I think we have a winner with regard to the design.  While the Cliff jack approach would work, I like the switch idea.  I have a few followup questions on that, though.

1) Is speaker wire 18 gauge?  I'll need to get some wire to connect the jack to the switch and the switch to the speaker.

2) Does it matter how I wire the speaker to the switch, the jack to the switch, and the Peavey power amp to the switch?  Since the wires going to the speaker carry AC voltage it seems like positive and negative aren't really meaningful in this context.

3) Some quick rough math says that if the amplifier puts 80W through an 8ohm speaker that it needs to be rated for a little more than 3A.

    P = I^2 * R
   80W = I^2 * 8ohm
    10 A^2 = I^2, => I = sqrt(10) ~= 3.1A

I think that I've seen DPDT switches online rated for 5A.

4) I measured the tabs on the speaker and the female connectors that go on the tabs, and they appear to be > 3/16" and less than 1/4".  A brief online search suggests that there is in fact a .205" size that is commonly used, which falls between the two.  I would think I could just go with 1/4" female connectors and squish them a bit to get them to fit.

Thanks everybody for your input above!

Hi phatt,

thanks for your input.  I think that what you mean by "6.5 sockets" (6.5mm?) are 1/4" mono jacks.  I'm pretty sure that it's safe for an SS amp to not be connected to the right speaker load; if the jacks aren't connected then the power amp sees infinite impedance.  I'm curious as to what kind of "enclosure" I would use. You mention using a bracket, is there something pre-made that you would recommend?  While it may be a little hokey, I'm picturing a small plastic box with a hole for the input wires currently going to the speaker lugs that would be connected internally to the jacks and then a hole going back from the "return" jack to the speaker lugs. I assume these would just be isolated (as in no electrical connection betwixt the two) mono jacks.  I could then attach either two sided tape or similar to the enclosure and stick it on the bottom of the amp's chassis.  I'm suggesting using an enclosure to protect the jacks from accidental shorts.

I found a thread in the tdpri forum that suggested a few other different options.  The first is very simple.  Just wire the two wires currently going to the speaker to a male mono plug, and add the female counterpart that connects to the two speaker lugs.  This way, I could just disconnect the plug from the Peavey power amp from the female jack leading directly to the speaker and insert the speaker cable leading from another amp.  There's an alternative suggestion to use a cliff jack which in theory would disconnect the Peavey power amp once the external speaker cable is inserted.

Here's a link to the discussion that perhaps more clearly describes what I tried to capture in the previous paragraph:
https://www.tdpri.com/threads/converting-a-combo-amp-into-a-cabinet.352192/#post-5487168

In particular, look at posts 5 and 6 on that thread.  For the cliff jack option, it would probably be best to have both amps powered off when making that connection.

I welcome your feedback.  Thanks so much.  Also, feel free to point me to a good resource where I might purchase suitable mono jacks and some sort of enclosure.  I imagine you were thinking of using some sort of metal scrap material and bending it into the shape of a bracket.

The attached image shows a cable that is almost what I need for the simple option I mentioned above, instead of a 1/4" plug I'm looking for a 1/4" jack or possibly could use a female/female adapter.

I recently bought a used Peavey Bandit in hopes that I can convert it into a multipurpose tool.  My other two amps are 8" combos, a Vox Pathfinder and a Randall RD1C.  Both have line and speaker outs, and the Randall has an effects loop as well.

I'm looking to do a couple of different things with the Bandit, but I'll focus on the first of those for now.

I'd like to use the Bandit as an speaker cabinet.  With the Bandit turned off, I'd like to simply connect the speaker outs of either of the other two amps to the 12" speaker in the Bandit.  What I'm envisioning is connecting the two wires coming out of the power amp and currently going to the speaker of the Bandit to some sort of switching jack into which I could connect the speaker out from either of the other two amps and then route back to the Bandit speaker.  The attached image shows the two wires to which I am referring.  I'd like to mount/attach this switching jack to the underside of the Bandit amplifier's chassis, preferably without having to drill into it.

So, how best to accomplish the goal of being able to connect a speaker cable from another amp to the speaker of the Bandit?

Being able to use the speaker in a combo as a speaker cabinet seems like a fairly common thing to want to do, but I'm just not sure what is the best way to go about doing this.

Thanks for your help,
Victor

Quote from: J M Fahey on November 10, 2011, 05:32:26 PM
Yes it will work.
Go backwards, from TDA2030 pin 1 , which will typically have a 22K resistor to ground then go through some DC blocking capacitor (usually 1uF).
You can safely cut the track "on the other side" of said capacitor and add your jacks.

I'm considering doing this type of crude FX loop cut in to my Vox Pathfinder.  It's also a TD2030 chip amp with 4558's in the preamp.  I'm also a novice...   I bought the amp for $50.  Running a Boss GT3 in front of it is not working out for me.  The Boss GT3 sounds much better through my low quality computer speakers.

One issue to consider is that depending on where you insert your effects send and return you may not be able to use the volume control on your amp.  I (believe I have) located a point in the circuit that is just before the volume pot which is itself just before "pin 1" of the power amp chip, the TD2030.  This would allow me to be able to use the volume control of the amp if I decide to only use the return running my guitar cable into the GT3 and its Left mono output into the return jack; in other words, just use the amp as a powered speaker with volume control.  By the way, there is a little jumper on the Pathfinder's circuit board labeled J6 that I believe is the location to insert the loop as I've described.

The steps that need to be tackled include what looks to me like some very precise soldering on the board (I need to use a fine tipped iron from work, perhaps); figuring out where to tie the jacks to signal ground; drilling holes in the pan that houses the PCB for the jacks; strain relief (?) for the new leads coming out of the board feeding my jacks; and actually finding a place to buy the jacks locally.  I think that I might use two switching jacks one for the send, one for the return, since these seem to be what is available and then just tie the two inputs on the send jack together.

Anyway, it all looks extremely simple on paper, but there are definitely a few details that make a rank beginner such as myself pause before proceeding.  I understand that it is a cost savings to not include these additional jacks on low price amps, but it sure would be a lot easier if they just made them that way...

I could conceive of a manner in which to buffer the send and returns with a couple of opamps on perf board, running 9V from the power supply out to the perfboard, connecting the send and return signal lines off of the PCB and to/from the send return jacks to the opamps in the appropriate manner.  However, as a simplification, the Boss GT3 should be able to properly buffer the signal.