Speaker Parameter Basics for Tube Guitar Amps
There is lots of confusion amongst players about amp speakers. There are so many of them out there and it's difficult to know what speaker to pick. When designing my amps I experiment with different speakers and speaker setups to get the best sound possible. Speakers make a huge difference in an amp’s sound thus speaker selection is critical to having a good sounding rig.
I learned this lesson with the first amp I ever built. It was a 1/2 wattTweed Champ type amp with a solid-state rectifier. After finishing it I plugged it into my trusty Celestion Vintage 30 loaded 2X12 and expected to hear that great Champ tone. Nope. What I got was a semi-clean sound that sounded really muddy. I was sure I did something wrong when wiring it. I went over and over and found nothing. I was using the wrong type of speaker to get the sound I wanted! So what went wrong? Why would a revered speaker like the Vintage 30 perform so poorly in one context and so well in a different context?
There are a number of parameters that shape the sound of a speaker. Here I will briefly cover them and how they affect tone. By understanding these parameters you will be better equipped to select your own speakers. You will even be able to look at speaker and get and idea of how it may sound. In a previous post I went over Ceramic and Alnico Magnets so I’ll skip that topic here.
Common speaker sizes are 8, 10, 12 and 15 inches. Some Gibson amps even used 6X9 Speakers. In general larger speaker reproduce more low end than smaller speakers and are louder. Different speaker size exhibit different sound break up as well. The smaller speakers tend to break-up quicker and usually (but not always) have lower power ratings.
Voice Coil Size:
There are different sizes of voice coils. For example many 50’s Jensen Speaker used 1" or 1.25” voice coils while most British speakers used 1 ¾” voice coils. Generally speaking larger voice coils have more bass and mids but roll off some treble. Smaller voice coils are more touch sensitive, brighter and tend to be found in speakers with earlier breakup.
Speaker Cone Paper Thickness and Type:
There are different speaker cone types. Without going into too much detail thinner cones are brighter, more touch sensitive and breakup easily. Thicker cones are smoother sounding because they roll of some highs and breakup less quickly. Paper cones are the most common but some speakers use hemp cones are darker and roll off treble. Some speaker have cloth surround that further dampens the high end.
Speaker Cone Ribbing:
Speaker cone many cones have ridges know as ribs and others have rib-less smooth cones. The more ribs the more detailed and brighter sounding a speaker will be. However more ribs means less speaker breakup. The shape and size of the ribs also affects the sound. Many late 40’s and early 50’s speaker were smooth cones (no ribs) which gave them a distinctive crunchy breakup. Many bass and hi-fi speakers also have smooth cones but are designed not to break up but they tend have large magnets and voice coils.
Speaker Magnet Size:
Speaker have different sizes of magnets. In general larger magnets mean a louder speaker with less breakup. Larger magnets are more efficient and control the cone movement more. Larger magnets tend to stiffen the sound and permit less speaker distortion.
A dust cap is the round piece of the cone material right in the center of the cone. Large dust caps diffuse highs and often used to tame treble. Many harp player add larger dust caps to speaker to help make the speaker sound less harsh with the harp. Paper is smoother sounding that the Aluminum dust caps you see on certain speakers.
As you can see there are lot a variables. To give an example I’ll describe two classic speakers: a late 50’s Jensen P12Q and the Celestion Vintage 30.
The P12Q used and small Alnico Magnet, thin paper cone material, 1 ¼” voice coil, a the cone had lots of ribs and a small dust cap. It was a 25 Watt speaker. So the result would be bright, detailed touch sensitive tone from the thin cone, small voice coil, small Alnico Magnet, and ribbed cone. Since it had low power handling, a small voice coil, etc. it also breaks up quite quickly and is not a efficient speaker. The P12Q was paired with vintage amps that had lot of mids and lows but not lots of high end. The speaker compensated for that and since most 50’s amp were relatively low gain speaker breakup is essential to the sound.
The Vintage 30 uses a large 50 oz ceramic magnet, thicker cone material, 60 watts of power handling, a larger 1 ¾” voice coil and plenty ribbing with a small dust cap. The result is a loud speaker that is hard to breakup, bright but with plenty of low end, and relatively smooth breakup. The Vintage 30 is often paired with Hi gain amps. These amp have lots of high end but often lack lows and mids when run clean. The speaker compensates somewhat for that. The speaker does not break up easily.
This explains my story about the first amp I built. You can see how a speaker like the Vintage 30 would be a poor choice for a Champ. Likewise a vintage Jensen would be an equally poor choice for say a JCM 800. I hope this is helpful to someone!