Amps 101 with Dan Boul of 65Amps, Part 1

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 (www.65amps.com), 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!

Part 1 – Input / Preamp Stage

SMG: Let’s start from the beginning and work our way through a basic tube amp. You get your guitar, you plug into the amp – what’s coming out of the cable?

DB: What’s happening is you have a magnet that’s charged on your pickup and it’s got copper windings around it. The more windings there are, the more it enhances the charge of that pickup. So you stick it perpendicular to your guitar string, which is metal, and it wiggles over that magnet. That vibration causes a disturbance in the magnetic field and that results in a tiny little charge. You’re talking about less than a few thousandths of an amp. That’s what you’re sending to your amplifier to make bigger.

SMG: What happens with that tiny electrical signal when you plug into the amplifier?

DB: Basically you have three elements in a tube: the plate, which is the positive side; the cathode, which is the negative side; and a mesh in between them called the control grid. When you run electricity through the cathode [negatively charged], filaments inside get hot – that’s what you see when the tubes light up – and release atoms [electricity] from the cathode. Electricity flows negative to positive so these atoms are received by the plate [positively charged]. The control grid is a negatively charged mesh that blocks the atoms from flying through until you send it a positive charge. When you hit that big chord, the signal goes to the control grid of V1 [the first pre-amp tube]. ‘V’ refers to valve which is what this is. You’re opening and closing the valve by charging the control grid.

SMG: What determines how much the valve is opened or closed?

DB: The volume knob on the amp. Because logically what you’re doing is feeding the guitar to the volume pot to the control grid of V1.

SMG: The atoms fly off the cathode and through the control grid. What happens at the plate?

DB: This is where the actual gain happens. So you’ve got all these atoms hitting this positive source (the Anode or “Plate”) and then it releases them on the other side in an electrical form. Whatever signal you’re putting in here to charge the control grid  gets increased anywhere from 10 to 100 times by the time it leaves the plate. The gain factor depends on how you structure the tube. It’s the relationship between the resistor you put on the cathode versus the resistor you put on the plate. If you put a 10K resistor on the cathode and a 100K resistor on the plate, that’s a gain factor of 10.

SMG: Most amps have multiple pre-amp tubes. Is this to add gain?

DB: Yeah. You run the output signal from one tube into the next. Every time you amplify the signal by running it through a tube you’re gaining it up. You can gain tubes up as much as 150 times before they start to misbehave.

SMG: How do you control clean and distorted signals?

DB: By adjusting the gain factor which is the relationship between the negative and positive side of the tube. This is what bias is. If the cathode and plate are closer together you get less gain; the further apart they are, the more gain there is and as you increase the gain in a tube it starts to distort. You can have multiple tubes in a row with a low gain factor to get really loud clean sounds or with the heavy metal stuff you’ll see five or six high gain stages.

Different amps have different levels of gain stages. Like the London, our first amp, only has a single gain stage – just one tube doing it. But a lot of our amps are two gain stages, and this high gain thing we’re working on right now has three stages, so it just depends on how you want to do it. You’ve got more options when you spread the gain across tubes. You can get a lot of really nice distortion, because you get a little bit out of each tube instead of having one tube just completely throw up. You’ve got more design options when you do that.

SMG: What choices exist for pre-amp tubes?

DB: 99% of people use 12AX7’s because they’re really gainey which means they’re the most flexible. There’s a whole family of 12AX, 12AY, 12AU, 12AT… They just have lesser gain. If I use a low gain tube like a 12AU it doesn’t matter what I do to it, I’m never going to get it up to the gain level that I would get out of a 12AX7 naturally.

SMG: Where does the EQ happen?

DB: It depends what kind of tone stack it is (the circuit that balances frequency response: treble, mid, bass). Just as a generic example, you’ve got your guitar signal going into V1, you amplify it up a little bit, and then run it into a tone stack. From there you go into V2. The volume knob can be here or anywhere along the pre-amp chain.

SMG: I thought the volume controls the charge sent to the pre-amp tubes. Wouldn’t you need that before V1?

DB: You can use volume in two ways. You can use it to drive the first tube harder or you can use it to squash the signal that’s coming out. The further it is in the chain, the more distortion you can get and then control the output. That’s what a ‘master volume’ is. The master volume is when the volume knob is placed between the pre-amp section and the phase inverter.

SMG: I’ve seen volume, gain, and master volume.

DB: There’s no difference – they’re just pots. You run a signal into a pot and it attenuates it, that’s all it is. You can use the volume control as a limiting device or as a gain device depending on where you put it in the chain. If you put a volume control in front of a tube it only controls the gain of that tube. If you put it at the end, that’s master volume.

SMG: Tell me about class A, class B, class A/B…

DB: Class A is usually cathode-biased stuff which means you’re getting more of the positive side of the signal’s sine wave. Both tubes are pushing at the same time where the negative side is a lot smaller. That’s why you get more power out of a grid biased tube. With a sine wave that’s class B, the positive and negative side are equal – one tube is pushing while the other is resting. Technically an amp with two power tubes can’t run class A. People throw that around. This is what’s called class A/B. So it really looks like class A but there’s a little bit of B. Even cathode biased amps are still technically running class A/B even though it’s hardly measurable because technically class A is just push, there’s no pull – no negative release. But you’ve gotta have some negative release or it just won’t work.

SMG: Is that an efficiency characteristic or a tonal characteristic?

DB: Both, actually. Class A has a different sound to it and the best comparison is like a Vox versus a Fender. Fender’s are class A/B – they’re very high fi and clean and stout sounding whereas a Vox is very soft and smooth, lots of compression, goes into distortion early… that’s more of a class A type operation. Now an engineer would ask you, “Why on earth would you ever run class A because it’s so inefficient?” Because it sounds good! It sounds great with a guitar. Not so much with playing recorded music through it, but playing a guitar through it, it sounds fantastic.

SMG: Why is that?

DB: Limited frequency range. The guitar is a midrange instrument – doesn’t go really low, doesn’t go real high. So in that operating zone, frequency-wise, class A sounds really good. But it’s inefficient in a very sexy way.

SMG: Tell me about effects loops.

DB: It’s a subjective thing. If you run your effects straight into the front then they’re going to get gained and distorted at each level [of the input stage]. But if you put them into the effects loop [between the input and output stage], it’s just going to affect what you’ve created going in. You’re not taking the effects and then gaining up and gaining up. That’s not good or bad, it’s just different. And if you’re playing clean and you want to have clean echo and clean reverb then you stick it in a loop. It depends where you’re getting distortion in the amp, too. Most of our distortion is coming from the phase inverter in the power section so the loop’s not really as effective. A lot of guys have in their head they have to have an effects loop, but it’s not necessary.

Stay tuned for next week’s installment of this 4-part series: Amps 101 – Output / Power Stage

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

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  • This is a great interview Dan. It’s awesome to learn a bit of the technical aspects about amps and what makes them tick. I look forward to reading the rest of the series…thanks!

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