Amps 101 with Dan Boul of 65Amps, Part 2

Most of us don’t know, and don’t need to know, how a barely audible plunking of strings mounted over a couple of magnets on a plank of wood can transform into a wall-shaking monster of musical expression. For those of us who like to get under the hood to gain a better understanding of what’s actually happening, you came to the right place.

SMG sat down with Dan Boul, co-founder of 65amps (, to break down the journey of the guitar signal from that first tinny strum through to its glorious amplification.

Dan Boul of 65Amps recently shared his amp expertise with SMG!


SMG: What happens after the pre-amp stage?

DB: The signal goes to a phase inverter which takes half your signal and flips it over before it reaches your output section – your power tubes. Every Push/Pull amp has at least two power tubes and they come in even numbers. One tube pushes half the signal and one tube pulls half. That’s called “push-pull”. Ultimately you want a waveform which is a sound wave your brain can enjoy. If you don’t correct the phase of the signal as it comes from the pre-amp section to the ouput section it’ll just sound distant and hollow. If it’s perfectly out of phase you won’t hear it at all.

SMG: The point of the power tubes are to…

DB: That’s big amplification. That’s when you get into watts.

SMG: Let’s say we come right out of the preamp tube into the speaker, what do you get?

DB: You would barely hear it. No tube can do that much amplification at once. If you take your signal and run it right into a power tube, it’s not going to be very loud. But when you take a bigger signal and feed it into a power tube, that’s when you get a net of 20-30 watts or more depending on the power tubes and the voltage they are utilizing.

SMG: It’s a matter of raising the volume in stages to get it to the point where you want to be?

DB: Yeah. And you choose how you want to do that. Do you want to do it in an ugly, mean way so you get ratty distortion? Yeah, sure – sometimes. Or do you want it to be big, fat, and sweet. You know, John Mayer’s kind of thing is not really gained up. It’s a big, fat, tube sound but it’s pretty clean. That’s ramping it up slowly. You take your power tubes to your output transformer which transforms the output signal into something that speakers can move.

SMG: There are different power tubes to choose from?

DB: Yeah, and they all have different characteristics. For guitar amps there are only six or seven that have ever been used that have traction. European tubes start with letters and American tubes start with numbers, which is your first clue. When someone says, “I have a 6L6 amplifier”, you can naturally assume that’s an American sounding or American type circuit vs and “EL34” amp would be a European amp generally speaking.

SMG: And that’s what’s meant by American-voiced or British-voiced?

DB: Yeah. American tube amps tend to focus more on Treble and Bass whereas European tube amps tend to focus more on the midrange frequencies.

SMG: Aren’t there also Russian and Chinese tubes?

DB: Yeah, but they use our numbering system. EL34’s and EL84’s are the two most popular European tubes. Marshalls all ran on EL34’s. Vox’s, for the most part, ran on EL84’s. EL84’s are the little skinny ones and EL34’s are big tubes. And there are little variations like KT88’s and KT66’s which are European tubes. The first Marshalls ran on KT66’s, which is sort of the European equivalent of a 6L6 but just sounds slightly different. If you look at them, KT66’s are sort of like a Coke bottle with a big fat plate on it. 6L6’s are very phallic – just a straight long tube. Physical construction makes them sound different. For American tubes, the two most popular ones are 6L6’s and 6V6’s. They’re very similar but the 6L6’s are bigger – they have more output. There’s no good or bad, it’s just personal taste.

SMG: The signal coming out of the power tubes, or output section, feeds into the output transformer. What does that do?

DB: The output transformer takes the sine wave electric signal and transforms it into something that’s usable by a speaker. That’s why it’s called a transformer. It increases the signal without gaining it up. Even though we’ve made the signal a lot bigger it’s putting, not gain, but an enlargement factor on it. I don’t know that you could drive the speaker without it because you’re still talking about really low level amounts of electricity, but there’s no gain or changing of the signal in the output transformer. The better ones are more transparent so the better the transformer, the more your original sound is coming out.

SMG: So if you have a crappy transformer it could color your sound.

DB: Yeah, because sometimes it filters out good stuff and adds non-musical distortion. With transformers it’s a direct scale – the more you pay, the better it is. You can do whatever you want before the output transformer, but if you’ve got a cheap transformer your amp sounds cheap. It’s also the most expensive part of the amp. These things are really simple. It’s kind of 8th grade intelligence but incredibly fun because of the results, you know like, “Oh shit – listen to that! It’s great!” And there’s really nothing more fun than blowing up amps. The best part of prototyping is blowing up amps.

SMG: You intentionally blow up amps?

DB: Well, no. We push them to their limits. We find out where the upper limit is because tubes basically hate voltage and like current so what you’re doing is irritating tubes on purpose. I’ll pound them with voltage until they give up and wherever that is I’ll back it down a little bit and see if it gives up later because I want something that’s reliable. So we’ll back it down to the most voltage the tube will take before it fails. When you irritate a tube, it distorts. If you irritate a tube in a precise calculated way it distorts in a pleasing musical way.

There is also a power transformer that takes the electricity from the wall. What does that do?

Here in the States electricity is rated at 120 volts, 60 Hertz. Depending on what you need for your application, your power transformer will take those 120 volts and turn it into 300… 400… 500 or more volts.

The electricity coming out of the wall is not enough to drive all the tubes and components?

No, not in a useful way at all.

SMG: But before it drives the tubes and everything it goes through a rectifier.

DB: Right. Most of the amp uses DC instead of AC so the rectifier tube converts AC to DC. It’s an on/off device. It shuts off on the negative cycle of the sine wave so you’re left with just the top. It does this at such a high frequency you can’t hear the dropout and all you end up with is straight DC which is what most of the amp needs to use.

For the components that require DC, they feed off the rectifier and for the components that use AC, they feed off the power transformer.

Yeah, that’s it.

There are analog tube-driven rectifiers and solid state rectifiers. What’s the difference?

A solid state rectifier is just done with diodes. It’s an on/off device. A tube is a diode but it works as a vacuum tube instead of a silicon chip, so it reacts differently. It’s a little smoother and softer and tends to add a little more harmonic content into the electricity because it’s operating exactly like the electricity coming out of the wall. Solid state is just on or off – there’s no rise or fall, whereas a tube tends to come on and swell down and swell up and it’s a little smoother and more organic feeling and sounding. You can hear the difference side by side.

SMG: Are there high end amps that use solid state rectifiers?

DB: Above about 50 watts you have to use a solid state rectifier because the highest a tube can really handle power is about 40 watts. So anything you see that’s 50 watts and above is mostly going to be solid state. Except for some Mesa Boogies – they use dual rectifiers.

You’ll get some amps that are small wattage and sound huge and then you’ll get amps that are 50 or 100 watts and don’t sound nearly as loud.

Right. People lie [laughs]. There are different ways to measure output. You can get three different answers measuring it three different ways. There’s a limit to what tubes will do. With EL84’s [for example], I can realistically get about 22 watts out of them. I’m not going to get 22 watts clean. They’re going to start distorting around 15 watts or so. But things like the cabinet and speaker that are involved…if you use a very high quality speaker which has high SPL – sound pressure level – you’ll get more sound for the same amount of electricity coming into it. The cabinet quality makes a lot of difference. The output transformer makes a huge amount of difference. Our 18 watt transformer is louder than most people’s 30 watt output transformers because it’s made with silicon-free steel and fine grade copper windings and all that stuff. It’s about how efficient the amp is at that point. You can still measure 20 watts out of an amp but it might be 20 watts of “who cares?” Then the type of cabinet and speakers. You can lose a lot there. Some people like low SPL speakers because they sound mushy and brown, and there’s a purpose for that, but they’re not loud. So if you hear a low power amp that’s really loud, that means it’s pretty efficient. And there are ten different ways you can be efficient. I’ve got a 12 watt amp that’s just louder than hell. People can’t believe its 12 watts, but its 12 complete watts. And two speakers are not twice as loud as one speaker, it’s just marginally louder. A 100 watt amp is not twice as loud as a 50 watt amp, it’s 10% louder. There’s a point of diminishing returns past about 20 or 30 watts. After that you’re just making it bigger and fatter.

Stay tuned for next week’s installment of this 4-part series: Amps 101 – Cabinets and Speakers

Dan Coplan is senior staff writer at SMG. Dan is a Los Angeles based cinematographer and self-admitting guitar junkie. Email:

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