Crazy idea to offer flexibility in speaker choices from a 4x12?

I was thinking late at night about my speaker limitations and whether I could get more options out of the speakers I already have.  I normally use a Marshall 4x12 with 4 x Greenbacks in it.  I have a couple of virtually unused V30s sitting around (yuk!) and also a couple of brand new Eminence 12" from a Fender 2x12 Combo, can't remember exactly which one.  I could not get on with the V30s with the GM36 but I was wondering if they would work with my Marshall JVM205H.  I connected this with an idea I already had to split the 4x12 internally with a simple solid baffle into 2 x 2x12 sections so as to have both 4x12 and 2x12 options on tap.  My cabinet is a genuine original 1968ish (maybe even a tad earlier) and I still have the original G12Hs stored but that's another story.  I wanted to keep it exactly as original casewise so no irreversible woodworking was allowed!  And that made a thought pop into my head.

Consider the speaker socket.  In my case it's a single small round plastic carrier with no room for a second socket and switch as some more modern Marshall cabs have.  But I could swap the socket for a TRS three terminal one and rewire it to offer more options by changing connections at the amp end.  There would be different options available for 16ohm and 8ohm driver pairs.

Think of the idea of having a TRS socket with 3 terminals.  These could be connected to the speaker pairs with the Tip as a common to both pairs and the other wire for each pair connected to one of the Ring or Sleeve.  Now you have options.

You can access each speaker pair individually by using either the Tip/Ring or Tip/Sleeve on a two wire TRS jack cable.

If you use a standard TS jack which would short the Ring and Sleeve together in the TRS socket, or connect the Ring and Sleeve of a TRS type together inside, you have a parallel connection of both driver pairs.

If you use the Ring/Sleeve and disconnect the Tip you have a Series connection of both driver pairs.

With care regarding the polarity of the speakers and whether you set up each pair for parallel or series you can get some useful alternatives which can be switched between very quickly with just a swap of speaker cable.  8ohm drivers can be set up in series for each pair to give 16ohm 2x12 pairs.  The parallel connection of both can then give you an 8ohm 4x12.  16ohm drivers can be paralleled for 8ohms each pair.  Then the series connection can offer you a 16ohm 4x12.  All of these are a match for the H&K amps.

It's a tad cumbersome at first glance but it is cheap and easy to set up and it may just give you an alternative which suits some unusual circumstances very well, as in my own case.  With differing driver types it can be very flexible. Worth thinking about if you have the need or the curiosity.

Here's a pic of what I was rambling about.....

The mixing of Greenbacks and V30s was my idea also Hwystar. I could set them up as a 2xG12M pair and a 2xV30 pair with each pair accessed individually, or as 2 pairs each with G12M + V30, and the mix used as a 4x12 in each case. It was just such a simple way to set these up in a single cabinet that I carry around anyway.

I know that some people don't like 4x12s on account of the size but there is no reason why this can't also be applied to a 2x12" to access each speaker individually. Though I suppose with a more modern cabinet you could have the usual two sockets and switch arrangement to do that.

Just a thought to ponder during the small hours.

Oh, and as to your open back..... I don't think I've never heard an open back Marshall 4x12 before so as far as I am concerned it could be the greatest sound since who knows when. And it would make access to the drivers really easy. As always, if you love it then you keep it.

I think I've now gone one better with this idea.  It occurred to me that it would be nice to offer the option of either speaker pair individually as a 2x12 or the combination of them as either series or parallel.  The impedance would be different in each case of course but they would fall at 4ohms, 8ohms and 16ohms with 16ohm drivers all round.  The parallel all 4 option wouldn't suit the H&K amps with their 8-16ohm range but for others like the usual Marshall and Fender culprits which include 4ohm taps it would give great flexibility.

I thought it would be difficult and require double pole switches until I looked into it.  I found all it needs is a pair of single pole double throw (SPDT) switches.  And it means no fiddling with RTS sockets and leads, I can go back to the original TS socket just fine.  Look at the diagram and trace it out.  If either of the switches is put in the upper position the speaker pair it controls is in circuit.  If the other one is down that speaker is off, if up they are both on in parallel.  If both switches are put down then they are all on in series.  Check out the phasing of the + and - terminals too, it all should match up.

What I have done is to buy a new handle for the Marshall 4x12 in order to keep its original 1968 intact, and I have mounted two slide switches in its upper surface well above the bar.  That way it is recessed and doesn't hack away at the cabinet.  Not bad for the price of a pair of plain old SPDT slide switches.

Yes, you're right, if Sw1 is flipped down it connects the -ve terminal of Speaker1 to the +ve terminal of Speaker2 so putting them in series. The current path, (which is really more what we are interested in with speakers rather than the voltage signal), goes: +ve Signal In --> Spkr1 +ve --> Spkr1 -ve --> Spkr2 +ve --> Spkr2 -ve --> -ve Signal Out.

Remembering that each of the Spkr1/2 really represents a pair in parallel, with 4 x 16ohm speakers you get 2 selectable pairs of 8ohms each or 1 paralleled setup of all 4 speakers at 4ohms or 1 series/parallel setup of all 4 speakers at 16ohms. With one of my Greenback pairs and my old original '60s G12H pair I can swap between them or mix them to suit.

And I haven't forgotten your advice on running open backed. That's something I'm planning to implement when I get all of this tied down. If it eases out a little of the heaviness of the bottom end it could be just what I need.

I gigged last night with the speaker setup I described above. I have the two G12M Greenbacks in the left pair and the G12Hs in the right pair. I chose to do it that way because I have built in an internal separating wall between the two sides based on a couple of pieces of real timber and the central support post. It isn't completely airtight but a very accurate close fit and will be fine for something which is not bass orientated. It is also completely removable leaving only a few small screwholes so as not to destroy the cabinet's value, (it's a genuine late '60s with original G12Hs). I am still unsure about using such rare vintage speakers in a gigging cabinet but I hate the idea of old warhorses being put out to pasture when they still have some fight left in them, so...

The tonal side of things is not massively different to my ears, (as you know I don't believe in searching for hyperboles to say just because I have been made to believe there SHOULD be a difference), but the G12Hs definitely break up a lot earlier than the G12Ms. This is very interesting as it means that I can get a louder clean sound on G12M and a lower level crunchy singing sound on G12H by selecting one or the other. The size of venue can also be factored in too of course as the G12Ms do break up eventually but at higher levels, and there are always the 4x12 "all parallel"/"series parallel" options too.

I'm looking into whether "hot swapping" between them is a good thing for the amp or whether it is necessary to switch off the amp first. My current feeling says it will not be a good thing, you can generate massive transformer destroying spikes by removing the load on a valve amp when it is in use, but with certain precautions it may be ok.

For a couple of £3 switches and a new cab handle it is a really good setup for me.

Now, maybe with Vintage 30s...

After a bit of research I have confirmed that hot swapping between speaker setups is not always a good thing, in fact it can be a very bad thing if you leave the amp output completely unloaded during the process. As I thought, this can generate massive flyback spikes to the anodes of the output valves which can cause severe damage. The output transformer is also at risk and that is expensive!!!

The solution is to put a highish value resistor permanently across the speaker terminals, about 470R 2W will do. This will mean that there is always a load even when the switches may both be in a "break" state during switching with neither connected. The load is low enough to reduce the spikes to a manageable level but high enough to not produce any mismatch in impedance when in use. (8ohms in parallel with 470R is... )

In my case the switching setup is more for venue matching anyway and would probably only ever be used during a gig for specific odd numbers, (Tom Petty's Running Down A Dream is crisper with the increased clean headroom of the G12Ms). The trick is to only hot swap between the two 8ohm pairs, never between a pair and all 4 (8ohms <-> 16/4ohms) which would require changing the impedance at the amp, and always to engage the second pair before disengaging the first one so there is always a load. You should also never do it while actually playing through the amp and producing a signal at the output. The 4ohm mismatch is a non-issue for the time it takes to change the switches and most well designed output stages will cope with that permanently.

I'm off to put in my 470R resistor now.