Marshall Plexi amps have lots of gain. Most of that gain is up at the front end preamplifier stages and it is in this area that things start to go wrong and have most detrimental effect in an amplifier of this age. One of the first things we are going to tackle here steps onto “hallowed ground” and may offend some of the purists out there, and that involves facing up to some brutal truths about the causes of amplifier noise.
Original 1960’s amplifiers were made using Carbon composition resistors. These resistors were the standard commercial low cost, low power resistors available at the time… so everyone used them.
Does that mean that they are the best choice to use today when you repair or rebuild your classic amp? That depends on your viewpoint. Do you want your amplifier to be “authentic” and look “original” when someone looks inside (and when does that happen?), or do you want it to sound even better than it did in 1965? Do you want the best performance from your amp, or do you want to persuade yourself that the degradation in performance contributed by outdated components actually contributes to your signature sound?
These kits focus on changing just a few key parts for more modern, higher specification components that can a) reduce the hiss and noise your amplifier produces to soften and mask your guitars sound and b) improve the reliability of the amplifier and protect it from horrible high voltage faults that can destroy valves and take the amp out right in the middle of your live guitar solo!
So…. Where do we start?
As mentioned earlier, most of the gain in these amps is in the preamplifier stages, and there are a few simple facts about the resistors used in high gain preamp stages that point the way towards what we need to do here.
1. The most noisy resistor type you can use in an amplifier is…… yes… Carbon Composition. The quietest type available in a wide range of values from low to high resistance is Metal Film.
2. The higher the value of resistance, the more noise is generated by that resistor. Plexi amps have a few key high value resistances in the preamp stages… input resistors, grid resistors and anode resistors.
3. The more current that is flowing through a resistor, the noisier it becomes and Anode resistors obviously carry considerably more current than input or grid resistors. In particular, carbon composition anode resistors are notorious for changing value with time, breaking down internally and becoming unstable and microphonic. Many vintage amplifiers that make sputtering, hissing and crackling noises do this because anode resistors in the preamp stages have gone unstable, so these are key components to change.
4. The noise produced by a resistor is also influenced by the physical size of that resistor (or in other words, its Wattage). Amplifier manufacturers used the smallest possible size resistors they could in order to keep production costs down and most vintage amplifiers were built using ½ W resistors wherever possible. A half Watt resistor can produce 3 times as much contact noise as a 2 Watt resistor of the same type!
As you can see from the above, the resistors we have discussed here are fundamental components, and changing them from tired old ½ Watt Carbon Composition resistors to modern and larger, more reliable lower noise 2 Watt Metal Film resistors is an essential first step towards improving the sound quality and reliability of your amplifier.
So now we have decided which resistors to change, what’s next?
Apart from resistors, the other components that work hard around the valves in an amplifier circuit are capacitors. These components block very high supply voltages from one stage of your amplifier from the next stage, but allow the relatively small signal voltage to pass through. Larger capacitors also absorb mains hum from the power supply and store and provide surges of boost current to the output stages when you hit that massive power chord, to give you a clean and powerful output sound at maximum output,
1960’s capacitors were very much built using yesterdays technology. Waxed paper and foil capacitors like the famous Hunts series were often used, but these were pretty vulnerable components over time. They swell, they sometimes split or burst open, they leak DC and they sometimes go short circuit altogether, passing the very high anode voltage of one valve stage on to the very sensitive grid circuit of the next valve, causing valve bias to go haywire and in extreme cases overdrive valves to destruction.
If the reliability of your amplifier is important to you and to protect the valves, especially the expensive output pair or quad, it’s well worth changing a few coupling capacitors. Our kits include those key capacitors to change to future proof your amp.
Likewise, the large can style smoothing capacitors and the output valve bias smoothing capacitors are chemical based, rather like a battery, and the fluids in these components dry up over the years. Some HT smoothing capacitors just lose capacity to do their job, resulting in a gradual increase in hum and a reduction in headroom…. But some capacitors will actually burst or in the worst cases explode inside your amplifier, often taking out other components as they go short circuit. The value and style of these components vary within different versions of Marshall Plexi amplifiers, but we do stock a wide range of suitable components to replace them.
Lastly, of course the valves themselves gradually age, losing gain. This results in a flatter, duller sound, less ability to overdrive the output stages and less crunch in your sound. Changing the preamp valves in particular sometimes makes a huge difference to the brightness and clarity of your sound, so your refurb should consider including the option to change two of the front end ECC83/12AX7 valves to bring back the focus and punch these stages bring to your performance.
Whether you would like to improve the sound quality of your classic Marshall amplifier, or ensure its reliability in the future, our kits offer a sensible, modern technology solution to making those little improvements to the circuitry that will refresh and renew it for years to come. Each of our kits comes with details about which components fit where and some safety and fitting instructions to help you.
Kit A: Contains the key preamp stage resistors and capacitors (high reliability Polyester Metal Film types rated at 630V and similar to the Mallory 150 series, thought by many to be the closest to the sound of the Mullard mustard capacitors), along with output stage coupling capacitors to improve hiss/crackle noise and reliability of your amplifier at an affordable price.
Kit B: Contains the same parts as Kit A plus two bias smoothing capacitors and two 12AX7B high gain low noise ECC83 replacement valves, manufactured by the same company that supply Marshall (see our other listing to purchase Kit B).
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