Bill Fitzmaurice

[quote name='fleabag' timestamp='1474748833' post='3140275'] If Bill says there's no perceivable volume difference between using four x 4 ohm drivers and four x 8 ohm drivers, i may as well just get the 8 ohm type ? [/quote]I would. You never know if at some point you might want to use it with a second cab.

[quote name='fleabag' timestamp='1474597460' post='3139131'] In which case Bill, i'm assuming i'll lose a little output power changing to 8 ohm drivers ? [/quote]Not enough to make any difference.

[quote name='fleabag' timestamp='1474588517' post='3139119'] ...its power is into 4 ohms, thats it. [/quote]600 watts indicates a SS amp, and if the amp says 4 ohms that's the minimum impedance load. It will have no problem with an 8 ohm load.

[quote name='markstuk' timestamp='1474382308' post='3137531'] as far as I understand ported cabs are more efficient [/quote]They can have higher sensitivity in the lows, but that's not always the case. [quote]and can be smaller[/quote]Or larger.[quote] sealed cabs have a flatter frequency response[/quote]Some do, some don't. [quote]and therefore can be more accurate..[/quote]Or not. You can no more make a blanket statement about ported versus sealed than you can as to which is faster, a 4 cylinder versus 8 cylinder engine vehicle. My 4 cylinder twin-turbo BMW is a lot faster than my next door neighbor's 8 cylinder two-ton Toyota pickup truck.

The Sunn 610L guitar cab used that baffle configuration. I've never seen a bass cab that did. It wouldn't work well for bass because that shape makes the rear chamber too small.

[quote name='Beer of the Bass' timestamp='1473329249' post='3129094']if I was to do a bit of reading, pick out a tweeter that extended down to around 2KHz (like some of the P-Audio offerings), calculate the inductor and capacitor values using an online calculator and wire it up with some form of switchable or variable attenuation, how badly could I go wrong? [/quote]The problem is that those calculators assume a resistive load at the exact value of the driver nominal impedance. That's not what crossovers actually have to deal with. It will work, but not necessarily well. As for running the woofer full range, if you do there can be destructive interference between the woofer and tweeter outputs. One award winning commercial cab I designed went through a full year of Beta testing with many component combinations before the manufacturer settled on the final crossover design. It can make that much difference.

[quote] I will be building the 1x12 very soon and intend to put a piezo in it. [/quote]One is insufficient. Part of the reason why commercial cabs with piezos sound bad is that they don't use high pass filters, the other is that they don't use enough of them. At the minimum you want two piezos per woofer, vertically aligned, wired in series for one pair, banks of series/parallel for more than one pair.

[quote name='Kevin Dean' timestamp='1472980546' post='3125674'] How hard can it be ? [/quote]A lot harder than you think. First off, I'd use either a midrange driver or a tweeter that can run to 2kHz. The usual tweeters used in bass cabs go no lower than 3.5kHz, if that, making them more useful for creating hiss than anything else. Then there's the matter of the crossover. Designing a crossover is a skill only perfected with years of experience. Yes, there are off the shelf crossovers, but they aren't very good. There are on-line crossover component calculators, but they're far too elementary to give a good result.

[quote name='tauzero' timestamp='1472736162' post='3123771'] I think they missed a trick not making it bridging. [/quote]It was bridged. Each 250w amp consisted of two bridged 62.5w amps. You can't bridge twice, and even if you could the minimum impedance load would have been raised to 8 ohms anyway.

They're not optional, they're critical. I should imagine your tone is very boomy without them.

That would work.

[quote name='bassman7755' timestamp='1472300606' post='3120158'] IME even budget floor standers sound much better than quite an expensive subwoofer + shelf/satellite speakers setup. [/quote]That's the opposite of my experience.

I use one channel of a Fly into an 8 ohm 1x12, but it's a high sensitivity Jack 12 cab, not your garden variety box. Depending on what your Hartke are a pair of them, one per channel, may be OK.

[quote name='Biglump' timestamp='1472301376' post='3120163'] I'm thinking of making my own because there doesn't seem to be any 4 ohm 100 watt extension cabs around. [/quote]Why 4 ohm? And why 100 watt?

[quote name='bagsieblue' timestamp='1472203893' post='3119363'] I'd like to think it's a debadged readily available option.... [/quote]It may be, but we can't look inside your cab. Confucius say "picture worth thousand words".

[quote name='Helderius' timestamp='1472133508' post='3118777'] Yes, it is open back. [/quote]I'd suspect the speakers.

In that case it's not a microphonic tube. Is this an open back '59 Bassman reissue? Despite the Bassman name they're guitar amps, useful for bass only at very low volume.

The short answer is that you want to use your bass speaker as part of a hi-fi, and where both response and low frequency extension are concerned bass speakers are very low-fi. Use the right tool for the job, in this case a hi-fi powered sub.

[quote name='MoonBassAlpha' timestamp='1472042537' post='3117929'] That's not quite what I meant Bill, I meant the rear-facing speaker is also open to the air, just facing backwards, or even upwards or downwards. [/quote]Without a picture I can't be sure of what you mean, but on the face of it that's not an isobaric alignment.

Possibly a microphonic tube. To test turn the amp on with no instrument plugged in, volume halfway, gently tap each tube with a pencil. If you have a microphonic tube you'll hear it.

[quote name='MoonBassAlpha' timestamp='1471871112' post='3116532'] Could you run an isobaric cabinet with the speakers magnet to magnet, one facing forwards, one facing backwards, both emitting sound? [/quote]Technically they don't both emit sound. The area of only one cone is exposed to the air, so the maximum output with two drivers isobaric is the same as with one in a standard configuration. The only advantage is a halving of the Vas, but due to the space taken up by the second driver and the connecting plenum chamber the net cabinet size is not halved, while cost is greatly increased by using two drivers. That's why manufacturers in general have not jumped onto this particular band wagon.

[quote name='Twincam' timestamp='1471788894' post='3115857'] I'm using maths and a very simple port tuning program. [/quote]Why? I could see it to some extent if modeling software was a major investment, but as it's free I can't comprehend not using it.

The tuning is the same as with a standard cab. The modeling software will take into account that it's an isobaric alignment.

Having done studio work during that period my first hand experience was that studios dumped most of the valve gear because they didn't want to be perceived as having fallen behind the technology curve. This was during the same period when they were changing to 16 and more track tape decks and consoles that were designed for the studio, rather than broadcasting. There was a 'clean sweep' attitude, and to their later regret many a perfectly good baby ended up tossed out with the bath water.

[quote name='Beer of the Bass' timestamp='1470841717' post='3108873'] Will they, though? It seems there was a period of uncertainty from the mid 80's to some time into the 90's when US and western Europe stopped production, but if anything they seem to have undergone a resurgence since then. It's not just Russia and China either - JJ are successfully making them in Europe under all of the environmental restrictions that go with that. So I would be surprised if they disappear within my lifetime. [/quote]The only use for them is in amps. As demand for valve amps goes down prices on valve amps and valves will continue to rise, further reducing demand, further reducing production, further increasing price in a literal death spiral. That can only make vintage valve gear, or at least what will be vintage someday, appreciate in value. A comparison can be drawn to Fairchild valve compressors, the studio standard of the 60s and 70s. After SS took over the compressor market studios literally tossed Fairchilds into the trash bin. Now those which remain command the same price as a very nice car. For instance: http://www.analoguetube.com/#!Vintage-Fairchild-670-For-Sale-44000/c1czm/5576e3fc0cf2312d79783c31 I doubt that valve amps will appreciate that much, but one never knows. .