I believe that every amplifier has a character or a natural presence, a tone that is fundamental to the components and the design of the amp. When modding, it is necessary to take a holistic approach with the goal of minimum changes and to have a plan to accomplish the objective. You cannot make a Blues Jr sound like a Bassman; you cannot force an amp to be something it isn’t. Keep it simple, and you will not be sorry. If you chase every mod recommendation on the forums, you will never be satisfied; you will spend more time modding than playing, your amp will be the worse for the wear and it will lose value. The approach I take is best described as “voicing an amplifier for harp.” This method includes setting the frequency response and the preamp gain of the amplifier for harp.
What makes a good harp amp?
To make it simple, there are three primary design features in a good harp amp: low end frequency response, correct preamp gain, and a good speaker. It really isn’t more complicated than that.
The number one component is the coupling capacitor; this, along with the speaker, sets the frequency response of the amp. The Fender Bassman is great for harp primarily because it uses .1uf coupling capacitors and utilizes 10 inch speakers; these features are ideal for harp because together they pass the lower frequencies and provide for a fatter tone. Coupling capacitors carry your signal from one amplifier stage to the next; there may be 2 to 5 coupling capacitors in an amp.
Next is preamp gain. Because guitar amplifiers are designed for the very low signal produced by guitars’ pickups, they have a lot of preamp gain. A microphone produces twice the output signal of a guitar pickup. The end result is we cannot turn our amp up past 2 or 3 on the volume control without feedback. How much gain do we need for harp? Only enough to saturate the power tube and no more; a good rule of thumb is that you should get to at least 7 or 8 on the volume control before feedback. Preamp gain can be tamed by a simple tube swap, the installation of a voltage divider, or the removal of the cathode bypass capacitors in the preamp.
The speaker is critical in achieving good harp tone, yet it may be the most overlooked component. How well the speaker breaks up, the frequency response, and the speaker’s sensitivity can make an average harp amp a great one. A couple notes on speakers: speakers with larger voice coils can be significantly louder than the same size speaker with a smaller voice coil; also, going up in cone diameter increases volume and low end response. Increasing speaker size can be the best way to make your amp louder. Another consideration is a smooth cone speaker will be warmer than a ribbed cone speaker, which sounds brighter. Two great harp speakers are the Weber Vintage Series and the Weber Signature Series in 8, 10, and 12 inch, smooth cone with the H dust cap. When comparing speakers with Alnico or ceramic magnets, the technical properties of the two are different and they produce different tones when overdriven. The Alnico breaks down slower and produces a smoother breakup than the ceramic, but the difference is very subtle and may not be worth the difference in cost.
When purchasing an amp, you can save money by passing on amps with boutique components like oil-filled capacitors and paper-wound transformers, which are not necessary because the human ear cannot tell the difference between them and less expensive components. One thing to remember is the vintage amps we love were built as cheaply as possible. Maybe an argument can be made that states there is a synergistic result from having all of the boutique parts; possibly so, I personally do not think the difference is discernible to the average human ear.
Low preamp plate voltage is sometimes recommended for harp amps. I have experimented extensively with low preamp plate voltages and found there are a couple things happening in the circuit when the plate voltage is lowered below 100 volts, and these same occurrences are magnified when below 90 volts. Under this extreme parameter, the tube draws much less current, which causes a drop in bias voltage. This is significant because it does two things that affect the tonal aspects of your amp: first is the gain of the tube drops, which is good for feedback; the second is the tube becomes highly overdriven, producing excessive even-order harmonics. Some even-order harmonics are essential for good tone, but under normal bias conditions, the harmonics produced are sufficient. When excessive even harmonics are produced, your tone becomes muddy and loses its texture. I recommend preamp voltages between 110 and 175 volts; there is really no magic voltage, just as long as you have good operating parameters for you tube.
The Relationship Between Wattage and Loudness
Sound is measured in decibels, which is a measurement of sound pressure; one watt equals 30db. Power has to be doubled just to reach 33db, which is an increase of 3db, and this is only slightly louder; less than 3db is hard to detect.
An increase of +3 dB or going from 5 to 10 watts = twice the power, but is only slightly louder.
An increase of +6 dB or going from 5 to 20 watts = twice the amplitude and is noticeably louder.
An increase of +10 dB or going from 5 to 45 watts = twice the perceived volume or twice as loud.
So, it pays to focus on tone rather than wattage when choosing an amp. I do not recommend choosing an amp 5 watts higher because you want to be louder when the one that sounds better is only 5 watts less. It is easy to gain 3 to 5db of volume by choosing a speaker with a higher sensitivity, larger cone diameter, and/or a larger voice coil. If you are not quite loud enough, changing your speaker or adding a speaker may be a better option than getting a new amp.
When shopping for an amplifier for harp or modding an amplifier for harp: consider the number of preamp stages, the fewer the better. Consider the preamp gain, can I swap tubes easily? And pay attention to the speaker type and size. As for the coupling capacitors, well, if you are looking at guitar amps, they will almost certainly be the wrong value for harp. But, if you pay attention to the aforementioned items and get at least a 10 inch speaker, then your amp can still be a great harp amp.
My name is Randy Landry, and although I am currently an operator at an oil refinery, I am an electronic technician by trade. I also own and operate Lone Wolf Blues Company, where we make effect pedals and amplifiers for harp players.