A Look Under the Hood, part II

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Continued from last week’s installment of TubeTalk:

The power section is where the hungry EL84 power tubes anxiously devour the signal and then blast it out in a monstrous fury of tonal bliss. These tubes work in a pair, each amplifying half of the signal sent along by the phase inverter. This so-called “push-pull” configuration is a more efficient way to use tubes to do the work of moving a speaker, which allows for higher wattage, as well as longer tube life. Before the signal reaches the power section, it’s got to pass through coupling capacitors, which make sure that the B+ feeding the plates of the phase inverter don’t mess up the signal. You might recognize these types of capacitors from the previous stage, where they did exactly the same work. Also, similar to what’s going on in the preamp tubes, there are grid load resistors, cathode bypass capacitors and cathode resistors.

The EL84s are pentodes, which means that there are five elements (cathode, grid, plate, and a pair of screens which catch rogue electrons and keep the tone flowing). There are resistors on the screens, which, just like the plate resistors, make sure that the voltage is at the proper level for the tube to operate correctly. There are a pair of resistors on the signal grids as well, which help to stop any radio frequencies which could get in there and cause squealing or oscillation. This resistor is called a grid stopper. They are often also used on preamp tubes. I’ve never used one on the input for The People’s Amp, in an effort to let as much signal as possible pass from the guitar into the amp. If there were ever a problem with radio signals, it would be easy to add one. Because we’re dealing with so much amplification in the power section, these resistors are necessary to keep things under control there.

And now, a brief word about distortion:
Distortion is what happens when the signal coming into an amplifying component (in our case, a tube), drives that component past its ability to amplify anymore. We hear different results depending on the type of amplifying component. If you have a boost pedal that heats your signal up before it hits the first 12AX7, it might cause that tube to distort, and we’ll hear what we call preamp distortion. It’s nice, for sure. To my ears, it’s sounds like a sizzle, or a buzz. On The People’s Amp, with the volume setting at lower levels, a harder pick attack can send the preamp into distorting. Some folks would call this “touch sensitivity.” The same results can be manipulated with the volume control on your guitar. Roll back a little bit, it cleans up. Full on, more preamp distortion. Most of the time when you hear a tube amp distort, preamp distortion is what you’re getting. On amps with a separate gain and master volume, the gain control is using one side of the preamp dual triode to overdrive the second one, then the whole shebang is shot down with the master volume before it hits the power section.

In your People’s Amp, the volume controls how much of the signal which has passed through the preamp makes it along to the power section through the phase inverter. The dual triodes in the preamp are acting as one gain stage, and it’s being run wide open. When all of the signal from the 12AX7 preamp tube is allowed to make it to the EL84s, it those power tubes saturate and they distort. Ladies and gentlemen, I’m here to tell you that power tube saturation is where it happens. That distortion is rich, thick, and creamy. You might notice with The People’s Amp that the amp doesn’t really get any louder from about 2 o’clock to maximum on the volume knob, it just gets more and more saturated. Amps that use a power soak, attenuator, or power scaling lower the volume after it’s been run through the power section. Personally, I’ve not found one that doesn’t also negatively affect the tone.

So now, your puny little guitar pickup signal is all beefed up and ready to go nuts. Unfortunately for your speaker, however a tube is a high voltage, low current, high impedance device, which is exactly the opposite of what your speaker is. They don’t play well together. Thankfully, our friend the output transformer is here to help. The output transformer is a hunk of iron that gets magnetized when there is current passing through the wires. Bigger pieces of iron handle low frequency sounds better, and prevent what is called “saturation,” not to be confused with the kind of saturation that happens to your power tubes. When an output transformer becomes saturated, it’s generating all the magnetic field it can, and won’t increase even if more current is present. This happens in low frequency signals first, and it 100% related to the size of the iron core. A larger transformer will open up low end, and make things more responsive, in general. The People’s Amplifier uses an output transformer designed for a slightly higher wattage amplifier. Through the magic of inductance, we get a low voltage, high current, low impedance signal that moves happily out of the amplifier into your favorite speakers.

“Well,” you might ask, “what about that other tube? What about the big ol’ power transformer? How about those DEADLY filter capacitors I’ve heard so much about? And, by the way, how do my tubes heat up, anyway?” These are all good questions, and you have stumbled into the power supply circuitry. This is the part of the amp that’s not really part of the amp, but the amp won’t work without it!

When you plug your amplifier into the power outlet and turn it on, alternating currnet “mains” power comes pouring on in (hopefully, it passes through a fuse on the way in, just to keep you safe). In the US, this happens at 120 volts at a frequency of 60 herz, meaning the signal swings positive to negative once a second. When you hear of 60 cycle hum or 120 cycle hum, you’re discussing what happens when this oscillation makes it into the signal path somehow. This is not good. What’s also not good is that most of the components of your amplifier run on direct current, not alternating current. I think it’s amazing that all of the electronics devices we use, not just tube guitar amplifiers, run on DC, not AC power. Pretty much everything you plug into the wall needs a transformer and some sort of rectifier that converts the AC current into DC current at the voltages required.

The first thing that happens is that the power transformer (on your People’s Amplifier, this is the tall black thing on top of the chassis), again through the magic of inductance, converts the voltage to things that the amp can use. In the People’s Amp, there are two sets of wires that we use on the amplifier side of the transformer. One of them is a low voltage (6.3 volts) that serves to light up the heaters, or filaments in the tubes. This is pretty much just like a light bulb. The other pair of wires carries 550 volts. This can kill you. Each wire carries half of that voltage to your EZ81 rectifier tube, which has had it’s heaters fired up by that other 6.3 volts. You might remember from From Glowing Wires to Amplifiers that one of the earliest discovered functions of a vacuum tube was to operate as a diode. Diodes are devices with two elements (in this case, cathode and plate) and they allow current to pass in only one direction. A rectifier tube does something else very special. It converts alternating current to direct current. Some amps use what is called a solid state rectifier to do this. The result is the same, but it takes four diodes to do the same work. Some folks will tell you that a tube rectifier provides an amp with something called “sag.” Without getting too technical, sag is what happens when the power tubes pull a lot of juice from the power supply, and it takes a little bit of time for the rectifier tube to recover. Solid state rectifiers recover much more quickly. Sag gives the amp a little squishiness, especially when things are really loud. It’s a nice subtle effect, and I think I like it, which is why I’ve used tube rectifiers in The People’s Amplifier.

From the EZ81 rectifier, the current then goes to your standby switch. What’s the deal with those darn things, anyway? One of the most frequently asked questions about tube amps is how to use the standby switch. The official answer is that when turning the amp on cold, leave the standby switch off for 30 seconds or so, to allow the heaters to come up to full value, then hit the standby switch to engage the high voltage. Some folks will tell you it’s safer, and that it will help to prolong tube life. That’s probably true. When your band takes a break, turn the standby switch off. This keeps everything warm, and prolongs tube life. When the gig is over, turn the mains off, then the standby. I’m not too sure what the reason for this is, but in your People’s Amplifier, this allows the draining resistor to pull the high voltage out of the filter capacitors, making your amp innards less deadly. The unofficial answer: it really doesn’t matter. I’ll probably stop making amps with one eventually.

Next up in the power train is those filtering capacitors that I just mentioned. These large electrolytic capacitors are like reservoirs, holding voltage until something downstream opens a “faucet.”  The current runs by three of them, filling them up. When we rectify alternating current, turning it into direct current, the voltage pulsates, carrying on the rhythm of the oscillation of the alternating current. When we run this pulsating current by those filter capacitors (think reservoirs!) it smooths it out to a constant voltage, which is what our tubes need to eat. The voltage at the first of our filter caps is around 388 volts DC, which gets fed to the plates of the EL84 tubes in the power section. It runs through the output transformer, which acts kind of like a resistor, and brings the voltage down to about 350 volts or so. That is a LOT of electron sucking power, which makes for an awesome amount of amplification. The voltage at the second filter cap has been reduced slightly by a resistor, and that gets sent along to the screens of the EL84. Those screens catch any stray electrons being sucked up towards the plates, and keep everything under control. Then, after passing through yet another resistor to bring the voltage down, the third filter capacitor supplies power to the plates of the 12AX7 tubes. This is somewhere around 190 volts. They don’t need as much, but they are more sensitive, so they need all the filtering that three caps can provide. By the way, this high voltage which is supplied to the tube plates is called “B+.” In the very old days of radio operation, there were four batteries used to supply the different levels of power to vacuum tubes. “A” for the filaments, “B” for the plates, “C” for the bias, and “D” for the screens. The “+” comes from its being a positive DC voltage, referenced to ground.

That’s it! I hope you enjoyed this stroll through each and every bit of The People’s Amplifier. All tube amps operate using these same basic structures. There are variations, for sure, but the basic theory is the same. In a future installment of TubeTalk, we’ll talk about how certain changes to component values may affect the output of the amplifier.

Public Service Announcement:
Tube amps deal with high voltages. If these voltages pass through you, they can stop your heart and kill you. Besides that, getting shocked feels awful. Please be careful. Those of us who are crazy enough to stick our hands inside amplifiers follow a few helpful rules. NEVER stick both hands in an amp at the same time. Your chances of giving that high voltage a path to ground are greatly diminished when only one hand is in there. Whenever possible, use a wooden chopstick rather than your fingers to poke around with. You should always be wearing shoes with an insulating sole when working on amps. Unless you are measuring voltages or adjusting bias, there is very little reason to be in there when the amp is on. Just to be extra safe, unplug the thing if you don’t need it to be on. Filter capacitors can hold charge long after the amp has been turned off. Using a large resistor connected to chassis ground, you can bleed the voltage off by connecting the other end to the positive side of each filter cap. I always check the filter caps with a multimeter to be sure they are at 0 volts DC before I get to work.

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