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25w practice amp powering 700w cab...bad idea?

Discussion in 'Amps and Cabs [BG]' started by Runlikegregg, May 30, 2018.


  1. Runlikegregg

    Runlikegregg

    Dec 31, 2011
    Brooklyn
    Hi!
    I'm hoping someone can let me know if this is totally stupid and/or dangerous.

    I am getting my usual amp head serviced and so brought in my little Fender Bassman 25 watt practice amp to my rehearsal space just to have something to play through. But then I noticed the 25 watt output for an extension speaker at 8 ohms so I plugged it into my Eden D410 XLT which is 8 ohms and rated at 700 watts or maybe 750 (I can't remember) and it sounded pretty good! I mean...maybe not really good, but certainly loud, and weirdly I think loud enough to compete with drums and everything.

    My question is, do I have anything to worry about? Can this do any harm to the practice amp?

    Thanks!
     
  2. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    No problem at all. Unless you want to drive the speaker cab into distortion. :laugh:

    It’s surprising how a little amp can drive a big cabinet.
     
  3. Runlikegregg

    Runlikegregg

    Dec 31, 2011
    Brooklyn
    Nice! Thanks for the reply. That was my gut but I just..I don't know!!
    But yes, it blew me away at how loud it is! I know it's more about surface area and moving air and all that, but holy moly!
     
    beans-on-toast likes this.
  4. johnpbass

    johnpbass

    Feb 18, 2008
    Glen Mills, PA
    I've run my Ampeg 20T through a pair of 210AVs many times. Surprising how loud 20 watts can be through a larger cab or multiple cabs.
     
    Runlikegregg likes this.
  5. lz4005

    lz4005

    Oct 22, 2013
    Yeah, as long as the ohms are right you're good to go. If it was a 4ohm cab you could damage the amp.

    It's not just surface area, though that's definitely a part. You're getting more watts with a 4 ohm total load than with just the internal 8ohm speaker, and the speakers in the extension cab you're using probably sound better in general than the inexpensive one in the combo.
     
  6. Runlikegregg

    Runlikegregg

    Dec 31, 2011
    Brooklyn
    oh thanks! yeah, I forgot about that part of it...that I'm getting more juice from the head now.
     
  7. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    When an external cab is plugged in to this amp, the main speaker in the combo is supposed to be disconnected. So the impedance remains 8 ohms. The 8 ohm cab is matched to the amp.
     
    Runlikegregg likes this.
  8. lz4005

    lz4005

    Oct 22, 2013
    Didn't realize that. Yeah, the manual says the internal is automatically disconnected when you plug into the extension, so the total load stays at 8. So the wattage doesn't change, just the number of speakers.
     
    beans-on-toast and Runlikegregg like this.
  9. johnpbass

    johnpbass

    Feb 18, 2008
    Glen Mills, PA
    An Eden 410XLT is one of the "loudest" cabs I've ever played through, IME. It has a lot of forward throw especially in the low mids.
     
    Runlikegregg and lz4005 like this.
  10. Runlikegregg

    Runlikegregg

    Dec 31, 2011
    Brooklyn
    I've been loving my cab for 20 years now!
     
    johnpbass likes this.
  11. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    The Fender Bassman 25 external speaker jack contacts need to be kept clean, free of oxidation. It’s a good idea to scrub them with Deoxit at least once a year.

    Over time, oxidization builds up on the external speaker jack shunt (it acts as a switch). Oxidization is an insulator, it acts like a resistor in the output signal path, and it can lower the level when the combo’s internal speaker is being used. It should be fine when the external cabinet is being used but it never hurts to keep the jack contacts clean. All the amp’s jacks should be periodically cleaned for the same reason.
     
  12. Jscriv

    Jscriv

    Feb 3, 2017
    Tonawanda NY
    This is kind of eye opening. I've been struggling to figure out if I should go with a tone hammer 350 or 500 with plans to split between a 800w410 and then get a equal wattage 15 to handle the lows. Ran in series not paralleled. Create a nice 410x15 deal. I wonder if the 350 would give me all the presence I could ever want? Or should I spend the extra cash on the 500??
     
    cjlembo likes this.
  13. Ross W. Lovell

    Ross W. Lovell

    Oct 31, 2015


    Doing that the other way around might be a problem?
     
  14. MarkA

    MarkA In the doghouse. Supporting Member

    Sep 26, 2008
    Depends on the efficiency and voicing of the cab or cabs and on the context (music, other players, venue, etc.) in which you'll be playing them...

    Wait. Just read your post for the third time. I missed "run in series, not paralleled" the first two times through!

    It is indeed possible that you might like the results with such a rig and that it might give you all the volume you want but, to me, it seems likely to be counterproductive on a number of levels:

    • If you're running two 4-Ohm cabs in series for an 8 Ohm load, you'll be getting about 250 Watts from the TH500 and about 175 from the TH350 (give or take). If they're 8 Ohm cabs... well, I don't know why you'd run 8 Ohm cabs in series with either of those heads, since either would handle the resulting 4 Ohm load of running in parallel, but you'd be getting very little power, relatively speaking, out of either head at 16 Ohms.
    • If the speakers are different impedances (say an 8-Ohm 1x15 and a 4-Ohm 4x10), I know how they share power in parallel, but wasn't sure it would work that way in series, so I had a look. According to this source (scroll down a bit, to "Different Impedance Speakers in Series"), it is the opposite of running in parallel. I.e., with 4 and 8 Ohm loads in parallel, the 4-Ohm load gets two-thirds of the power and the 8-Ohm load one-third of the power. With 4 and 8 Ohm loads wired in series, the 8-Ohm cab will be getting twice the power that the 4 Ohm cab is.
    If you already have an 8-Ohm 4x10 and and a 4-Ohm 115 (with most 115s handling less power than a similar "level" 410 -- though I know you said "equal Wattage 15") and want to run them together, hooking them up in series might be a good way to go, as the 8-Ohm 410 will get more power (which is usually what you want and the opposite of what you'd expect running them in parallel). If you don't already have the cabs, though, this is not a combination I'd buy (because of the requirement of series wiring and because you won't get a ton of power out of either head at the resulting 12-Ohm load).

    I invite those more experienced with such matters to check me on the above. I don't want to mislead anyone in a way that might involve blowing up their cabs. I'd also like to be sure that I'm understanding things correctly.

    If I am, though, with either the TH350 or the TH500 (which are 4 Ohms minimum) I would either run one, big, 4 Ohm cab or two smaller 8 Ohm cabs in parallel. I wouldn't look to wire them in series unless I already had cabs that I really wanted to use and the resulting load in parallel would be less than 4 Ohms and/or I had to account for some weirdness like the 4 Ohm 1x15 and 8 Ohm 4x10 -- and, then, if the cabs were different, I'd be very careful about the power distribution between the cabs to be sure that it made sense.

    In general, I think you'd be better off running a 4 Ohm load with two of the same cab (or one big cab, if you prefer that) with either of those heads. If you let people know what cabs, specifically, you're looking at, you can get more specific advice. You might or might not want to start a new thread for that (or peruse the many Tonehammer threads on TB to see what folks are using with them).

    Finally, depending on the cabs, you might find that the 410 has more "lows" than the 15. You might also find that you get what you want with less schlep (and maybe less total cost) with the TH500 and a single cab... again, depends!

    Good luck!
     
    Last edited: May 30, 2018
    instrumentlevel likes this.
  15. Jscriv

    Jscriv

    Feb 3, 2017
    Tonawanda NY
    With parallel input Jacks on the back of a cabinet, if I Daisy chain they would be in parallel. Meaning my two 8 ohm cabs would deliver a 4ohm load. That's my entire bad. Daisy chain at work = series. Daisy chain on my cabs =parallel. I mix em up from one to time
     
  16. MarkA

    MarkA In the doghouse. Supporting Member

    Sep 26, 2008
    Dude... well, at least it was an interesting mental exercise for me! I think I learned something and I hope it lead to something that someone else finds helpful down the line.

    Yeah, in that case, with two cabs, 4 Ohms in parallel would be optimum with either of those heads. With most 15s, I'd question the 410+15 setup because most 15s are closer in power handling to a 210 in the same line. An 8 Ohm 15 with a 4 Ohm 410 can make sense, but then you need a head that will do 2.67 Ohms. I also don't know about the 15 necessarily doing lows better than the 10s -- depending on the specific cabs, you might actually find the 15 to be more middy.

    Some guys are into the 410+15 thing (though I think 210 + 15 usually makes more sense) and some are militantly against mixing speakers... I think that two of the same cab is more of a sure thing, but if you like the way a mixed stack sounds, you like the way it sounds. I would do some research into the specific cabs beforehand, though. Again, it'll depend on what you're looking for for volume and tone. You might also find that you get better results with a single good cab than with two lesser ones (again, don't know what cabs you're looking at).

    Gah! Too much typing. Good luck!
     
  17. Now you know why 800w bass amps are a headscratcher.
     
  18. alaskaleftybass

    alaskaleftybass Will Hanbury, Jr. Supporting Member

    Mar 21, 2012
    Sitka, Alaska
    This was tried before and the results weren't pretty. Ladies, please turn your heads.

    explosion.jpg

    All kidding aside, it should be perfectly fine. People forget in the early 60's most Fender Bassman's were only 50 watts! And I played many a concert with one-- it was my first big amp. As long as both the amp and cab are 8 ohms you're fine.
     
  19. mexicanyella

    mexicanyella

    Feb 16, 2015
    Troy, MO
    I've practiced with a speakerless 15 watt/4 ohm Dean Markley guitar practice amp driving an 800 watt/4 ohm 4 x 8" AudioKinesis cab a few times, and I too was surprised at how loud it was, and how good it sounded. Haven't tried it at band practice, but maybe I will one of these days.
     
    Wisebass and Runlikegregg like this.
  20. 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.

     
    Apr 21, 2021

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