Mark Baier—Understanding Your Output Transformer

November 08, 2013
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In today’s world, the role of an amplifier's output transformer (OT) is well known. As guitarists discover this critical link between your power tubes and the speakers, the important effect it has on your amplifier’s character becomes obvious. In response, an entire cottage industry has arisen to provide tweak happy guitarists with upgraded power and output “iron." Indeed, the choices for replacing the output tranny for a typical 6L6 powered amp are legion, and discriminating one from another can be confusing to say the least. Comprehending the complexities ain’t for the faint of heart and mind! However, there is one attribute that is easily recognizable, and understanding it is surprisingly simple. Folks, the size of your transformer matters!

A transformer has two basic components, the stack, which is the steel part you see, and the windings, which are the thousands of turns of copper wire tucked around the stack. Both the stack and the windings will determine the performance of an output transformer. To understand the role of the stack, a trip back to Leo Fender’s world of the 1950s is in order.

Back 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. Because 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. Fender knew that the low frequencies were the factor most responsible for speaker overheating. 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. Now the 40 watts of low end that the tubes are delivering will never get to the speaker because it’s saturating in a smaller OT. Doing this had the unintended benefit of creating the creamy compression that’s so sonically satisfying. There is a specific, sweet distortion to be found here, and it’s the type that’s only revealed when the amplifier is driving the output transformer into saturation. The sense of a tube amp “opening up” and producing a singing overdriven note is certainly a function of the transformer compressing and putting it’s signature on things.

The ’50s amps that are most renowned for having this smaller OT are the (tweed) 5E5 Pro, 5F4 Super, and 5E7 Bandmaster/ Prior to this, the OT’s used for two 6L6s were larger than those found here. In fact, the original data sheet for a Triad 45216 Super OT (dated 03/57) specifically notes a “restricted low frequency response” in the description. It also lists the wattage as 40 watts from two 5881s in push-pull. Comparatively, the spec sheet for a 5F6 Bassman indicates the use of a larger stack for the same configuration of 5881s. This basic transformer architecture has remained unchanged to this day. The Blackface Vibrolux, Vibroverb, and Pro Reverb all share this smaller OT, and it is a key ingredient to the vibe of these amps. (The part that Fender produces today is literally identical to the 1950s specifications).

Who doesn’t think a Vibrolux is sweeter sounding than a Super Reverb? Now you know why. Amps that use this smaller 28-watt transformer are always going to have that certain compression and dynamic that defines an output transformer going into saturation. In contrast, the larger Bassman/Super Reverb trannies will relate the vacuum tube and circuits’ clipping before the tranny hits the ceiling, which accounts for the fast, grainy bite that the larger amps display. It should be noted that the actual volume difference is indistinguishable between the two. It is the tonal dynamics that are discernibly different.

The next time you have an urge to experiment with aftermarket transformers, and Lord knows there are a lot to choose from, consider the performance enhancing possibilities that the smaller tweed Super or blackface Vibrolux output transformer has to offer. It might be exactly what you need to tame an unruly Super Reverb or 4X10 Bassman. Not only will you be traveling the same path as Leo, you’ll be mining some great tone in the process!
 
Mark Baier is the owner and founder of Victoria Amplifiers, victoriaamp.com

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