2.65 ohms with a tube amp?

Discussion in 'Amps and Cabs [BG]' started by Son of Bovril, Sep 28, 2005.

  1. Is it alright to run a Tube amp at 2.65 ohms? Specifically regarding the Fender 300 Pro/ Sunn 330T ?

    I remember reading in the manual for the Trace Elliot V* that it could run at 2.65 ohms...
  2. I think people generally say up to a 100% mismatch is ok, but im not really sure :s
  3. Kelly Lee

    Kelly Lee Yeah, I'm a guy! Supporting Member

    Feb 17, 2004
    Marana, AZ, USA
    On a tube amp it is safer to run lower a lower impedenance than higher like in a SS amp. :meh:

    OK, now let me make sense of what I just said. If your tube amp is set to 4 ohms you should be able to run 4 ohms and lower (2.67, 2 ohms) supposedly were a SS amp you could run 4 ohms and higher (8 ohm, 16 ohm).

    Not that made it easier to understand but to answer your question, yes. I have been told by numerous people that it would be safe with a quality piece of gear like Trace, Mesa, Fender, etc.

    Hopefully one of the tube gurus can pop in here and confirm what I'm saying. Calling PhyscoBassGuy.... :D
  4. bump. any tube amp techies around?
  5. Mika

    Mika Guest

    Nov 29, 2002
    I'm far from what I would call a tube amp techie but I know enough to say that you can safely run a 2.65 ohm load off the 4 ohm tap (impedance selector switch setting).
  6. kuys


    Jun 29, 2010
    I'm curious of this too.

    I'm think about making two 3 10' cabinets based off the cab of my Kustom Groove 310c which I believe is 5.3 or 2.65 with two cabs. I'm wondering if my Peavey Tour VB-2 will be ok with the load on 4 ohms.
  7. dmrogers

    dmrogers Supporting Member

    Jan 26, 2005
    Eastman, GA
    The Fender 300 Pro specs show that it operates safely at 2, 4, or 8 ohms.

    I wouldn't run any amp at an impedance lower than specified (whether tube or solid state).
  8. BigOldHarry


    Aug 11, 2008
    San Diego, CA
    you can run any amp rated to handle a minimum load of 2ohms at anything more than 2ohms.
  9. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    As already noted you can't run a tube amp into too low an impedance, or a SS into too high. But there's no advantage to running a tube amp into a very low impedance either. Half the lowest rated tap is about as low as you want to go.
  10. Nightlyraider


    Sep 30, 2009
    Select the 2 ohm tap or switch and play away.

    There may be slightly less power tube life from mismatching impedence, but nothing that is going to hurt the amp.
  11. eastcoasteddie


    Mar 24, 2006
    So if you were to run a 2.6 ohm load, what tap would you use , 2 or 4 ohms? I'd guess the 2 ohm tap...since that is the "minimum" load. If you use the 4-ohm tap, that is the minimum load for use with that tap.
  12. BigOldHarry


    Aug 11, 2008
    San Diego, CA
    That's what I'd do.
  13. As someone who has blown up a lot of amps and speakers let me just say that it almost always ahppened when I was running the wrong impedance.
    EG, I have never blown up my Mesa Bass 400 running 4 or 8 ohms. I have at 2 ohms.
    I have never blown up a YBA1A at 8 ohms. I did blow it up twice at 4 ohms.
    So my earned wisdom on that is what you can do and what you can do with the amp running hard are very different things. I wouldnt hesitate to run my Mesa at 2 ohms or off the wrong tap, at bedroom rockout levels.
  14. okcrum

    okcrum in your chest Supporting Member

    Oct 5, 2009
    Verde Valley, AZ
    RIP Dark Horse strings
    That should be safe running on either a 2 ohm or 4 ohm tap on the output transformer. Up to a 2:1 mismatch is *usually* OK, 2 ohm tap and 2.67 ohm speaker load is marginally safer than 4 ohm tap and 2.67 ohm speaker load. Reflected power from the load mismatch back to the amp is about 2% using a 2 ohm tap and 4% using a 4 ohm tap, so either is fine. "Tap" is the output from your amp. :D

    A 2:1 impedance mismatch is 11% reflected power (back into the amp) and is about the max most well built amps will safely do consistently.
  15. Stumbo

    Stumbo Wherever you go, there you are.

    Feb 11, 2008
    So. Cal.
    This is my understanding:
    Tube amps are a different story. Efficiency is assured by the correct output impedance being reflected onto the primary of the output transformer, which in part determines the amount of current flowing through the output tubes. If you put on a wrong impedance (EITHER higher or lower), the output tubes "see" an impedance mismatch and actually put out LESS power into the cabinet. It can also cause transformer flyback and tube socket arching leading to a quick death to either the output tubes, cathode resistors, or the output transformer (maybe all!). In short, DON'T mismatch cabinet impedance with a tube amplifier!http://www.duncanamps.com/technical/speaker_cab.html

    I was told the same thing previously by an amp tech. You can add transformer taps to utilize different ohm rating but you had to match the ohms for reliability purposes.
  16. Holy Zombie Thread batman!

    I recant my earlier answer, these days I wouldn't risk having a mismatched impedance
  17. eastcoasteddie


    Mar 24, 2006
    In the Ampeg FAQ, on their site, it says that mismatching impedances is safe to do, but you will have a noticable mismatch in output between the cabs. But I guess that depends on what kind of cabs you're going to run. If you have a pair of 4x10's, one 4 and the other 8 ohms, you'll have the mismatch. But as per some recent threads, running a 4-ohm 4x10 and an 8-ohm 2x10 will actually balance the output due to speaker arrangement.
  18. okcrum

    okcrum in your chest Supporting Member

    Oct 5, 2009
    Verde Valley, AZ
    RIP Dark Horse strings
    Whatever. Friends of mine have run 8 ohm loads off the 4 ohm taps on their SVTs for years w/o difficulty. I do so myself with no problems. But go beyond a 2:1 mismatch and you risk the horrors you describe. The myth here is that a 2:1 mismatch is 50% reflected power, instead of the 11% it is in reality. It's not like the unused power builds up in the amp or anything. If a 2:1 mismatch can kill the amp, then it was marginally or inadequately designed in the first place.

    EDIT: I'd add that I've driven synchronous motors from tube amps without difficulty in the past, from OTs with 110 volt taps. Given the motor impedance can fluctuate wildly as its load changes, and given my other experience with tube amps, I'd say one could safely run the mismatch the OP describes.
  19. We used to deal with reflected power in RF systems in the Air Force (Ground Radio Maintenance). Has anyone measured reflected power in a bass amp with a Bird inline watt meter? Is it the same as measuring reflected RF? If different, how? Thanks!