Blackline SVT problems from impedance of a 7-speaker fridge and not using standby to warm up?

Discussion in 'Amps and Cabs [BG]' started by 863127, Feb 12, 2021.

  1. 863127


    Jun 29, 2019
    What might happen to a '73 SVT from being played through an SVT cab (an old one with the 32 ohms CTS speakers) that only has 7 speakers, doing that for maybe about 50 hours over several months?

    The amp's starting to sound a little flubby (you know what I mean, right?) and muddy. I haven't taken it to a tech to check values of things in it. So I'm wondering if the 7 speakers might've been messing with something, or if it's just that it needs a normal tuneup.

    Also, it had been at someone's house who played on it sometimes without putting it in standby for a while first, and around that time I think it started sounding a little less loud and also distorting earlier. This was early on in the ~50 hours it's been played through the 7 speakers. So is the result of not using standby that it wears some tubes? Other things?

    The calculation of the impedance from 7 speakers is below.

    The formula for calculating impedance of speakers wired in parallel is:
    (Speaker1 ohms x Speaker2 ohms) / (Speaker1 ohms + Speaker2 ohms)

    And then for the impedance with 3 speakers, repeat the equation with the result from the combination of speakers 1 and 2 as the new value for "speaker 1" in the equation and speaker 3's ohms where "speaker 2" was in the equation; then for 4 speakers, repeat with the result from the combination of speakers 1, 2 and 3 as the new value for "speaker 1" in the equation and speaker 4's ohms where "speaker 2" was, etc.

    So for the 32 ohms-each CTS speakers in an SVT cab...

    1 and 2 -- (32 ohms x 32 ohms) / (32 + 32) = 1024/64 - 16 ohms

    1, 2 and 3 -- (16 x 32) / (16 + 32) = 512/48 = 10.6667 ohms

    1-3 and 4 -- (10.6667 x 32) / (10.6667 + 32) = 341.3344 / 42.6667 = 8.000 ohms

    1-4 and 5 -- (8.000 x 32) / (8.000 + 32) = 256 / 40 = 6.40 ohms

    1-5 and 6 -- (6.40 x 32) / (6.40 + 32) = 204.8 / 38.4 = 5.3334 ohms

    1-6 and 7 -- (5.333 x 32) / (5.333 + 32) = 170.656 / 37.333 = 4.5711 ohms

    1-7 and 8 -- (4.5711 x 32) / (4.5711 + 32) = 146.275 / 36.5711 = 3.999 ohms
    Last edited: Feb 12, 2021
  2. JimmyM

    JimmyM Supporting Member

    Apr 11, 2005
    Apopka, FL
    Endorsing: Yamaha, Ampeg, Line 6, EMG
    Neither should mess with the amp. Most likely you just had a tube or two wear out over the natural course of events.
    863127, Wasnex and biguglyman like this.
  3. Wasnex


    Dec 25, 2011

    AFAIK that impedance is still going to be so close that the amp probably won't care. But you are going to be beating the H out of the poor 7th speaker. The cab has four chambers and you really need two (matching) drivers in each chamber so they balance each other out. If the 7th driver is still good, keep it as a spare and put a pair of matching drivers into that chamber. The new drivers don't have to match the other 6 drivers, but they should match each other.

    Also keep in mind that CTS alnico loaded cabs are really only good for about 240W when all eight drivers are healthy. So if you have been cranking the rig, you may have thoroughly trashed all of your drivers; this could be why the sound has degraded.

    It could just be time for some new tubes. You could check the bias. If it's way low, a couple of the output tubes may be dead. It's pretty common for a SVTs to blow a screen resistor when the tubes go bad.

    Good luck!
    Jim C, agedhorse and 863127 like this.
  4. PraiseBassing

    PraiseBassing Supporting Member

    Mar 6, 2017
    The 7th driver could have been damaged running in the same airspace as a blown driver and the resulting 6x10 you have now could sound different.
    agedhorse likes this.
  5. beans-on-toast


    Aug 7, 2008
    I wouldn’t worry about it. The change in sound is most likely a preamp tube that needs changing. Let the tech go over the amp, repair or change the speaker.
    Wasnex and Raw N Low like this.
  6. JimmyM

    JimmyM Supporting Member

    Apr 11, 2005
    Apopka, FL
    Endorsing: Yamaha, Ampeg, Line 6, EMG
    I do agree that the broken 8th speaker should be replaced or reconed, too. Get a used CTS from that 1969-74 era if you can.
    Wasnex likes this.
  7. Primary

    Primary TB Assistant

    Here are some related products that TB members are talking about. Clicking on a product will take you to TB’s partner, Primary, where you can find links to TB discussions about these products.

    May 25, 2022

Share This Page