To understand the role of the stack, a trip back to Leo Fender’s world of the 1950s is in order.īack in the day, the most common source of amp failure was the speaker. A high powered Altec industrial speaker was maybe 25 watts, and top of the line Jensen guitar speakers were rarely over 15 watts. A vintage P10R was a whopping 9 watts! This presented a problem for Fender. His wide range, high fidelity guitar amps were driven by two 6L6s, which by mid decade had evolved into a 40 watt power plant. It doesn’t take a genius to know that when you push 40 watts of amplifier through a 15-watt speaker, something’s gotta give. Eventually, the speaker overheats and fails, and when that happens, it can wreak havoc in a vacuum tube amp. Leo Fender’s problem was obvious: How to get the maximum from the available Jensen speakers without risking catastrophic failure. Fender knew that the low frequencies were the factor most responsible for speaker overheating.īecause of the basic physics of output transformers, he was able to solve the problem and create many of the classic tones we now treasure at the same time. It’s elementary the lower you go, the more difficult it is for the electronics to swallow. And here’s where the transformer comes into the equation. In an output transformer, the low frequency response is largely governed by the size or mass of the stack. The bigger the stack, the lower it will go without saturating. (That’s why an SVT weighs so much, its OT is the size of a Oldsmobile!). What Leo did, his genius, was to deliberately inhibit the low frequency response of the amplifier by diminishing the actual size of the output transformer.
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