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CBS learned this lesson when it took over Fender in 1964. In pre-CBS Fender amps, the placement of every part and wire was the result of successful experimentation. Fender techs spent long hours perfecting their wire placement, moving parts around and running ground wires to the right spots. This resulted in amplifiers that were free of hum and oscillation – it was craftsmanship on a large scale.
CBS, however, was interested in production and sales volume, and that meant producing more amplifiers faster to lower labor costs. They cut some corners in component placement, which resulted in amplifiers that sometimes hummed and screeched. The CBS response was not better craftsmanship, but adding parts to tame the new noise. To make more units at a lower cost, adding parts was cheaper than adding craftsmanship.
The amps worked well enough to sell, but buyers noticed that the new amplifiers didn’t sound the same. The extra parts with different values reduced the amplifier’s sensitivity to hum and oscillation frequencies, but also changed the tone of the amplifier, ultimately leading to “blackfacing,” a process in which the changed parts are reverted back to blackface-era values and the extra parts are jettisoned. A thorough blackfacing might include changing out the output transformer for one with more care for tone, but the problem of layout still remains in many cases. Very few amp techs will replace the exact parts and wires relative to each other, as this amounts to remanufacturing the amp.
So with a silverface Fender that has been blackfaced, it’s often the original blackface- series parts but with the same layout that led CBS to make the circuit changes, which can result in the same CBS-era problems. This leads us to some principles of electronics layout that always matter, and if you’re going to mess with building or modifying effects or amps, you need to know them.
Rule 1: Separate inputs from outputs as far as possible
Inputs are sensitive points that take in delicate, small signals. Outputs are places that have very strong, powerful signals. If you put an input next to an output, it shouldn’t surprise you that some of the output can bleed over into the input and cause oscillation, hum or noise. The bigger the output is compared to the input – the greater the gain difference between the two – the more likely oscillation is unless you are very careful – today’s monstrous high-gain amplifiers make this a real concern.
Rule 2: Keep wires short and direct from place to place
There are wires to inputs, and any point on the wire leading to the input is just as sensitive. Likewise the wires from big-signal places are just as “loud” electronically, so the placement of wires matters, too. Long, looping wires are invitations for crosstalk, RF interference and oscillation. One thing that makes true point-to-point layouts so effective is that the component’s leads are typically short, keeping the distance between parts small and discouraging the use of long wires simply to make things look neat.
Rule 3: Place AC power as far away from the inputs as possible
Wires from the AC power coming into an amp contain the amp’s biggest signals. That’s why you tend to see power transformers as far away from the input jack and circuits on the chassis as is reasonable.
Rule 4: Don’t mix reference, shield, and “sewer” grounds
Grounding deserves an article all its own, but here are some quick tips. There are three kinds of grounds: reference grounds that serve as a source of zero volts that some signal swings around, shield grounds that fence out interfering signals, and what I call “sewer grounds.” Sewer grounds are what drain the electricity back to the power supply after it’s been used by the circuit elements. If you cross-connect reference grounds with sewer grounds, some of the “sewage” – the burbles of used electricity – will show up in your signal.
Rule 5: Keep the output transformer separated from the power transformer and from the inputs
Another thing to watch for is magnetic interference and coupling. A tube amp will always have at least two, and often more, sources of magnetic radiation. The power transformer is a big one, belching out lots of power-line hum. In severe cases, the output transformer can produce hum in the speakers when the power transformer is near it, even without any tubes in the amp! Tubes only make matters worse. The power supply inductor can both pick up power line hum and emit power supply ripple hum.
There are many more layout rules that need consideration if you’re doing a design from the ground up, but you can get a long way with just these basic principles. The same rules apply in effects – place the input leads too close to the output buffer and it just may oscillate uncontrollably.
If you do your own amp mods or effects hacking, keep these principles in mind. They could save you a lot of debugging time.