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SVT OHM Question

Discussion in 'Amps and Cabs [BG]' started by 57pbass, Jan 27, 2005.

  1. 57pbass

    57pbass Supporting Member

    I have a 1972 SVT head.

    It reads 4 ohms on each speaker output...

    I have not used this head in a very long time

    Can I connect a single 8 ohm 15 cab to each side or do I need to connect a 4 ohm load to each side?

  2. Ben Clarke

    Ben Clarke Liquidating to fund a new business. Buy My Gear!

    Jan 6, 2005
    Western NY

    Impedance ratings on tube amps are minimums. 8 ohms on each won't hurt a thing, and you won't lose an appreciable amount of output. However, running that amp after an extended period of inactivity might. The capacitors in the power supply may be suspect, as they tend to deteriorate without somewhat regular use.

    Worst case scenario: blowing a transformer. Not good.

    If possible, slowly bring the amp up to line voltage with a variac, or at the very least, keep a close eye and ear on the thing. A once-over by a trusted tech would be good idea regardless. Those heads are worth the few bucks to keep in nice shape.
  3. NO! NO! NO! NO!

    One 8 ohm cabinet on each output jack will have the amp expecting a 2 ohm load, not four!!!!!! The SVT uses a switching jack, so when you plug into the "Extension Speaker" jack, it trips a switch and connects BOTH outputs to the 2 ohm tap off its output transformer. While two 8 ohm cabinets are in parallel ARE 4 ohms, those are NOT normal paralleled jacks. And for about the fifty billionth time:


    While MOST tube amps can TOLERATE a 100% mismatch, it's not good for them, nor will they sound their best. And if you want to talk about risking an output transformer, the inductive feedback spike generated from having too high a load connected to it will do FAR more damage than any kind of cap short.
  4. 57pbass

    57pbass Supporting Member

    Ben, Thanks..I plan on bringing this in for a once over...

    Psyco.. So the answer is not to the single 8 ohm cabs..
    How about a 4 ohm Goliath into jusy one jack... Thanks
  5. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    PBG is right on...MOST tube amps will tolerate it, but the SVT isn't one I would advise mismatching.

    The SVT is pushing the ratings of the tubes already. So some of the "usual margin" for mismatches etc is used up. And, do you REALLY want to risk a nice old SVT?

    Yes, the 4 ohm cabinet is OK to use, assuming it is power rated suitably.

    BUT.....make sure you plug it into the right jack. As PBG mentioned, one jack is for a single 4 ohm load.

    But, when you plug into the other, BOTH are connected to the 2 ohm tap........the system assumes you will be using 2 4 ohm cabinets. Back in the day, there weren't very many choices at this power level, so that wasn't a bad assumption.

    If you plug 4 ohms into the second jack with nothing on the main output, you will have the same problem as using 8 on 4.
  6. Ben Clarke

    Ben Clarke Liquidating to fund a new business. Buy My Gear!

    Jan 6, 2005
    Western NY
    Well, I guess I stand corrected. I'm much more experienced in the home audio side of things, and had never heard of impedance being too high for a tube amp. My home speakers have wild impedance swings, from just over 4 ohms to about 40 ohms at one point. Most guys I know that use these run tube amps, with no related problems. Of course, they do have different design goals, so I can understand the disparate results. Any quick links where I can read up on this issue? I'd appreciate it.

    Am I wrong in assuming that this question has come up before? ;)
  7. Ben Clarke

    Ben Clarke Liquidating to fund a new business. Buy My Gear!

    Jan 6, 2005
    Western NY
    Also, to solve Dan's problem, it would make sense to just daisy chain the two 8 ohm cabs to present a nominal 4 ohm load to the main out jack, right?
  8. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    Yes, that should work.

    Of course the speakers do vary in impedance. They are rather high at resonance, and also increase with frequency, so a 4 ohm speaker may be 8 ohms at 3 or 4 kHz.

    But, starting with 4 ohms and going up is one thing. The 8 ohm speakers will have a larger maximum impedance.

    I can tell you that following the nominal impedance ratings will be fine if the amp has no other problems. There is a fair amount of history on doing that.

    If you use a different impedance of speaker, the results may also be fine......or not.....I don't know exactly, because it depends on the speakers, the amp, how you play, etc.

    I'm not gonna tell you to run your nice vintage amp skating at the outer edge of its limits, or maybe over them. What's the point? Use it as intended.

    If it has been out of use for a long time, it probably needs to be inspected for dry capacitors etc. Even if they are OK and the seals are good, they may need to be brought up slowly to full voltage as mentioned.

    Electrolytic capacitors can have problems if full voltage is slammed onto them after there has been no voltage on them for many years.
  9. Luckydog

    Luckydog Supporting Member

    Dec 25, 1999
    You can replace your 8 ohm speakers with 4's from Avatar, and sell the 8's on Ebay.
  10. icks


    Jul 12, 2001
    Charleroi, Belgium
    put them in series, you will get a 4 ohms load in one amp jack output.

    is that correct ? Ampeg insider or Psycho Bass Guy ?
  11. 57pbass

    57pbass Supporting Member

    I plan on using a Single 4 ohm Goliath cab through the main out and will also daisy chain my two Bag End S 15 Ds ( 8 ohms each )through the main channel as well.

    Thanks for the info folks...
  12. IvanMike

    IvanMike Player Characters fear me... Supporting Member

    Nov 10, 2002
    Middletown CT, USA
    nope. daisy chaining will hook them up in parallel, which will end up with a 4 ohm load.

    connecting cabs in series requires special wires or modifications. 2 eight ohm cabs in series will result in a 16 ohm load. plug that into an svt expecting a 4 ohm load and you can kiss it goodbye pretty fast.
  13. I forgot to mention that you can just hook the two cabs togther normally (in parallel) and run them both off the "Speaker" jack of the SVT, which will not trip the switch and the amp will be happily running into a four ohm load, just as intended.

    RE: Speaker impedance and resonance- keep in mind that those measurements are taken WITHOUT the influence of an amplifier, which damps the load and changes the overall properties of the speaker which is why an amp will always sound like "itself" even through various speakers. Resonance peaks are part of a speaker's reactive load and those figures WILL change when hooked to a running amplifier.
  14. Ben Clarke

    Ben Clarke Liquidating to fund a new business. Buy My Gear!

    Jan 6, 2005
    Western NY
    I just don't understand the inductive feedback issue. I'm no schmuck, but as I said, I'm used to dealing with hifi tube amps which are by and large not run near their limits. I'm sure the SVT is optimized for output, while my McIntosh are designed for low distortion. I'd imagine this enters into the picture with this.

    I'm not disagreeing, and I'm sure not trying to be argumentative. I'd love it if you could either explain it to me, or point me to some reference materials. I really have little affinity for sand amps, so any knowledge I can attain regarding tube amps is really helpful.

  15. The issue of output feedback is one of the last "black magic" secrets of audio. It can encompass a whole host of issues from speaker resonance to amplifer output topology and is the reason why anybody can't just use WinDSL to make a cabinet that's better than EAW or Meyer. Speaker voice coils are inductors (transformers). As they move through the magnetic gap in a speaker, they generate feedback voltage that works in conjunction with whatever amplifier is powering them to produce that particular speaker's tone. Things like cabinet tuning and driver impedance plots are the STARTING point but to think that they tell the whole story of a how a cabinet will perform with any amp is a VAST oversimplification. There really aren't any refernce materials as most of the info on this stuff is propreitary and only recently coming into widespread application, but its the primary reason that live PA sound has made QUANTUM leaps in the past five years or so.