Repair Log: 1981 MusicMan 75 Head 2100 – 75 EX SN: xxxxxxx

23/06/18 Copyright retained Terry Relph-Knight

RRP in 1980 $445 Value today – £ 200

Supplied with a 1M Fender speaker jack cable.

This MusicMan head is a hybrid transistor / valve design, rated at 75 Watts r.m.s. !! from a pair of 6L6GC output valves (it currently has a pair of JJ’s).

The B+ voltage must be very high (yup it’s 700Volts, the JJ 6L6GC is rated at 500Volts maximum). 

This would seem to be pushing what can be achieved from a push-pull pair of 6L6GC, which are rated at 30Watts maximum anode dissipation each, to almost insane levels. It is set for 220Volt mains, which probably means it is over-volting even further. Power rectification is of course solid state.

Like apparently all Music Man amplifiers this amp can be switched to low power or (75 Watts) high power on the three way Standby switch, which is standby in the middle, up for Hi power and down for Lo power. This is done by switching between two voltages for B+. Hi power is 700volts and Lo is 350volts. Actual measurements into an 8 ohm resistive load show 63Watts and 30.25Watts, so no you can’t squeeze 75Watts out of a pair of 6L6GCs and Lo power is half the high power output

The pre-amp is entirely solid state. It has two channels – Normal and Bass. Each channel has two inputs – 1 and 2 with 2 being -6dB quieter than 1. Both channels have a Bright/Normal rocker switch and Volume, Treble, Middle and Bass controls. The Bass channel has in addition a Master volume and a Deep/Normal rocker switch.

 1981 MusicMan 75 Head 2100 – 75 EX Repair Log

According to the dscription on the Reverb site this design is intended to provide good clean tones with a little added warmth from the push-pull valve output stage. However the circuit shows it has a master volume mixing stage that has back to back transistor / diode pairs in the feedback loop of an op-amp, which looks very much like a distortion circuit to me. This model was manufactured from 1980 to 1984. It uses a mix of op-amps (LF353 or TL072 in the pre-amp) and transistors right up to the the output valves. The push-pull output circuit has a pair of identical JE1692 transistors driving signal into the cathodes of the output valves, preceded by two op amps (dual op-amp LM1458) one of which provides signal to drive one transistor, and the other a phase inverted version to drive the other transistor. There is an internal adjustable bias control which sets the output valve idle current via the two cathode drive transistors. All four op-amps are fitted in sockets.

The inputs to this amp are quite low impedance, around 300K, rather than the ‘standard’ for guitar amplifiers of 1Megohm. Each channel uses one half of a dual op-amp as a pre-amp before a ‘classic’ passive tone stack. The second half of the op-amp buffers the tone stack. A dual gang channel volume has one half in the feedback loop of the input amp, controlling it’s gain and the second half as a passive volume on the output of the tone stack buffer with the buffer driving the wiper of the passive volume.

The outputs from the tops of the two passive channel volumes drive the input of a single op-amp stage that appears to be configured as some kind of distortion stage with transistor / diode pairs in its feedback loop. The output from this drives the top of a master volume and the wiper drives the input of a two op-amp phase splitter for the power amp.  The two outputs from the splitter drive common emitter NPN transistor stages, each of which drives the cathode of an output valve. The suppressor grids of the pair of 6L6 pentodes are both connected to the 350 volt rail via 470 ohm resistors and the control grids are both connected to a +22 volt rail via 220 ohm resistors. The anodes drive either side of the centre tapped primary of the output transformer. 

Problems – The output valves have been replaced with new JJs without re-biasing. The Bass channel hardly passes any signal (it’s possible that corrosion on the op-amp pins and socket has caused loss of contact). Amp is quite dirty and has some rusty bits. Looks like it was left standing in a shallow pool of water for a while.

Work done

Output stage biasing checked and re-set. Replaced the dead TL072 dual op-amp in the Bass channel. Cleaned the cabinet, knobs and the amplifier front panel. Much of the original knob numbering had worn away so I rubbed white wax into the knob engraving to improve the visibility of the numbers. Tightened several loose nuts on the controls. Measured the output into a resistive load while observing the output on an oscilloscope for power measurement, purity and clipping point.

Diagnosis –

The cabinet of this amp is quite dirty and various metal parts are quite rusty, with some rust on the transformer laminations! It looks as though the amp may have been left sitting in a shallow puddle for a while. The chassis is a welded steel tray with an angled front for the controls, suspended upside down in the cabinet from four long bolts in the four corners in the fashion of all the old Fender amplifiers. Getting the fourth nut, which is tucked behind the power transformer, into place on the thread of the bolt is an absolute PIA!! 

Hand written on the pre-amp PCB is – BC 1/12/81. Assuming the American convention of month/day/year this probably means the amplifier was assembled or final tested on the 12^th of January 1981. The chassis has a paper self adhesive label 81-4.

Since the dead ‘Bass’ channel has a dual TL072 op-amp as the main active component and the op-amps are socketed, the easiest thing to do, rather than poke around measuring things, was to try replacing the op-amp. Seems to have worked.

An online source says this about bias

Section I DRIVER TRANSISTOR BIAS CALIBRATION PROCEDURE

A. This applies to all models containing the following circuit boards:

DB-2, DB-3, DB-4, GP-1, GP-2, GP-3, GP-3A,GD-1,GD-2 AND GD-2A.

B. Adjustment is as follows:

1. Turn the amplifier to “ON” with the HI / LO Standby switch in the HI

position. No Signal.

2. Using a voltmeter measure the voltage from emitter to ground on each of

of the two driver transistors. Across the 3.9 OHM emitter resistors is a

convenient measuring point.

3. Adjust the bias trimpot (TR-1) until you read 25mv DC across the 3.9

OHM emitter resistors. If there is a difference in voltage between the

emitters of the two driver transistors, set the lower of the two to 25mv.

The higher of the two should not exceed 55mv DC. 

Voltage across the 390 ohm nearest the back panel is 16.4mV and 14.4mV across the in-board (which at some point had burnt out and has been replaced by two 680 ohm).

Reset to 25.7mV and 24.3mV. Remeasured 28.9mV and 28.1mV. 

Voltage on either side of the Standby / output power switch is 242.5V and 362.2, basically the voltage is the same on either side and it changes as the switch is flipped ???? Hmm, seems there is some odd voltage doubling circuitry. The actual B+ to the red wire on the output transformer does what it is supposed to. Pin 3 (anode) on the output valves flips between 483volts and 722volts as the switch is thrown.

The amplifier is set for 220V mains so yes the B+ will be even higher than the specified 700V.

Output transformer primary has a red wire to B+ a blue to the anode of V2 and a brown to the anode of V1 (V1 is the valve nearest the end of the chassis, anodes of 6L6GC are pin 3).

Red to Blue = 212.4 ohms, 1.207 on Hi     Red to brown = 191.0 ohms, 0.99volts on Hi

Red to ground = 726volts Hi, 485 Lo

Anode current V1 = 1.207/212.4 = 0.005682674 amps = 4.1Watts dissipation  !? doesn’t seem right

Anode current V2 = 0.99/191 =  0.005183246 amps = 3.758 Watts dissipation

Output power tests

Standby switch up Hi power measured output into 8 ohms = 63 volts p-p = 63 Watts r.m.s.

Standby switch  down Lo power measured output into 8 ohms = 44 volts p-p =  30.25 Watts r.m.s.