MUSIC MAN 210-RD-100 guitar combo amp

210-RD-100

Welcome back to the blog. We continue to carry out service & repairs/restorations to the still popular original production amps from the Music Man company, built in the USA during the late 1970’s – early 1980’s era. This example is the 210-RD-100 model 100 watt 2 x 10 combo from the early 80’s period, owned since new by Peter Tos, and gigged heavily back in the day in Sydney’s nightclub scene.

front panel after new standby switch installed

The amp was delivered to the workshop with a broken standby switch, which is a 3-position toggle switch which selects either off (standby), high power or low power functions. Therefore, the switch needs to be a single-pole/double-throw (SPDT) with centre-off position, capable of switching high voltages. The standby switch achieves this functionality by routing higher or lower AC voltages to the voltage-doubler rectifier/power supply.

rear view new standby switch installed

In the high position, the power supply delivers approx 725V DC HT (no signal) to the anodes of the pair of 6L6GC output valves (tubes) !! No wonder this amp configuration easily achieves 100 watts audio output from a single pair of output valves. The screen grids are powered from half the HT voltage (otherwise they would certainly self-destruct). The valves are run very close to pure Class-B or in other words they are biased for very low current draw (no signal).

rebuilding the voltage doubler power supply

Having dealt with the primary problem, we knew from previous experiences we would need to check the condition of the high voltage electrolytic capacitors in the voltage doubler power supply. As expected, they had both ruptured and were no longer capable of doing their job. We replaced them with a pair of those very fine F&T 50+50uF/500V capacitors, bedded down in silcone for stability. The original capacitors have axial leads, so we have to hand wire the F&T vertical mounting units to the existing eyelet board.

another view of the voltage doubler eyelet board

The 500V F&T units in series give us a nice safety margin of 1000V DC maximum rating compared to approx 725V DC (as measured) high volts supply. We didn’t get a good earth test result, so we replaced the 240V 3-pin plug. The amps pots & jacks were cleaned with FaderLube & DeOxit, respectively. The Music Man amps were known primarily for their clean (“enhanced” Fender-style ??) tones, and this model is no exception. The overdrive channel produces what could be best described as 80’s distortion tones, which is nothing to get too excited about. The channel switching arrangements work very well. The original footswitch unit still works fine !! The 3-band EQ plus bright deep switches are shared by both channels.

the only valves in this amp – a pair of 6L6GC output valves

You may have noticed there are only two valves (tubes) in this amp design – the pair of 6L6GC output valves ! There are no pre-amp valves – the gain & signal shaping functions are performed by IC/op-amp’s. Even the phase splitting function to provide a pair of drive signals to the output valves, is performed by IC’s. How can an IC provide the level of drive signal to a 6L6GC valve you might wonder ?? Well, it’s achieved by configuring the 6L6’s in grounded grid mode of operation, whereby the drive signal is applied to the cathodes of the 6L6’s, utilising a pair of small NPN power transistors.

the well designed board still all original, as are the pots & jacks

The control grids still have a bias voltage applied, but they are grounded as far as signal is concerned. This arrangement works quite well, although you miss the warmth & colour of a valve phase-splitter stage. The only drawback that we have seen with this configuration in a guitar amp is that in the event of a catastrophic failure of an output valve, the drive transistors will surely be destroyed. The earlier 1970’s Music Man amps utilised a 12AX7 valve for the driver/phase splitter stages. Those amps are naturally considered more desirable, but lack the channel switching functionality. Thanks – IR.

the ruptured pair of high voltage filter capacitors

(the original footswitch still works fine !)

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