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SVT & 8 ohms?

Discussion in 'Amps and Cabs [BG]' started by geggy spoiger, Apr 23, 2005.

  1. Hi all,

    I have an SVT 2 Pro on hold for a trade-in on my Mesa head, and when I went to try it the salesman asked if I'd like to hook my cab up to it. I said I didn't think I could because the 2x15 is an 8 ohm cab. He said that as long as the impedance switch was set to 4 ohms it would be ok because you can run DOWN TO 4 ohms with the switch set to 4 ohms. I thought I had heard differently...anyhoo

    Is this ok to do? or not? :confused: I did hook it up and all and it did sound like the amp of gods and titans. Will prolonged use like this shorten tube life or hurt something more in the head over time??

    I hope it's ok, this amp sounded soooo much better than my Mesa...and was soooo much heavier :scowl:

    Thanks in advance people!!
  2. Tash


    Feb 13, 2005
    Bel Air Maryland
    If a head is rated down to 4 ohms you can use an 8 ohm cab, you will just get less power out of the head.

    Its bad to go BELOW the rated ohms, i.e. going down to 2 ohms on a 4 ohm head is bad.
  3. Nope It's not a solid state amp. The rules are different. Here's why:

    To understand WHY tube amps don't like higher impedances and why they are MUCH more dangerous to tube amps than lower ones, you have to understand what it is in both types of amps that actually does damage.

    In solid state amps, the output transistors are connected directly to the speaker load. Since solid state amps put out a constant voltage, the variable that determines power/current is the impedance of the load. Ohms law: P (power)= i (current)/r (resistance). The lower the load (resistance), the greater the amount of current is passed by the output transistors. Current also generates tremendous amounts of heat, which is why if the load is too low, the heat stress on a solid state's output transistors can damage them.

    The same rules apply to tube amps, but there is a deciding factor that makes a huge difference. Tubes are extremely high impedance devices, and in order to be connected to a speaker, must be coupled through an output transformer which takes the high voltage/low current output of the tubes and makes a high current/ low voltage output that can drive a speaker. Instead of being constant voltage amplifiers, they are constant current amplifiers.

    A transformer is an inductive coil that works by amplifying current and/or voltage. If the transformer is not connected through a circuit on both its primary and secondary taps, it will feed back on itself and generate a large voltage spike at its connected end. What happens with too high a load connected to the transformer's secondary is that it acts like an open connection and causes feedback within the transformer. Since the primary of the transformer is connected to the plates of the output tubes, this voltage spike hits them first. As long as it doesn't exceed their limit, you're OK, but if it does, you can short them AND your output transformer.

    The reason a lower impedance is not so dangerous has to do with the nature of the tubes themselves. If the transformer secondary is connected to a lower than expected impedance, the transformer obeys Ohms law and tries to maintain a constant current, which causes a drop in the voltage of the plates of the tubes and they emit less, so power actually decreases. Some tube amps short themselves to ground when no load is present as a means to protect themslves from inductive feedback. That's as low an impedance as you can get!

    The short of all that is that 8 ohms is a 100% mismatch, and depending on your playing style, volume, and the actual electrical response of the speakers you hook to it, you strongly increase the possibility for SERIOUS damage to both your amp and the speakers connected to it just by using the amp. The impedance ratings are there for a reason.
  4. I had thought the ampeg tube amps could be switched between 4 and 8 ohms?
    Maybe thats just V4B's and classics.
  5. illidian


    Jul 2, 2004
    As I understand it, the current production V-4BH is 4/8, and the SVT is 2/4, and the 2-Pro is 2/4

    You definately don't want to flip the switch to two and use a four ohm load (or to four and an eight ohm load). But if the sales guy said you could and you hate the store, :ninja: Go for it! :bag:
  6. Rockinscott


    Nov 24, 2003
    Psycho Bass Guy is correct in all of his theory. The thing that prevents an SVT from being able to safely work at 100% mis-match is it's EXTREME power. Smaller, lower power amps can usuallly be run at that type of mis-match without much damage. The SVT and it's transformers are producing too much voltage and current to do that safely for any length of time. ONLY RUN YOUR SVT AT IT"S RATED LOAD! That is unless you have a thing for rebuilding a perfectly good paperweight. :D
  7. aw3645


    Apr 18, 2005
    If your amp, lets say can produce 500 watts at 4 ohms and you connect two 8 ohm cabinets to it, you would be driving two cabinets of equal impedence so each would receive half the power your amp can deliver, or 250 watts. The cabinets should be able to handle 250 watts each under clipping, that is where the amp runs out of headroom and begins to distort.
  8. I know someone who used to have an SVT-II and he used it at 8 Ohms, even though it was set to 4 (the highest it can be set to). This setup worked fine, but I think it shortens tube life a bit. Just don't go over 100% mismatch. And don't listen to any of this BS about fewer watts, thats only for solid state heads.
  9. Rockinscott


    Nov 24, 2003
    I'm sorry "Plain Old" but you are mistaken. While the drop in output power for a solid state device is much more significant as you increase the load, if you operate a tube amp at a load higher than what the winding on the output transformer is set for, it will most certainly reduce the output power due to the exponentially increased resistance against the entire output circuit (please refer to Ohm's law.) Here's a perfect example: Take an older (original) SVT and plug a single 4 ohm cab up to it and play it. Note the volume level. Now, shut it off, don't change any settings, just put a dummy 1/4" plug (one that has nothing soldered to either of the lugs) into the other speaker output then turn it on. Now when you play it you'll note that it has significantly more volume and punch. The explanation for this comes from Gerald Weber, founder and chief engineer for Kendrick amplifiers and author of the books "Tube Amp Talk for the Guitarist and Tech," and "A Desktop Reference of Hip Vintage Guitar Amps:" Mr. Weber writes "Ampeg realized that people were only using one cabinet with the SVT and were blowing speakers so they wired the impedance switch on the extra speaker jack so that when using one 4 ohm cabinet, the transformer was actually set at two ohms. This reduced the power somewhat and kept the speakers from blowing." He then goes on to explain the process I outlined above for increasing the power. Now I may be biased but having talked to Mr. Weber on a few occasions, having been to his factory and listened to his amps, and having read both of his books and his numerous articles in Vintage Guitar magazine, I tend to believe he knows what he's talking about but please, don't take his word for it, ask your local Amp Tech. Better yet, call Ampeg and speak to George McKale, the engineer that designed the SVT II.:D
  10. I was actually referring to the comment right above mine, but your comment was (I believe to be) correct. However, from experience this doesn't change the volume too much, but I wouldn't run a tube amp at max in any case because I like to hear things clearly ;)
  11. .

    Wrong. ANY tube amp with a transformer on its output has this problem. It varies in danger with tube type and plate voltage. The SVT is not a "special case."

    Wrong again. Running a higher load INCREASES power, as it dramtically raises the amount of plate volatge on the output tubes. If the tubes can take the stress, you're OK, BUT most bass amps are already running their output tubes at the limit of the operation voltage in order to maximze clean power.

    I have referred to this VERY passage before. Gerald Weber may make nice Bassman copies, but he is entirely WRONG on this and if you follow his advice, you risk destroying your amp. I cannot stress strongly enough how WRONG this is!!!!! If you read the info in my initial post, it explains EXACTLY why this is not the case.
  12. Running the amp at lower volume and avoiding clipping distortion is all that saved it from destruction. It's NOT a safe practice for the average player who wants SVT roar. You are also entirely correct about fewer watts.
  13. The guy actually ran it quite loud (not at 100% but occasionally at about 80-85%). But he had military surplus tubes, (which he said he paid a fortune for) but I dunno if the tubes had anything to do with the amp not blowing up...
  14. JAN (Joint Army Navy) milspec tubes were made to withstand extreme operating conditions and most other makes could not have taken it.
  15. Rockinscott


    Nov 24, 2003
    Now, now, let's just agree that we disagree, that's okay. Guys, please don't take my word, or anyone elses on this forum when it comes to what your amp is designed to do, or not to do, as you have no idea what experience or education any of us have with these amps. You should only trust your local tech or the engineers that built your amp. ALWAYS contact one of them when in doubt. Peace and Rock on!
  16. That's a bold statement from someone with all of six posts to his credit. My word is as correct as possible, and when there's a doubt, I have no problem pointing anyone onto references other than myself. I REFUSE to agree to disagree. This is physics and all well-documented since the 1930's. There IS an absolute right and an absolute wrong, and no matter who says what, that doesn't change. I have made my education and experience plain time and again. If you happen to not agree with what I say, unless you show some SERIOUS cause as to why not, I find it offensive that you recommend the advice I give be discounted out of hand.

    There are MANY tube amp manufacturers who have a strong working knowledge of them and how to build on someone else's work (in Gerald Weber's case, Leo Fender, who had built on Western Electric's designs), but lack the underlying BASIC electronic theory to explain even the most rudimentary concepts of their operation. A book and a successful business does not make you a technical expert; it makes you a successful buisnessman.

    If you want straight answers and you're into heavy reading:

    This should take care of everything.

    The Navy takes tubes VERY seriously. Also take note this document comes from 1998.
  17. Thanks to everyone for the replies and great info!! I do have two other 8 ohm 4x10's, so i'll just run one of them with the mesa cab, its just more crap to lug which is why I didn't want to in the 1st place - but if it saves my amp :smug:

    Thanks again!!
  18. I'm going to read that sometime. I really am. :D
  19. Rockinscott


    Nov 24, 2003
    To anyone who red this thread and was offended because I recommend that they use a qualified local technician, I apologize. :(
  20. I have absolutely NO problem with recommending someone to find a qualified local tech. I have made that recommendation myself MANY times. My problem is that you made the blanket statement to ignore the advice I gave because it disagreed with yours, irrespective of my qualification to render it. This may be the internet and it may be a semi-anonymous forum, but I have had an active visible presence here for five years and have helped many posters and even had a few fellow TB'ers in my shop. I would like to think that gives me a small margin of credibility. If you want to discuss credibility, maybe you ought to do some research into this forum and the topic at hand first.

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