Coverting an open back 2x12 cab for bass use

Chris Marinelli

Chris Marinelli

Jun 1, 2021
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#1
So I have been out of the bass game for about 15 years and am just getting back into it. I have a P Bass but no proper bass amp. Well, maybe not…

I have an excellent high powered tweed twin clone that was optimized for pedal steel (so highest headroom possible before distortion). For those unaware, it’s a Fender Bassman preamp into 100 clean tube watts. The bass sounds wonderful through it but it just isn’t going to get loud enough to compete at a gig with a drummer and guitarist. I suspect much of this is due to the speakers (2x Jensen Neo Tornado 100s) and the open back guitar cab design.
So here’s where it gets interesting - I have a spare larger 2x12 open back cab (28x20x10.5) that I could load with 12” Bass drivers and either use along with the tweed twins speakers or alone.
This is where I need some help: is this cab volume good enough for two bass woofers? Would I definitely need to seal it or could it work open back? I’m happy with a warm vintage tone so I don’t need tons of clarity necessarily. Just looking for some general advice on whether this is a feasible plan, or a big waste of time and money!
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#2
An open back cabinet, no matter what you do to it or what you combine it with, isn't going to work even passably well for bass.

It's a waste of time and money, especially when you can buy decent used (larger, heavier, but that doesn't seem to be an issue if you already have a Twin) real bass cabinets for 25 cents on the dollar.
 
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#3
Chris Marinelli said:
So I have been out of the bass game for about 15 years and am just getting back into it. I have a P Bass but no proper bass amp. Well, maybe not…

I have an excellent high powered tweed twin clone that was optimized for pedal steel (so highest headroom possible before distortion). For those unaware, it’s a Fender Bassman preamp into 100 clean tube watts. The bass sounds wonderful through it but it just isn’t going to get loud enough to compete at a gig with a drummer and guitarist. I suspect much of this is due to the speakers (2x Jensen Neo Tornado 100s) and the open back guitar cab design.
So here’s where it gets interesting - I have a spare larger 2x12 open back cab (28x20x10.5) that I could load with 12” Bass drivers and either use along with the tweed twins speakers or alone.
This is where I need some help: is this cab volume good enough for two bass woofers? Would I definitely need to seal it or could it work open back? I’m happy with a warm vintage tone so I don’t need tons of clarity necessarily. Just looking for some general advice on whether this is a feasible plan, or a big waste of time and money!
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Waste of time. If you went that route, you would definitely need a solid piece of plywood to create a sealed cab, if you wanted the speakers to last, and to create the proper soundwaves(an open back cab would enable too much cone excusion, and allow too much low frequency cancellation). That being said, what you would end up with is essentially the first version of the blackface Bassman cab. Woefully inadequate in the 60’s, and useless now. Fender made iconic basses and amplifiers(although often underpowered), but they missed the boat with MI speaker cab design, which was just becoming an art in that decade. The biggest problem is that they were too shallow, but also, now, whatever drivers you’ll be using, the box needs to match their specs for maximum performance. Fender made great guitar amps, but bass players in that time, if they didn’t move to Showmans, went to Sunn, Kustom, Acoustic, and Ampeg for the most part. If that cab has no other use for you, sell/trade it for a cab specifically built for bass. What drivers were you going to use? You might sell those as well, take all that money, and just start fresh; there are a few 2x12 options if that’s your preferred configuration.
 
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#4
agedhorse said:
It's a waste of time and money, especially when you can buy decent used (larger, heavier, but that doesn't seem to be an issue if you already have a Twin) real bass cabinets for 25 cents on the dollar.
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Thanks for the advice, and I sort of had a feeling this was the case. Thankfully the combo weighs in just over 40 lbs so it’s very manageable.

So, do you all think it would it be best to bypass the internal speakers altogether and just go out to a bass cab, or would the existing 2x12s at least give some volume and upper-end content to go along with say a 1x15 or 2x10/2x12 bass cab? The amp is somewhat unique in that it accepts a 4 ohm cab on each of the two speaker output jacks, so it is 4 ohm to the internal speakers and switches to an 8 ohm tap for 2 4 ohm cabs wired in series when an extension cab is plugged in.
I’m not really looking to go up to a 4x10 as the size and weight starts to become an issue for transport. My gut tells me a 1x15 paired to the existing open back 2x12s could sound great, but I’m also scared the 12s will fart out and distort before the extension cab gets loud enough.
 
#7
Although technically you could run the internal speakers along with an external cab, you'll find that doesn't work out well. The biggest problem -- the open back enclosure -- doesn't change if you're using an external cab. The 12's in the Twin-clone will still start to exhibit signs of over-excursion at a relatively low level so the overall rig will still fart out before it gets loud enough. The Twin amp is actually an OK amp for a head -- just plug it into a decent bass cab and the rig will suffice.

You could use both the internal speakers and external ones if you had a second amp channel -- using the 12s to enhance the mid and high-frequency response, but you would need some way to filter the dangerous low frequency content out of the signal going to the internal speakers.
 
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#9
agedhorse said:
I’ve never heard of a Twin 212 combo at 40 lbs, I owned one and it was closer to double that w/D120’s.
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It’s pretty custom - pine cab with the dimensions of a LP Tweed Twin but with a HP Tweed Twin chassis. Also the Jensen Neos help a lot. It was made by Fox Vintage Amps (out of NC I Think?) and I had the cab built separately. He doesn’t build any more but was pretty well known in the pedal steel world. It’s a great double duty amp for 6 string and steel guitar, and is made to stay clean and loud down to the low B of a 12 string steel.

I’m pretty confident it will work well as a bass head. The biggest issue is going to finding a 4 ohm cab - 8 ohm seems to be the standard. I’ll have to ask him if a dummy plug in the internal speaker jack will activate the 8 ohm tap in the extension jack, or if that will like blow up the amp or something!

so now the question is, 2x10s, 2x12s, or 1x15? If it’s going to be the only cab I’m feeling like the 15 might not give me enough volume.
 
#10
You could make a removable back panel and try it out. It may not give you the sound you are looking for, but you will learn from it.

One of my cabs is an 2x12 Avatar that I've used for bass as is. It's partially open back like yours. Sounds crisp and clear. It's loaded with two different Celestrons.
 
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#11
74hc said:
You could make a removable back panel and try it out. It may not give you the sound you are looking for, but you will learn from it.
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I’d love to but I didn’t but speakers for the extension cab yet. I don’t really want to invest $150 into speakers to find out it’s not going to work for me.

I think sell/trade the empty 2x12 for a bass cab is my best bet. Now, who has recommendations for cheap 4 ohm cabs that sound great on their own?!
 
#13
Respectfully, I’m going to call BS on the weight. Stock Twins are going to come in at between 60 and 65 pounds, the amp chassis alone is almost 40 pounds. The cabinet itself under the best conditions is going to be 25 lbs. the difference between a Neo and stock drivers is 3-4 lbs each.

I have owned them and also worked for Fender, so I’m quite aware of what a twin is, how much they weigh, and what open backed speaker cabinets are capable of for bass applications.

Just find an older, used 4 ohm 212 and call it a day. There are plenty out there that are plenty cheap.
 
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#14
agedhorse said:
Respectfully, I’m going to call BS on the weight. Stock Twins are going to come in at between 60 and 65 pounds, the amp chassis alone is almost 40 pounds. The cabinet itself under the best conditions is going to be 25 lbs. the difference between a Neo and stock drivers is 3-4 lbs each.

I have owned them and also worked for Fender, so I’m quite aware of what a twin is, how much they weigh, and what open backed speaker cabinets are capable of for bass applications.
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I mean I could put it on a scale and take a picture for you but I’m not sure why someone would lie about that. Anyways, not the point of this thread but thanks for the unsolicited accusations! Any helpful advice on cheap 4 ohm cabs that would do the job at a gig with a drummer and guitarist playing hipster indie rock through a 50 watt 2x12?
 
#16
Chris Marinelli said:
I mean I could put it on a scale and take a picture for you but I’m not sure why someone would lie about that. Anyways, not the point of this thread but thanks for the unsolicited accusations!
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Just keeping it factual.
 
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#19
If you can handle the weight, grab an older Traynor or Peavey 2x15 BASS cab. Either should be well within your budget and likely to be 4 ohms. I've tried open back guitar cabs for bass before and it is underwhelming, even at lower volumes. It is also a great way to fry a decent speaker intended for guitar use.
 
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