The front panel is gold anodised and the rear panel is black anodised and acts as a large heat-sink for the power transistors which are recessed safely between the rear panel flanges. The whole chassis assembly fits into the cabinet housing by pushing the chassis into the cabinet from the rear of the cabinet. The chassis is secured to the cabinet by two functional large OBA Posidrive screws inserted from underneath.

Circuit Layout


Open and accessible and extremely practical should servicing be required. The mains transformer and output transformer are secured with shake-proof nuts and sealed with Locktite which is a first class method of attaching heavy chassis components. The large bridge rectifier and smoothing capacitors are also well fixed to the chassis. All wiring is neat and tidy and secured with plastic ties in places; all soldered joints are well made. The printed circuit board (P.C.B.) is to a high professional standard using epoxy-glass and annotated throughout for the board components. The mains transformer is a double wound 'C' core type suitably varnished; the winding style of the transformer reduces hum fields within the amplifier chassis. The components used have been well selected as they are of good quality and easily obtainable.

Front Panel Layout


Looking left to right there is a mains toggle switch, incandescent lamp, master volume control, presence, bass, middle and treble control, two slider switches for cut and boost with a normal position, volume control 1 with volume control 2 and three input jack sockets marked 1, 1+2, and 2.

Rear Panel Layout


The components are all mounted on a very high quality black anodised extrusion which acts as a highly efficient heatsink for the power transistors. Again looking from left to right there is a footswitch socket, an echo send and return socket, a direct injection socket, two speaker sockets in parallel, an impedance selector plate, a voltage tap plate and the mains cable wired in. The selector plates need a screw driver to change the settings which is a good practical point preventing the knob twiddlers from changing the pre-set positions. The power transistors covered with plastic cover guards are recessed on the rear panel between the D.I. socket and the effects socket.

The voltage tap plate is coded 120 V. & 240 V.
The impedance selector plate is coded 4, 8, No Connection, and 16 ohms.

Power Output


Measurements made into a 4 ohms resistive load (measured with a Wheatstone bridge at 4.06 ohms), and at 1 KHz.

T.H.D. 0.5%, gave 95 watts sine wave at onset of clipping. We measured the mains voltage and found this to be 230 volts and calculated that this would account for a loss of 8 watts. The amplifier would therefore easily deliver 100 watts at low distortion.

Power output at 1% T.H.D. gave 100 watts.
Power output at 10% T.H.D. gave 121 watts.
Similar figures were confirmed for 8 ohms and 16 ohms.
This is an unusual feature for a solid state amplifier due to the output transformer.
Many amplifiers give higher outputs into 4 ohms but much lower into 8 and 16 ohms.

Total Harmonic Distortion & Noise


0.5% at 90% power at 1 KHz into 4 ohms.
0.5% at 50% power at 1 KHz into 4 ohms.

Signal to Noise


Measured -69 dB (excluding hum) with Volume control 2 at maximum and no input signal. This is extremely good.
Hum measured -66 dB below peak power.

Sensitivity


In order to reproduce a good square wave and therefore a good level frequency response for measurement purposes we set the tone controls as follows:
Presence 0, Bass 10, Middle 10, Treble 0. The controls are coded 0,2,4,6,8,10.
Also see details on the boost and cut switches below.
The master volume control was set to maximum throughout the sensitivity tests.

With Volume 1 maximum (10) & Volume 2 at 0. Load 4 ohms.
7.3 mV r.m.s. required for full output.

With Volume 2 maximum (10) & Volume 1 at 0. Load 4 ohms.
14.2 m V r.m.s. for full output.

Some interaction occurred between volume controls affecting measurements slightly but this would have not the slightest significance from the musical point of view.

Input 1


Input 1 Includes treble boost (added brightness for lead guitar).

Input 2


Input 2 A normal input with L. F. response tailored to the lead guitar (not recommended for bass instruments or background music etc.).

Rise Time


With square wave measurement the rise time was 40 micro seconds (8.7 KHz for -3dB).

This is more than adequate for most speaker systems and can contribute to an overall warm sound from the amplifier.

Tone Controls


Presence control +13.8 dB at 10 KHz.
Bass control 9.2 dB swing at 50 Hz.

The treble and mid-range controls are probably the heart of the Marshall sound.
This was the area where we had a little difficulty establishing figures and if there's any one area responsible for producing the sound that's made the name Marshall famous it's probably here. The manufacturers quote the following figures:
Treble control 30 dB swing at 10 KHz.
Middle control 9 dB swing at 500 Hz.

We feel that measurement within 2 dB of these theoretical vales should be considered excellent.

Protection


Tests proved that the amplifier was fully protected from both open circuit and short circuit conditions.

The amplifier ran into a short circuit for two minutes under full drive and operated normally upon reconnection.

Direct Injection


Output 0dbM into 600 ohms.

A footswitch can be supplied as an extra for booster effect. The footswitch will increase the boost if the boost is already switched in, or switch on to boost if the boost switch is off on the front panel.

Conclusion


The amplifier is professionally constructed from expensive materials and should stand the rigours of transportation very well. The combination of sounds obtainable from this highly sensitive amplifier should please the most discerning lead guitarists. There are many features and facilities that will make this amplifier another Marshall leader. We would like to say that the amplifier was supplied to us by the Rose-Morris showrooms and was not specially provided by the Marshall factory.



Previous Article in this issue

Test Bench - Ampeg G212 Amplifier

Next article in this issue

4 Guitar Test


International Musician & Recording World - Copyright: Cover Publications Ltd, Northern & Shell Ltd.

 

International Musician - Mar 1975

Donated & scanned by: Mike Gorman

SoundCheck

Review

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> Test Bench - Ampeg G212 Ampl...

Next article in this issue:

> 4 Guitar Test


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