Connecting a Head unit to an existing amp

Personally I’d like to recycle my old equipment if I can. When I decided to jump up to a tube amp, it seemed natural to just get a head unit, and use my existing amp as a speaker cabinet. With about $20 in parts from Radio Shack I was able to convert my existing $100 amp into a great sounding speaker cabinet for my tube amp. Below is my thought process that led me to the route I took. Below the cut is the actual, very easy, how-to.

So as soon as you get your first three chords down consistently and have an appreciation for tone (or at least you think you do!) you immediately stop browsing the bargain aisle and start perusing the tube amp section. So now, if you (I) were smart, you initially bought the crappy amp with the 12″ speaker instead of the 6″, 8″ or 10″, even though that upgrade cost you all of $10.

Let’s review what you get real quick with that extra $10 bucks:

  1. More low end sound
  2. Won’t break up at high volumes (play louder without “bad” distortion)
  3. More durable speaker
  4. Replacing the speaker is really easy

Let’s look at number 4 there. So you bought (or I bought, in this case) a Crate GTD15R (now called the FW15R), which is a 15 watt amp with a gain knob, reverb knob, and headphone jack/ipod line in. Really good price/feature for $100. Probably the most important feature is that it has a full size, 12″ speaker. 6, 8 and 10″ speaker amps don’t make a good investment and tend to sound terrible. Also doesn’t have a shit-ton of confusing amp synth adjustment crap on the front. An amp should never, ever have an LCD display. WTF. If your instruments need an instruction manual You’re Doing It Wrong. Go buy a Line 6 POD if you’re that desperate.

Rant over. So you bought a starter amp with a 12″ speaker, and now you’ve decided you want that glorious, warm vintage tube tone. In the all-tube amp $200 and under realm (very small realm) you have two options a) the Epiphone Valve Jr. ($150 anywhere) and b) the http://www.amazon.com/gp/product/B002L7F72G?ie=UTF8&tag=neadea-20&linkCode=as2&camp=1789&creative=390957&creativeASIN=B002L7F72G ($200 anywhere). The Epi comes in two flavors, head only ($150) or Head ($150)+Speaker ($150 = $300). Well hey, you (I) say, I already have a box with a 12″ speaker in it, why not just plug the Valve Jr head into that?

8.jpgActually it’s that easy. Most lower end amps come with a vented, or open back, giving you easy access to the speaker cone and the soldier points on the back. You can litterally solder and/or crimp some wires from a 1/4″ phono plug (radio shack pt# 274-255) to the speaker cone contacts and be finished in about 60 seconds. This has some drawbacks though; you can never, ever have both on and connected to the speaker (likely to fry your old amp, speaker, tube amp (in that order) or all three), and for safety’s sake you’ll probably have to unplug one from the other incase your friend decides to try one out without asking first. As always, there’s a work around.

IMPORTANT! Working with electronics can kill you if you’re not careful! Anything involving a soldering iron immediately voids your warranty on any product. That said, unplug everything you’re working on, you should be ok, there’s nothing high wattage you’ll come into contact with and unless you start unscrewing sealed metal boxes, is perfectly safe. We’re working with open backed (ventilated) amps here, so all the dangerous stuff is nearly tucked away from you already by the manufacturer.

So you simply stick a switch on there and soldier this thing up. Essentially what you’re doing is creating a slave circut for your old amp, and a switch to go between analog or digital. With a switch in place, both can be on with zero interference or damage taking place. Additionally, everything is DC so as long as you don’t cross wire anything (pretty difficult to do) you don’t have to worry about which is the positive or negative terminal(!). Easy-peasy “my first electronics experiement” stuff.

WARNING! Make sure the number of Ohms (omega symbol) on the speaker matches the number of Ohms coming out of the Amp Head. In the case of the existing amp, it’s matched to your speaker. The Epiphone Valve Jr. (version 3) has three different outputs (4, 8 , 16 ohms) depending on your setup so you should be ok. Other amp heads may not have this luxury. Look at what you’re doing. You’re not going to blow anything up in a shower of sparks if you mismatch ohm ratings, but you’re going to prematurely wear out either a) the speaker or b) your amp. Be careful.

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This is what’s called a DPDT switch, or Dual Pole (+ and – on each flip of the switch) Dual throw (on A or on B) switch. Radio Shack also has things like SPDT (Single Pole, Dual Throw) and a variety of other options. You want the DPDT switch (radio shack pt# 275-653) . The pins are arranged like this -> : : : <- The left and right colons (:) are the inputs (Crappy digital amp, Tube Amp), and the middle colon is the output (Speaker). So! Find or buy about 12 feet of black wire, 12 feet red wire, and start soldering up!

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In my case, the connector between the digital amp and the speaker was a crimped connection, so I simply slid it off the speaker and slid it onto the switch. Soldered a pair of black and red wires from the 1/4″ phono plug to one side of the switch, soldiered a pair of B&R wires from the center of the switch to the speaker, and then slid the digital amp’s crimped connectors onto the other side of the switch. The trick to soldering is to lay the hot tip of the iron on the electronics you’re about to apply soldier to for a good 10 seconds to let the surface heat up. This helps prevent the sauder from balling up (cooling down/solidifying too quickly) in the wrong spots and helps it “flow” into nice, neat looking solder pads. This is probably the most overlooked piece of advice ever when it comes to soldering. Anyways plug the new amp head into the 1/4″ phono plug you just soldered up, and voila! Working slave circut!

But it looks ugly as fark, and exposed metal circuits hanging out where they can bump into your (magnetic!) metal speaker cone support is bad. Project enclosements Good! I got one of these at Radio Shack (did I mention ALL my parts are from Radio Shack?). Part number 278-2092 Snap-In 2-Port Surface Mount Box. They run about $4 and beside the solder (also $4) it’s the most expensive part of this whole project. Snap it apart, mount the bottom to the side of the inside of the amp. Drill some holes in the top half for your switch and 1/4″ phono plug (or speaker jack is the technical term at this point), paint it black with some black laquer spray paint ($3, home depot – glossy black FTW). The switch and phono plug have threaded tops just like most non-Strat style phono plugs, so unscrew the bezel, take off the washer, push the plug/switch through the back of the hole, and then screw the washer/bezel to lock it in place.

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The above pic is the only detailed “wiring diagram” I have for you. If you follow the red wires it’s easiest to understand. The switch, on the left, has three pairs of red and black wires feeding out of it. The center pair output to the speaker directly. The 1/4″ phono plug (right of switch) feeds into the far left pair of connectors on the switch. The prexisting amp (shiny metal thing in upper left corner) feeds into the right pair of connectors on the switch. With the flip of a switch you can go from internal amp, to external amp feeding the speaker!

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Carefully thread the wires through the back of your mount box, and snap the top to the bottom piece. You now have a functioning half stack(!!!). I have my switch set so that if you flip the switch up towards the jack, you get the jack input. If you flip it away from the jack, you get the alternate (Crate Amp) input. I should also note that the mount box (for the switch, phono plug) isn’t flush with the outer edge of the speaker box – it’s set in quite a bit, so that the switch won’t snag on anything, and the phono plug won’t interfere when pushing the speaker cabinet against the wall.
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Things to do:

  • Add “4 ohm” label to jack
  • Upgrade internal speaker to Celestion 100 dB sensitivity speaker
  • Rock out
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One Response to Connecting a Head unit to an existing amp

  1. Xth says:

    just letting you know – your site is fantastic!

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