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Tube amps without a load

Discussion in 'Amps and Cabs [BG]' started by i_got_a_mohawk, Oct 11, 2005.

  1. Soo, i did a bad thing, my firebass is always sitting ontop of my SVT, and it usually has the speakers plugged in, so i took the cables out of the firebass, then put them back in, and turned on my SVT, let it heat up for about 5 minutes (but thats only the heater elements that are powered on standby, so thats ok right?), and then it was on for about 10-15 seconds, while i was trying to figure out why there was no sound, then i turned it off, checked round the back, what a damned fool! :mad:

    I plugged the speakers in, the load was already set right (2 Ohm) and then it worked fine after turning it back on (was being used for about half an hour)

    I just want to know if ive managed to screw something up badly, i feel like such an idiot :mad: :scowl: :rollno:
  2. The 0x

    The 0x

    Aug 24, 2003
    Timonium, MD
    I've done this before by accident, as long as you turned it off right away, there shouldn't be any damage. It's after long periods of time that it can get messed up.
  3. anyonefortennis

    anyonefortennis Supporting Member

    Jun 28, 2005
    Lincoln, NE
    If it sounds the same, it prbably is fine. This scenario would porbably damage the OT so you could easily hear it if something was worng.

    I did something similar once when I hooked up the cabinet to the footswitch jack instead of the speaker out of an old marshall.

    That's why 1/4'' plugs suck. they can be plggued in all sorta wrong places.
  4. Yeah, i supose, but they are all thats on there, and its my fault for being a tit (they werent even plugged into the right amp!)

    But yeah, i thought, aslong as its still sounding good its fine, cheers guys :)
  5. hello fellow svt II'r..was just down @ bass nw in seattle yesterday & happen to have this same discussion..although it was an acutal "theory" or fish-story. apparently, some studio guru in LA has done this as standard practice to "age" the tubes to how they were "back in the day." in the age of upgrading/replacing strings & tubes whenever there's a good sale or when GAS sets in, the contention has been made that in the early days, the tubes probably rarely got replaced, & had countless hours on them.
    so in the drive-thru mentality tone-quest, the no-load, leave 'em on to age pursuit may be the quick solution to the desired tube vibe.
    i wanna be clear that i don't & won't endorse this & have not ever tried it..as i concluded @ bass nw yesterday, the agreement was that since we don't have a few hundred $$ laying around to spend on tubes we probably will never try it...but it does sound tempting. BTW, what power tubes are you running? i can't seem to tell what mine are..no real distinct markings..maybe GE? also, the first in line doesn't even have a glow so may not even work.
    keep the svt going n going!! :cool:
  6. On the Marshall VBA400 "internal power" is not supplied unless a speaker is plugged in.

    I'm assuming the speaker jack at the amp end is acting as a switch to the high voltage rail on the power tubes. I'm guessing if the speaker cable was plugged in at the amp but with no cab on the other end it'd do no good.

    I will not knowingly test this out to let you know.
  7. No load on a tube amp is damaging to the output transformer. That big hunk of iron at the heavy end of your SVT II is expensive to replace should it have a melt down.
  8. My Ampeg V4, and I suspect the SVT, has been designed with a shorting speaker jack. When the speaker isn't plugged in, the circuit is closed yielding a 0 ohm speaker load as seen by the transformer and keeping the tubes happy. Unlike transistor amps, which don't want to see a 0 ohm load...

    So I wouldn't recommend deliberately firing up a tube amp without a speaker load, but there's enough protection there to keep things from frying.
  9. luknfur


    Jan 14, 2004

    I've got a THD and the speaker was inadvertantly uplugged once but the THD has an automatic dummy load applied for such occassions (and others I suppose). Have no idea whats involved in that but if I ever had an amp worked on, I'd definetly inquire about it and see if such a critter couldn't be installed if it wasn't already.
  10. edited--oops, you said THD, which is a dummy load, though any dummy load must handle the full power output of the amp...

    I did a quick search on Google and found several articles which explained how a shorting jack on the secondary would protect the transformer.

    edited: this is from musicianshotline.com

    I tried plugging my guitar into a Peavey tube head vtm 60 (no speakers, no dummy load) I then connected the Peavey via the effects send to the input jack of my Fender Blues DeVille. The Peavey sounds great as a preamp. From what I've been reading this is BAD. Is it?
    Thanks, Jeff Schwartz

    Yes BAD is a good word to describe what you're doing. You see, a tube amp, unlike a solid state amp, needs to have a load on its output. The AC current and voltage on the primary and secondary sides of the output transformer are directly affected by the load on the secondary side. The optimum conditions exist when the secondary (or output) side of the transformer is properly loaded. Some tube amps use shorting jacks for the speaker outputs. When there is no load connected to the speaker jack, the output of the transformer is shorted to ground. Now while this is far from an ideal condition, it will, for a short duration, prevent damage from occurring in the output transformer. It will, however, cause damage to most transformers if this condition is sustained for any extended period of time because too much current is being drawn from the transformer. Since I believe your Peavey uses open jacks for the speaker outputs instead of shorting jacks, an opposite but potentially more dangerous condition exists. When there is NO load on an output transformer, it attempts to develop more voltage on the secondary side. This can cause the internal insulation of the transformer to break down and high voltage arcing can occur causing shorted turns or open windings
  11. I was (trying) running two heads one night and plugged one of the SVT's (70's) into the wrong jack and played both for a few minutes before I realized it, it blew, was probably only a few minutes, whether they have shorting jacks or not I wouldn't advise it. The OT was ok but it was a very expensive fix and lesson and it came back worse than when it went which prompted me to fix them myself from then on.
  12. BurningSkies

    BurningSkies CRAZY BALDHEAD

    Feb 20, 2005
    Seweracuse, NY
    You should be OK, as long as your amp was in 'standby'. I wouldn't try it in full power conditions though.
  13. luknfur


    Jan 14, 2004
    Yeh the THD is a horse of a different color and only 15 watts so may have something to do with it. Nevertheless, it plenty loud enough for a respectful unmic'd drummer on the hi gain channel. I bought it to experiment with a tube amp - which it is ideal for. That and recording which is one of the reasons for the dummy load. Has a line out with an output control so can be used for silent recording as well as a preamp using the same outlet.
  14. anyonefortennis

    anyonefortennis Supporting Member

    Jun 28, 2005
    Lincoln, NE
    My amp tech described this situation with the most unusual example bt I think it gives us (the electronics/physics stupid) a good idea of what's happening.

    "It's like playing Tug-o-War with no one pulling on the other side"

    Hope that clears things up like it did for me :)
  15. popinfresh


    Dec 23, 2004
    Melbourne, Aus
    I did that the other day. Had it on for 30 seconds, even near a minute because I thought it was my tuner pedal messing up. The speaker lead was in my cab, but not properly :( I was getting a VERY faint signal. Hope I didn't mess anything..
  16. tombowlus

    tombowlus If it sounds good, it is good Gold Supporting Member

    Apr 3, 2003
    Fremont, Ohio
    Editor-in-Chief, Bass Gear Magazine
    Trace Elliot employed a similar output transformer protection circuit in the VA and VR series. They may have used something like this on other tube amps, but I don't think that my Valve series amps or my V8 has anything like this. My VA400 and VR350 definitely do, though.

  17. Joelc73

    Joelc73 Supporting Member

    Nov 13, 2000
    New York
    If it was only 10-15 seconds you are probably OK - particularly if things seem to sound OK. It is a good lesson learned though!
  18. metron

    metron Supporting Member

    Sep 12, 2003
    Thank goodness for that... this is pure idiocy!!! :eyebrow:
  19. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    Actually, I have never seen damage to the transformer from hanging the speaker jack open, such as with no speaker on the far end of the cable.

    Theoretically, it might happen, especially if you turn up all the volume controls wondering why there is no sound........

    What usually 'goes" is the output tubes..... your first clue is usually the orange glow from the plates of the tubes..... There is no need for any input to cause this, usually it happens with no input.

    What goes on is high frequency oscillations, which are caused by the internal feedback being wrong due to no load. That ends up fouling up the bias and drawing too much current. Too much current tends to damage the emission material on the cathode of the tube.

    The result is usually a set of "way too soft" tubes with low emission, low power output, etc, etc.

    If you catch it fast, maybe no problem. But if the orange glow goes on for very long, usually there is a problem. I have seen just 10 or 15 seconds about ruin a set of tubes.
  20. Great, not filling with hope!

    I think its fine, because it was just for recording, so the volume was set at 1/10, and it worked fine after that, next time i fire them up, ill check the glow, make sure they are glowing the same as before

    If not, and the output tubes are gone, well, they have a 6 month warranty, and i kinda want another set instead of these svetlana's :smug: