Amps 101 with Dan Boul of 65Amps, Part 3

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!

CABINETS AND SPEAKERS

SMG: Continuing along our journey, what happens when the signal leaves the output transformer and joins the speaker?

DB: The electricity connects to the speaker’s magnet and the voice coil. When the magnet gets charged, it heats up and becomes more magnetic and that moves the voice coil back and forth. The voice coil is glued to the cone so as the voice coil moves back and forth, the cone moves with it. The cone is nice and round and oscillates at the same frequency of whatever signal we’ve taken and beefed up.

SMG: Tell me about cabinets: the importance of open back designs vs closed back designs, wood choices…

DB: The wood is a good place to start. The cabinet vibrates in sympathy with the speaker. So just like this wood sounds different from that wood [knocks on two different desks], the same thing’s going to happen when you make a cabinet out of a harder or softer wood. The wood quality makes a difference, like an acoustic guitar. A guitar with knots all over it doesn’t sound very good. Same with a speaker cabinet. We use furniture grade Baltic birch so there aren’t any knots or voids (gaps in the wood), because it sounds better. If you use lesser grade wood, it sounds less musical. You don’t get much sympathetic vibration, you just kinda get mud.

SMG: It sounds like you should find out what kind of wood is being used when shopping for an amp.

DB: Yeah, not only is the wood important but the joinery is important, too. We use half blind dovetail joints, which is technically stronger than the wood itself, versus nailing a cabinet together. It’s a lot cheaper to make it with nails but nails don’t sound very good. The wood really makes a difference. An acoustic guitar is the best analogy. If the top on your guitar was nailed down it probably wouldn’t sound nearly as good as one that was joined correctly. You’re not going to get that vibration through the whole instrument. As far as the back goes it’s really about building up air pressure in the cabinet. The more air pressure you have, the more bass you’re going to get and the punchier it’s going to sound. And you can create different types of ports to adjust and massage that. We do a horizontal port across the back that’s specifically tuned to keep a lot of the air pressure inside but then let the speaker breathe. If you build up too much air pressure, then the speaker doesn’t move very well. There’s back pressure on it, so this way the speaker can still throw really far but it keeps enough air pressure inside the cabinet that it sounds fat and chunky.

SMG: How about speaker impedance and series vs parallel.

DB: Impedance is how much push the speakers are going to give back to the output transformer. If you don’t have those matched then the output transformer won’t work correctly. If I tell the output transformer it’s either going to see a 16 ohm load or an 8 ohm load it will beef up the signal accordingly. The load is what the output transformer is going to see from the speaker. It’s a balancing act. There’s a primary and a secondary. The side of the output transformer that puts the signal out is called the secondary. If you have two 8 ohm speakers in series it adds up to 16 ohms. If you have them in parallel it divides. It’s like a weight bar. If you put two 8 pound weights on the end of the bar you’ve got a 16 pound weight, but if you spread them out in parallel it’s easier to lift. You want to use the highest ohm rating you can because it uses the whole transformer. To balance for a 16 ohm load it has to use the entire thing. When you set it at 8 ohms it’s only going to use about 2/3’s of it. And the more windings you use, the better it sounds. There’s a trick to it. Guys will set up 2 ohm and 4 ohm loads because if the amp is misbehaving, you only want to use a little bit of the output transformer because it’s going to reveal all of the non-musical crap that’s going on in the amp if you use the whole transformer. That’s a big tip-off on the quality of your amplifier. If it runs down to 2 or 4 ohms then they’re getting by with a cheap output transformer or cheap parts on the amp.

SMG: I’ve heard you can blow up your amp if it’s not plugged into a speaker and you turn it on.

DB: If it’s a cheap output transformer, yeah. It’ll cook if it doesn’t have a load on it. It’s not balancing so if the output transformer is pushing really hard with no load on it, it just goes wild. If you stand on the accelerator of your car and it’s not in gear, there’s nothing pushing back against the engine and it just goes “WHRRRRRRRR!”. You want to make sure it balances. With [high quality] Mercury transformers, you can run without a load for days and it’ll be fine. With cheap ones you can cook it in 20 minutes. It can be dangerous. You want to get the load right. If the output transformer is expecting a 16 ohm load and it has an 8 ohm cabinet, there’s an imbalance there and that output transformer and the tubes are going to run really hot.

SMG: Is that what impedance matching is? Matching the output transformer to the speakers?

DB: Yes. You also want to match the impedance of the output tubes to the primary of the output transformer. When we make ours, we tell Mercury “We’re going to use tubes that fall in this very narrow range.” so they can really bring the detail out of an EL84. If you don’t tell them that and they build one that kind of has a wide net, then it really doesn’t make much difference what kind of tubes you use.

Stay tuned for next week’s installment of this 4-part series: Amps 101 – Features and Wrap up

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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