18” combo?

Discussion in 'Shock Brother's DIY Amps' started by LightningPhil, Jan 10, 2020.

  1. LightningPhil

    LightningPhil Tele-Meister

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    Has anyone been silly enough to make an 18” combo?

    I ask because a have a spare driver kicking about and I’m tempted to give such foolishness a go.

    The driver is the inexpensive qtx unit:
    https://www.simplysoundandlighting.co.uk/qtx-18-low-frequency-driver/

    For an amp circuit, thinking something single ended under 20W (yep, I know it’s a 350W driver). Will need to push the treble a bit, so can implement a presence type control in the feedback.

    To do or not to do?
     
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  2. printer2

    printer2 Poster Extraordinaire

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    Forget it for any high end.
     
  3. jsnwhite619

    jsnwhite619 Friend of Leo's

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    I think I may simply hook it up to any amp for a test run before I spent the time, money, and effort on it. Either an amp you already are completely familiar with or the brighter thing you can find, or both, but see what it sounds like.
     
  4. LightningPhil

    LightningPhil Tele-Meister

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  5. printer2

    printer2 Poster Extraordinaire

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    It might be good for bass. I had some 18" speakers that were used in W horns. To get that large and take the power the diaphragm is not the lightest. Even then I thought for bass a midrange speaker was a must.
     
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  6. LightningPhil

    LightningPhil Tele-Meister

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    Gave it a go.

    Yep, the treble is attenuated. Doesn't sound boxy though. And amusingly the lows are almost OK with a bare driver.

    So on balance, while it remains a silly idea, it's on.

    Shall report back as the project progresses.
     
  7. schmee

    schmee Poster Extraordinaire Silver Supporter

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    I'd just try that speaker first. Some low end speakers cannot recover highs by just turning the treble up. Some aren't bad at all.

    whoops, I see you did!
     
  8. AJBaker

    AJBaker Friend of Leo's

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    Sounds silly, but heaps of fun :)!
    I look forward to seeing what you come up with, I imagine it will be ridiculously impractical, but quite amusing.
     
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  9. LightningPhil

    LightningPhil Tele-Meister

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    There be the goals.

    Today's progress was working out the parameters of the driver. Not strictly necessary as it'll be open back, but handy to know.

    Here's what QTX claim:
    • Model: 902.558
    • Type: Bass
    • RMS (W): 350
    • Max (W): 700
    • Driver Size (Inches): 18
    • Impedance (Ohms): 8
    • Resonance (Hz): Unknown
    • SPL @ 1W/1m (dB): 99
    • Frequency Range (Hz): 37Hz - 3KHz
    • Overall Diameter (mm): 500
    • Depth (mm): 250
    • Voice Coil Diameter (mm): 76.2
    • Weight (kg): 8.5
    • Magnet weight: 2.15kg
    • Voice coil: 3 inch (76.2mm)
    • Dimension: 250 x 500mm Diameter
    And here's what an hour of faffing about with REW, a sound card, a resistor, some bits of wire and play dough resulted in (consider these numbers for interest and not gold standard, that would take 2 hours...):
    • Zmin 7.81 ohm
    • Le(f3) 1.602 mH
    • fs 25.6 Hz
    • Qms 8.183
    • Qes 0.595
    • Qts 0.554
    • Fts 46.2
    • Mms 131.13 g
    • Vas 599.99 litres
    • Bl 16.261 Tm
    • Lp (1W/1m) 94.36 dB
    • Sd 1200.0 cm^2
    If compared to a Celestion G10 Vintage, which has great highs but not a lot of bass (resonance around 130 Hz before break in), it's quite interesting. The area is 3.5 times bigger but it's 4.4 times heavier. Thus 25% heavier per unit of surface area. So yes, heavier, but by that metric, not too silly. The inductance is 2.5 times higher though. That'll nobble the treble a bit.

    Also ordered a sacrificial second hand amp for parts.
     
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  10. LightningPhil

    LightningPhil Tele-Meister

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    Sacrificial amp arrived.

    It was the obvious choice for a bass amp - a pro junior. Wow do they have no bass at all! But makes for a great bass fuzz effect :)

    Interestingly though, a back to back comparison made by plugging the 18" speaker into the pro junior showed that it should make for a reasonable guitar speaker. It sounded much, much, much nicer than the stock speaker in the combo even though the 18" is not mounted in anything (lent on a piano stool).

    So there is now more hope that the foolish thing will work - but not with the sacrificial amp. It'll need bigger cajones. That means the pro junior (which does not impress) will live to be modified. And a new plan will have to form for the combo.

    Unless...

    Shall give this a go - replacing the output TX of the pro junior with something much heavier:
    https://www.diyaudio.com/forums/tubes-valves/327541-6n2p-6cw5-design-toroidal-outputs.html
     
  11. jsnwhite619

    jsnwhite619 Friend of Leo's

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    Yeah a Pro Jr has an OT the physical size of a vintage Champ OT. Not going to get a lot of impressive low end out of it.

    I've never modded mine but have played it in & through lots of different cabs and speakers. They can sound great in bigger cabs, but mine is always pretty bright no matter what it plays through, and never has jaw-dropping bass.

    Sent from my moto g(6) using Tapatalk
     
  12. LightningPhil

    LightningPhil Tele-Meister

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    This is the TX I'll try:
    https://cpc.farnell.com/multicomp/mcta050-06/50va-toroidal-2x6v/dp/FF01545

    8 ohms reflected through the wingdings turns out to be 3000 ohms. Which is a bit low for the stock voltage, so will have to lower that a little (sag resistor here we come) and tweak the bias a bit.

    If that doesn't work, then time to come up with a bespoke circuit using all the boxes of rubbish I've been collecting in the garage.
     
  13. LightningPhil

    LightningPhil Tele-Meister

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    Not a useful update on the 18" combo, but the pro junior is now much less noisy after following everyone elses mods.

    Interesting to note that loosening or removing the top rear screw really does reduce noise. When it's fully would in, it shorts the chassis to a static shield, causing a ground loop around the volume and tone pots. Shall shorten the screw tomorrow.
     
  14. Dismalhead

    Dismalhead Poster Extraordinaire

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    Yup, be the ball Danny.

    Do it, should be fun.
     
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  15. LightningPhil

    LightningPhil Tele-Meister

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    A further pro junior interlude...

    Discovered that the bias circuit was injecting noise (and generating it with its diode). So now it's cathode biased. And negative feedback has been eliminated too. It's not silent, but now mainly resistor and valve noise.

    A very important improvement was throwing away the valve sockets as they were quite loose and made for horrid noises if the valves were touched. The new ones have a much better grip on the valve pins.

    A sprinkling of larger coupling caps finished off the tinkering and now it sounds really rather good into a bass cabinet. So maybe it'll be sacrificed after all.
     
  16. LightningPhil

    LightningPhil Tele-Meister

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    Brief update.

    The TX arrived today and was hurriedly installed for a test - dangling happily out of the chassis by its leads. Test now done, and it's powered off pending a proper install tonight.

    Wow! What a change! Not sure at all if any of the highs were lost, but they remain great so no problem. But the lows. It now has them in big fat dollops if plugged into a bass cabinet - where it contentedly makes things in the room vibrate and rattle. And when in the 18" speaker on its own (still not in an enclosure and leaning on a piano stool), it sounds much better and more rounded than the stock TX.

    Really it should have a 16 ohm speaker as the reflected impedance is about 3k ohms from an 8 ohm load and it should be quite a bit higher for maximum power coupling (circa 5 or 6k ohms). But it works and cost £11.15 delivered.

    There is a possible bonus of having a low impedance load - it can apparently extenuate the 2nd harmonic a bit and make it sound even more lush :) at the expense of binning off some power :(.

    So, if anyone wants to tinker with their pro junior, I can a similar TX. The specs for the one I have is:
    • 50VA
    • 2 x 115V in (wired as a centre tapped for PP operation)
    • 2 x 6V out (wired in parallel)
    • Ratio 19.2
    • 8 ohm reflected impedance = 2949 ohms
    • 16 ohm reflected impedance = 5898 ohms
    Ideally, a Toroidal TX would have 4 to 5 V output to up the reflected impedance. But the one in the link in the previous post was the lowest I could easily find.
     
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