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Tube Amp Problem (Mesa 400+)

Discussion in 'Amps and Cabs [BG]' started by Matteran, Apr 16, 2005.

  1. Matteran

    Matteran Banned

    Jan 1, 2005
    Santa Rosa, CA
    My girlfriend has a Mesa/Boogie 400+ (which i am incredibly jelous of), and today, she went to band practice, and i was hanging out there a bit. She goes to play, and the output is really quiet, and there's lot's of hum. We change cables, same thing. Replace her 9V in her bass, nothing changed. I check out the amp, and she had her amp outputing 4ohms into her 8ohm cab.

    Now, I told her that she needs to make sure she puts it in the right plug, because she's done this before. I said that it could destroy the amp, but it's less of a problem with tubes, but not much less. And for some reason, she took the "less of a problem" thing too far, and I suspect that for a while she's been doing this. At least once before, maybe more.

    The power tubes were bright purple, brighter than i've seen them, but with the fan on high they weren't hot enough to blow the fuse. Anyway, I'm starting to think that she's ****ed her power tubes. Any advice?
  2. coolest girlfriend ever.... :bassist:
  3. AlembicPlayer

    AlembicPlayer Im not wearing shorts

    Aug 15, 2004
    Pacific Northwet, USA
    that wouldn't cause the problem you describe..also, she is not hurting the amp running a 8 ohm cab thru the 4 ohm tap. The problem is elsewhere.
  4. Ralphdaddy

    Ralphdaddy Supporting Member

    Nov 6, 2003
    Chicago, Illinois
    Take it to a certified tech brother. But yeah, she is cool, I love my 400+!
  5. Wrong. That's EXACTLY the problem. At the very least she's looking at a new set of tubes installed by a tech who'll check the amp out for further damage (which there probably is). Higher impedances than rated damage tube amps. For the reason 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 varible 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 transfromer 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!
  6. AlembicPlayer

    AlembicPlayer Im not wearing shorts

    Aug 15, 2004
    Pacific Northwet, USA
    I respectfully disagree..
    I've run Boogies for years...impendence mismatch will cause slightly lower output and may shorten tube life...but wouldn't cause the problem he describes. Even in the Mesa/Boogie manuals, Smith tells you it's okay to mismatch and experiment with different ohm taps.

    from Mesa's manual for MarkII -
    from Mesa's manual for the Bass400+
    I'm not advocating running mismatched cabs for any prolonged period of time...but I have never had the problem he describes from running my Boogies in the same fashion..including their guitar amps.
  7. You're talking about GUITAR amps with much smaller amounts of both relative plate voltage and current. When you raise the impedance on an 6L6 based amp running at 300-400 volts B+ and passing nothing lower than 80 Hz, it's a whole different ballgame than a bass amp running the same tubes at nearly 600 volts under NORMAL conditions before a reflected impednace spike raises it even further. Why exactly do you think doing so "may shorten tube life"? It also won't LOWER output; it raises it, but in a VERY dangerous fashion.

    I'll highlight MAJOR point you're missing:

    8ohms on 4ohm tap is a 100% mismatch. That's quite a bit more than "slight." Not all nominally rated loads are equal either. One kind of speaker may have a significantly larger impdenace resonance spike than another of the same impdenace rating, but don't take my workd for it. Here's what Jerrold Tiers, an engineer for Ampeg had to say about this very subject, complete with a link to the entire thread it came from:

    You can see the whole thread here.
  8. BassGreaser


    Aug 22, 2002
    Austin, TX
    I agree 100% with Psycho....... :bag:
  9. BurningSkies

    BurningSkies CRAZY BALDHEAD

    Feb 20, 2005
    Seweracuse, NY
    Ok...so I have a somewhat related question...I too have a 400+, and am currently running 2 8ohm cabs properly in the 4ohm outs...but soon I'm going to have a Schroeder 1210 cab...which I believe is only made with a 4 ohm impedance...Soooo...I had been planning on using it with an Avatar SB112(8ohm) for a bit, until I can get a new 4ohm driver for it...making the load 2.666ohms...Is this a very bad thing for the 400+? I had been told it would be ok to run it off of the 4ohm outs. True? Not true?
  10. Plain Old Me

    Plain Old Me

    Dec 14, 2004
    Heres my $.02... I know a guy who has run an SVT-II for years into a cab at 8 Ohms, though the amp has a max of 4. Tubes can take a 100% mismatch with some loss of tube life, but as long as you don't go over 100% nothing else will happen. I dunno if there is a difference in the 6550s of a SVT-II ot the 6l6s in a Mesa, but my experience is that they will both work.
  11. tombowlus

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

    Apr 3, 2003
    Fremont, Ohio
    Editor-in-Chief, Bass Gear Magazine
    I asked Geof at Aguilar about the correct setting for running a 4 ohm HT310 and an 8 ohm HT115 (2.67 Ohm load) off of my DB 728, and he told me to use the 2 ohm setting.