Set your echo to produce a single repeat at about a tenth of a second or less, as if your sound was reflecting off a nearby wall. Reverb should not last more than a few tenths of a second. After that, it begins to muddy up your sound and instead of filling it in, starts to cloud it over.
We cringe at having to use a guitar to check this, but it's a good source of ultralow frequencies. Just bump the strings, right over the pickup, with the heel of your hand and release slowly. A flashlight helps to see the cone action if you can't see the back. The speaker cones of both the amp and extension should jump in the same direction. You need to consider matching the impedance, however.
Tube amps are more tolerant of low impedance mismatches, but are intolerant of opens and higher impedance mismatches. Using too high an impedance for a tube amp can turn the tubes or output transformer to smoke. If any of this sounds confusing, there's a good reason -- it is. Those of us who don't know what they're doing in this area should consult a lot of books or someone who does know.
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A mismatch is like hooking a big pipe to a small one (or small to big). Either way, not enough water flows and, in your amp, not enough current flows to get the most sound out of your microphone. OK, so an impedance matching transformer changes one to the other (at this point, the analogy doesn't hold water).
The Honker output is high impedance. If you need to plug in to a "direct-inject box" on a stage where the mixer board / P.A. system is some distance away, the lines from this "D.I. box" to the rest of the system will invariably be low-impedance. This will require an impedance-matching transformer. There are other ways of doing this, but a simple transformer is the most widely used. Avoid cheap ones, you should pay $30 - $50 for a decent one. If you need other signal processing, like a preamp, then get something that also has a low-impedance output. You'll save a few bucks and it'll be more convenient.
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The (Full) Honk sound is based on clipping and will sound quietest because it is limiting the peaks of your signal. This is where you get the most compression, that is, your volume won't drop out when you open your cup up for wah effects. At cleaner settings, you will get greater dynamic range and whether your'e blasting or pianissimo, that's what will come out of your amp.
You may find a "sweet" spot on the Honk control at about "12-o'clock" where it's loudest and you get a fair balance between Clean and Honk sound.
A starting point for harp amp settings is with the BASS all the way up, the TREBLE and MIDDLE (if any) all the way down. Listen to that, and add a little MID or TREB a little at a time if you need more bite.
Standing to the side of your amp, a little to the front (or back) and as far away as you can get (and still hear yourself) will also help. Mic'ing your amp into the PA can also allow more loudness because the PA speakers will be farther away from you than your amp. You will also want to keep your signal out of the monitor mix, since this can cause its own feedback loop. Use your own amp for a monitor, it's easier to hear yourself when it's coming from a source that's separate from the rest and it can even be pointed right at you if it's mic'd. Now, the HEARAMP will give you a Personal Monitor built into the Honker.
Room acoustics will have a great effect on feedback as well as your overall sound. When you're playing a new place and you get a chance while everything is reasonably quiet, clap your hands (once every few seconds) and listen to the resulting echo. This will tell you how "live" the room is.
A "live" room will have a definite echo, maybe off a back wall, or maybe just a "reverb"-type echo. If you don't hear much of anything back, the room is "dead", acoustically, (normally a good thing) and will tolerate more volume out of your amp before feedback happens - at least from room echoes. (There are several more ways feedback can happen.) Remember that when the room is (hopefully) full of people, it will be "deader" than when it's empty.
A "dead" room will also require a little brighter tone because the highs disperse (get absorbed) more quickly than the lows. This is all for rooms that aren't designed for sound. If you listen to the sound of one hand-clap in a concert hall, it will sound like a symphony.
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