Another beauty came in for some TLC. It doesn't keep the volume up and lately it just cuts out.
In the 70's amp builder were taking the solid state route as that was hip and happening. Marshall also followed this route by releasing a number of transistor amps in both combo's as well as tops. The model 2195 100W Lead and Bass was released in 1976 and was produced up to 1980. It uses a combination of transistors and a single FET to mimic the sound of a Marshall in both clean as well as overdrive. Although these amps are very reliable people tend to shy away from Marshall solid state amps and prefer the tube versions so these amps go for good prices on the second hand market.
The circuit
The amp is housed in a small and low head that is not similar to the normal size of the heads of Marshall amps. Therefor is really stands out. Yet again, it has a similar look of a Marshall amp with the golden colored faceplate, big Marshall logo and known knobs.
It has a single power on switch (no Standby is needed) and controls for Bass, Middle, Treble, Presence, Volume (which is the gain knob) and Master Volume. Two inputs select the 'Normal' and the 'Bright' channel and on the backside are two outputs for the speaker. When opened up you can spot a long and narrow PCB (70's style with swirly traces) that is filled with known Marshall components in both resistors and caps (Mustard caps / Tropical fish caps, etc). Controls and the inputs are directly mounted on the PCB. A single trimpot sets the BIAS of the poweramp. The amp can be connected to any speaker cab with an impedance of 4 ohms or higher. For this model the Marshall 2196 was designed.
Online a schematic can be found:
As you can spot the amp uses a single preamp that is fed by two inputs that adjust the amount of brightness in the preamp. The signal follows throuigh three transistors and then go into a tonestack that mimics a lot of the Marshall tones. The FET that is placed after the tonestack buffers the sound but also add the known overdrive sounds you can get from this amp. The poweramp is a standard solid state poweramp with the addition of a presence knob that, juist like the tube amps, adjust the amount of brightness in the poweramp. The single trimpot can adjust the BIAS of the poweramp / powertransistors by measuring the voltage over the big 5W resistors.
The fix
This amp came in for some TLC. It was noisy, did a lot of cutting out and didn't respond that well to the controls.
- I recapped the amp for moe stability and a better sound. The original caps were leaky and old. By adding new electrolyte caps in the power-supply and the audio path the amp became more stable and most of the noise went away;
- I rebiased the amp so it would run a bit cooler. I think the further degrading of the electrolyte caps made the BIAS higher then necessary;
- I cleaned most potmeters and jacks so the contacts would be 100%;
- A big part of the problem was that the PCB sloped too much down due to the way the amp was build and to the direct connections of the potmeters to the PCB. The biggest problem was on the input jacks that were loosened and didn't properly ground the input signal. Resoldering of the PCB and readjusting the level of the PCB was needed to less stress was on this point. These points though remain a weak point of the amp.
- The trace to the input of the FET is tiny and weak. I noticed that the stress of the level of the PCB made this trace come loose and cut a clear connection. I've fortified this location with some glue and tried to enlarge the trace a bit for more stable connections.
With these repairs the amp came up running again and sounding great!
The sound
The amp is a rare beast in the Marshall catalogue and follows the standard building blocks of a solid state amp in the 70's; these amps were modelled after the tube sound. And Marshall succeeded in this good. The clean tone is Marshall like and the volume and response of the amp is good and sound a bit tube-y. When some gain is introduced the amp begins to add another character that reminds me a lot of the vintage Marshall like tones but with a more modern character. The limited controls though give you enough adjustment to emphasise that vintage or even that modern side making it a very versatile amp for both guitar and bass. It sings in the mids making it push and cuts though the mix nicely. A great addition to any guitar rig or backup!
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