Converting closed-back to open-back

[Chuckles]

So, I bought this 1x12 K120 in a cab off eBay. It's in great shape, the speaker sounds great... but it's closed back and is way too-bottom heavy. You can see the pics on the completed listing page here:

http://rover.ebay.com/rover/1/711-53200 ... =824&kw=lg

Can I ask the sound/speaker engineers in the house if I should leave the jack where it is in a maybe 2" strip going across the back and cut out openings above and below... or would that somehow screw up the sound? I only get to cut this thing once...

Thx!

[Chuckles]

Oh, yeah... one other quesiton...

Cool to put this thing on top of my 2x12 E120 cab and get 3 12's rollin'?

[JonnyBoy]

I have always made my own cabs by learning from other cabmakers over the years. I have had your same question before, but most of the time I was using pictures without verbal help. I opted to move the jack using a plate to the top corner, due to how flimsy the back material was.
I also played with a situation comparing the new hard truckers that were using an oval cut back which is about 1/2 of the total area, to give it a different sound. I tested out a bunch of backs using cheap 1/2 ply and found no change in sound from the HT open back to the situation I wound up using, an open area like a cross up down side to side. The cross is open not wood.
a 2" strip going L 2 R or a 2" going U and D, compared to a completely open back. What you are asking to do will have extremely insignificant results on the sound, even if the strip runs from top to bottom/side to side directly in the center or around the sides. 2" is not enough area to give audible results, IMO.
If your amp can handle the ohms, GO 4 It with the Jerry tower. You may have to hook two in series and one in parallel or maybe the other way around. Here is a link to a wiring for dummies situation. It has helped me in the past hooking up multiple speakers on a mono channel. Good Find!!

http://www.usspeaker.com/speaker%20wiring-1.htm

[Chuckles]

Cool; thanks for the come-back. So, just to make sure I've got this right and don't melt my MosValve... I wire the 2 E120s in the 2x12 in series to get a 16 ohm cab, then adding the 1x12 K120 extension cab gives me a 5.33 ohm load, correct? Because the K is rated at 200 watts, that also gives me a 600w power rating (even though the E's are rated at 300)? Am I anywhere in the ballpark with this?

[JonnyBoy]

When I called the US Speaker People they said some amps handle odd speaker ohms better than others, and they couldn't tell me about Mosvalves. On a Mosvalve, you just don't want to get below 4 ohms If its like my RT-2100 100watt (from the manual). If it is above 4, and below 8, on my amp, your fine. I have talked to B.K. Butler through Email a few times about random shit, I can ask him if that amp can handle a speaker setup like that. We're not buddies or anything, but he's like Brad from SMS, a nice guy that appreciates his customers. I sure have picked his brain a few times. I know he must be thinking,"oh no not that crazy Dead Head again with his towers of 300 watt JBLs, butchered twins and priceless Macs used as guitar amps.." I know some amps are fickle and optimized for certain loads and some can handle it all within reason. I'll send him something about this and see what he says. I think you'll be OK...I'll be back with his answer tomorrow.
Peace, Jon

[Chuckles]

That's very kind of you Jon! Yeah, the 1500 is the same as all the MosValves... nothing below 4 ohms, please.

I'm just hoping I have the math right on the ohms and power rating...

[JonnyBoy]

Hey Chuck, I got a reply, he's always punctual with his responses. This is what he said:

You need even numbers of speakers for the best match to the amp.
3 x 8 ohm speakers in parallel = 2.7 ohms (too low); possibly will overheat the amp.
In series, it's 24 Ohms (too high); it will work, but won't sound very loud.
If you can add one more 8ohm speaker you can then at least get 8 ohms by seriesing 2 of them and then paralleling the two resulting 16 ohm loads back to 8 ohms.
3 speakers really doesn't work well on these amps.
Best,
BK

Now he didn't address a 3 speaker parallel/ series situation. I would check your overall ohms at the end of the hookup with an ohm meter and make sure the speakers have 5-6 ohms. I'm sure he would mention that if it was possible. Your amp should have 2 speaker out jacks, right? you can hook up one to there and two to the other. Sorry his response wasn't helpful.
Maybe Jerry had 3 speakers at high ohms to tame the volume on his 350 watt MAC and have the amp clip? I would bet there is something going on there other than 3 speakers wired parallel/series. Healy could have some kind of buffered system to change the ohm input or something of the sort. It could be that the Mac can see funky loads. Maybe one of our historical electrical gear heads can chime in on how Jerry had those towers wired. Peace bro, Jon

[Chuckles]

Hey man, appreciate the effort! According to some, there was at times a fourth speaker backstage for the road crew running from Jerry's rig; but since he was coming off the MC, he didn't really have to worry about mismatched loads. I'm gonna give the series/parallel thing a go and see what it reads - and not play a note through it if it's under 4 ohms. I *think* in practice I should get 5.33... but we'll see.

Cheers, brother!

[Chuckles]

Fwiw, with the 2 E120s in series and the 1 K120 as an extension, it hovered between 5.0-5.9 on the multimeter. And sounded freakin' great! SO psyched to take this monster out to an outdoor Earth Day gig this Saturday! The cab is rated at 600 watts and I'm going to be pumping 300 through it; just have to decide whether to clip the MosValve or the Twin!

[FretfulDave]

Just wondering per the thread subject if you did anything to the cab yet re: opening up the back?

Cool that is sounds good in whatever setup you got.

Thanks,

Dave

[Chuckles]

Pulled the speaker out and cleared out the insulation (or whatever the technical term for it is) and will cut out the back tomorrow. Only thing that I barely caught on this was that I remembered at the last minute about the checking for polarity with JBL's... did the 9v check and sure enogh, the K went in when the E's went out...

The K sitting on top of the f/x rack (which sits on top of the 2x12) seemed way loud to me, but I think it was either just because it was at ear level or because it was canceling out some frequencies from the E's - or maybe closed-back cabs are just louder? (never had one). I know the K will sound louder because it is rated 100w lower, and wonder if I should make a E/K 2x12 and have a single E120 in the extension... ah, the fun of experimenting!

[JonnyBoy]

Sweet!! I know it sounds good! Its not hurting the amp one bit, The mosvalve probably prefers it more than the 4 ohms. Have you ever seen a picture of Jerry's 4 speaker cab with the bottom speaker turned the other direction or inside out? I wonder what the purpose of that was....

[strumminsix]

You need even numbers of speakers for the best match to the amp.

I'm thinking bullshit on this one. Try taking an ohm meter to your speakers and you'll find that your 8o is really anywhere between 6-10 depending on the speaker.

Also I'd be surprised of an amp that can handle say 2o load and also a 4o load but will struggle with a 2.67o load.

[JonnyBoy]

That's what BK wrote, No bullshit. but that answer would only explain a parallel OR a series hookup, and what he said is true. He must not thought of the fact you can do a series and parallel hookup, I was a bit confused and wonder why he didn't address that either, since that was something I asked in the email and he is a pro amp builder. That was the method Chuck and I talked about before I sent the message and hoped to get the answer to. I agree, his opinion was a waste of time.

[strumminsix]

That's what BK wrote, No bullshit. but that answer would only explain a parallel OR a series hookup, and what he said is true. He must not thought of the fact you can do a series and parallel hookup, I was a bit confused and wonder why he didn't address that either, since that was something I asked in the email and he is a pro amp builder. That was the method Chuck and I talked about before I sent the message and hoped to get the answer to. I agree, his opinion was a waste of time.

WTF? He said an even number of speakers but that doesn't guarantee a specific ohms.