In the first part of this mini-series, we checked out a variety of basic preamp topologies. And in the second, we looked at the output stage. This time, I’ll dive into their unsung partner in tone crime: the power supply stage.
Defining The “Power Supply Stage”
The power supply stage is the part of the amp that generates the electrical voltages that the tubes run on while also conditioning this voltage supply by filtering out artifacts that can lead to noise and other performance issues.
Note that your guitar signal doesn’t actually pass through this stage in any way. For that reason, it’s usually the last part of the amp that players give any thought to. If you’re hoping to develop a comprehensive understanding of how tube amps work, though, you ignore the power supply stage at your peril. While the preamp and output stages may be responsible for shaping the gain level and frequency content of your guitar signal, nearly everything each of those circuits does relies on the performance of this humble workhorse — and things will happen quite differently in those parts of the amp when they are coupled to dramatically different power supply stages.
The Parts of the Power Supply Stage
The essential components of this part of the amp are:
Fender '65 Super Reverb Power Transformer
Power Transformer: The power transformer (or PT) receives the 120 volt AC supply from your wall socket and ramps it up to two or three times that amount in the first stage of many that provide the voltage that the tubes run on. Meanwhile, the PT also produces a low-voltage, but high-current, 6.3V AC output that powers the tubes’ heaters. Finally, if the amp has a tube rectifier, it also produces a different 5V AC feed for that tube’s heaters.
Rectifier: This can be either a tube or a string of solid-state diodes, but all tube amps have a rectifier of one type or another. Tubes need DC voltage in order to accomplish their amplification duties, and this is where it comes from. The rectifier receives two strands of the doubled or tripled AC voltage from the PT, as mentioned above, and converts it to a single strand of DC voltage, at a level that is higher still.
Filter Capacitors (aka Electrolytic Capacitors): Following the rectifier tube or diodes, the power supply runs through a series of large electrolytic capacitors that filter out the remaining ripple from the DC voltage, which can cause unwanted noise and other detrimental effects. A number of large filter caps are used to clean up the voltage supply before it is split off for delivery to different parts of the amp.
Choke: Some amps also have a small transformer-like coil called a choke, which is usually placed among the first few filter caps to further reduce power-supply-induced hum. Many smaller amps dispense with this component, and use filter caps only, but most amps of 20 watts or more will have one.
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It All Begins with Voltage Levels
Whatever he or she intends to do in the rest of the circuit, any thoughtful amp maker generally looks to the power supply stage first when considering a new design from the ground up. That’s because the voltage levels that this stage delivers to other parts of the amp — the preamp and output stages — play an enormous role in determining how the amp sounds and feels.
As a rough rule of thumb, the higher the voltages applied to the tubes, the more they are able to increase the signal passing through them, producing a higher signal voltage, assuming all else is handled correctly within the circuit. Higher signal voltages in the preamp tend to mean a tighter, crisper, more hi-fi tone. Higher signal voltage in the output stage means more power — higher wattage levels delivered to the output transformer and, as a result, the speaker.
Lower voltage levels, as you might guess, tend to equate to less overall output power — yes, along with a somewhat “browner” sound, too — which means slightly easier breakup (quicker onset of distortion) and a little more grit and texture.
'55 Fender Tweed Deluxe
To get where they want to be, amp designers might, for example, engineer a power supply stage that delivers relatively high DC voltage levels to the output tubes, but relatively lower voltages to the preamp tubes. By combining the different characteristics of these two signal-amplifying stages, the designer determines the amp’s sound and feel. They will achieve these differences by properly selecting their power transformer specs, the type of rectifier they use, and the type and amount of power filtering applied to the stage.
To better understand how different voltages contribute to different sounding amps, consider these two common examples:
A Fender tweed Deluxe of the late ’50s runs its 6V6s in the output stage on about 340V DC, give or take a few volts. There are other things going on in the amp too, of course, but that’s a significant part of what determines these old combos’ characteristically round, gritty, touch-sensitive sound.
The Deluxe Reverb of the mid ’60s, however, runs its 6V6s on around 420VDC. Along with other changes, that increased voltage helps to make this iteration of the popular Fender combo sound somewhat tighter, crisper and clearer, and enables it to produce around 22 watts vs. the tweed’s 15 watts. (Yes, there are many other changes from tweed to blackface aside from the different DC voltage supplies, but it’s a significant factor in the evolution of the design.)
Efficiency and Playing Feel
In addition to how they might encourage an amp’s preamp and output stages to sound, power supply stages are even more of “a feel thing.” That is, the way in which this part of the amp functions plays a huge part in the touch sensitivity of the amp — the way it feels under your fingers.
Any time an amp set to moderate or high volume levels is hit with a demand for power — say, you hit a series of heavy power chords, or you’re digging into a fast solo run — there’s a slight “lag and catch-up” effect going on in the power supply stage. That is, when demand for power is high, the power supply components have to work harder to provide it, and they aren’t always able to do so instantly. In playing-feel terms, we generally refer to this lag as “sag;” it’s a compression-like feel that puts a little softness around the pick attack at the front of the note. In some circumstances players love it; in others, it can be detrimental to their playing style.
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The Rectifier’s Role
In addition to the amount of voltage supplied by the PT, as discussed above, the type of rectifier used is a major factor in this whole touch-sensitivity thing.
Groove Tubes GT-5AR4 GZ34 Rectifier
Solid-state rectifiers are extremely efficient in meeting a high demand for power, and their lag in doing so is usually almost imperceptible to the human ear, or to the hand of the player. As a result, amps made this way tend to have a fast, detailed response, making them great for everything from fast country chicken pickin’ to eviscerating shred and metal riffs. (Note that output tubes can also lag behind when demand is high, creating some sag of their own as distinct from power supply sag.)
Tube rectifiers, as a rule, are relatively slower to respond to high demand than are solid-state rectifiers, and they tend to induce more compression-like sag as a result. There are many different types of tube rectifiers in use, though, and each has different characteristics and efficiency levels. The most powerful and efficient of the common types, the GZ34 and equivalent 5AR4, are extremely efficient and might only exhibit sag when an amp is cranked up and demand for power is high. Lower-efficiency types like the 5Y3 and EZ81 might begin to sag at moderate demand for power, yielding a particularly “squishy” feel when hit hard.
The Role of the Filter Capacitors
The size and configuration of the filter capacitors (aka electrolytic capacitors) also plays a part in determining the playing feel of any tube amp. As discussed above, all amps need some filtering to maintain relatively quiet and satisfactory operation, but amp designers can further tweak power filtering to craft a desired response one way or another.
Lighter filtering tends to induce a softer low-end response, with relatively more looseness in the feel of the amp overall. Light filtering can also, to some extent, affect the harmonic content of an amp, inducing a little dissonance and “ghost noting” when the amp is pushed hard, which can be a desirable part of any given amp’s overall texture in some instances, and undesirable in others, depending on your playing style and the tone you seek.
Heavy filtering helps to firm up an amp’s bass response while also leading to a tighter, punchier sound overall.
Makers will sometimes lean toward lighter filtering for simpler vintage-style amps that seek to replicate the sound and feel of the classics from the ’50s and ’60s. For heavier or more contemporary sounds, with thumping lows in particular, makers will usually go with larger filter caps, and more of them.
Add up all the variables, and you quickly see how the power supply stage plays a big part in shaping the personality of any tube amp. Partner it up with the right preamp and output stage for your needs, and you’re off.
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ABOUT THE AUTHOR: Dave Hunter
Dave Hunter is a writer and musician who has worked extensively in the USA and the UK. The author of "The Guitar Amp Handbook," "Guitar Effects Pedals, Guitar Amps & Effects For Dummies," "The Gibson Les Paul," and several other books. Dave is also a regular contributor to Guitar Player and Vintage Guitar magazines.