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Vintage threads from the first ten years

Installing a Lineout Jack

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11/5/1997 11:05 AM
David Axt	Installing a Lineout Jack I would like to install a 1/4" line out jack on my amp. This way I could use my ampt for recording or for headphones. It would also be nice to install an XLR jack so that I could be patched directly into the sound board. How can I do this? Thanks.

11/6/1997 12:31 AM

Installing a 1/4" unbalanced line out jack is actually quite easy. It's especially easy if your amp has an extension speaker jack that you don't need. Converting an existing ext. speaker jack to a line out means you don't need to drill a hole in the chassis and install a new jack.

If you're converting the ext. speaker jack, first remove any wires that connect it to the main speaker jack(s). If you're installing a new jack, try to locate it as close as possible to the speaker jack(s). Now connect the hot (tip connection) terminal of the line out jack to the hot terminal of the speaker jack using a 2K2 ohm 1/2 watt resistor. Next, connect a 100 ohm 1/2 watt resistor between the hot and ground terminals of the line out jack. If the line out jack is a type that makes a ground connection to the chassis, you're done; otherwise, you'll need to connect the ground tab to chassis ground via a piece of wire. The two resistors form a voltage divider that create a line-level signal from the speaker signal. There will be absolutely no change on the tone coming from the speakers.

For direct connection to a sound board, I recommend using an external direct box, connecting the line out jack to the box using a short patch cord. You could also install an XLR jack on the amp, but you'll definitely need to cut a big hole for it. Also, you'll need to install an impedance matching transformer (which is small) that goes between the line-out signal (derived the same way as I described above) and the three terminals of the XLR jack.

I would stick with the external direct box. Then, you have a choice of trying out different ones to see which sounds better. For example, the Hughes & Kettner Red Box is a direct box that has active circuitry that simulates the coloration created by a dynamic microphone placed in front of a 4x12 speaker cabinet - a much better sound for direct guitar IMO than using a standard direct box. There are other cab simulators on the market as well.

11/6/1997 5:53 AM
Mike Oriente	Will the same Line Out trick work on a combo amp w/out a speaker jack by taking the signal directly from the speaker connection?

11/6/1997 8:16 AM

Yes. The positive terminal of the speaker (usually marked by either a "+" or a red dot) corresponds to the hot terminal of a speaker jack and the negative speaker terminal corresponds to the ground (or common) terminal of a speaker jack. This is a great way to use smaller, low-powered old tube amps as preamps without doing any alterations to the amp itself. You can really capture the sound of a small amp that's cranked! (As you probably know already, you still MUST have a load - either a speaker or load box - connected to the amp output.

By the way, this line out mod is described in the Tweed Deluxe section of Gerald Weber's amp book.

11/6/1997 10:50 AM
Andy	I am absolutely amazed that this is all you need to do to install a line out on an amp. Learn something everyday. Thanks!

11/6/1997 5:15 PM
Mike Oriente	What's in a load box? I haven't seen any schematics for that on the web. Of course I'd rather make my own then buy one.

11/6/1997 11:05 PM

Jim S.
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I second your motion about using a direct box like a H&K Red Box for running a signal into a recording board. The speakers filter out a lot of the extraneous noises which would otherwise go right into the board (and would be that much more noticable if you are using headphones). The Red Box filters out a lot of that garbage. The line out circuit you describe is more appropriate for running into the power amp input of a slaved guitar amp, or maybe a PA board if the sound man isn't too picky. (They'd usually rather mike your amp than let all of that garbage into their board...)
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Steve Ahola
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P.S. Would you use the same value resistors for small and large amps? I guess the voltages are roughly the same, whether you have 5 watts or 100 watts, right?

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