Psst... Ready to join TalkBass and start posting, make new friends, sell your gear, and more?  Register your free account in 30 seconds.

Can this be done?

Discussion in 'Amps and Cabs [BG]' started by bassboysam, Feb 18, 2005.


  1. The other day someone mentioned to me that I should remove 1 or 2 of my power tubes from my amp to reduce its power and therefore enabling me to overdrive the tubes without having to go to insane volumes for practice, small venues or recording. Is that OK to do or would that damage the amp in any way?


    thanks
     
  2. With some amps you can pull two power tubes and change the impedance selector and effectively 'half' your power. Keep in mind it's only like a 12% decrease in volume, so you're really not accomplishing a heck of a lot.
     
  3. nonsqtr

    nonsqtr The emperor has no clothes!

    Aug 29, 2003
    Burbank CA USA
    Ouch. My favorite question. :eyebrow:

    The answer is "yes", you can pull some power tubes, but in that case you need to pay careful attention to the speaker impedances, and the "load" on the amp.

    It's especially important if you're running at full power (or "close to it"). And, it also depends on the amp. If your amp is already running on the hairy edge of self-destruction, the load impedances are probably even "more" important.

    I'll give you an example. I have a Marshall Major, that's probably one of the best sounding tube amps I've ever heard. It has such punch and presence, it's unreal.

    But, when you crank the amp, the output transformer gets some serious stress. It gets "hot", real hot sometimes. It's really easy to blow the trannie, if your amp isn't "tuned up" properly.

    Now, I've pulled a couple of the output tubes, for recording, sometimes, to get the volume down to a more reasonable level. But, in doing that, I have to make sure that my speakers are adjusted accordingly.

    In other words, I'd need to plug a 4 ohm load into the 8 ohm speaker jack, or vice versa, depending on which way I'm going.

    The general rule for tube amps seems to be, anywhere between 50% and 200% of the rated impedance. But, those boundaries get a little tighter when you're operating at full volume.

    "Be careful", would be my take. Yes it's possible to do that kind of thing, but make sure you understand the ramifications. :)
     
  4. It woul be much safer to get an attenuator.
    They work great !

    JJ
     
  5. so if i pull out 2 tubes and i am using an 8 ohm cab where should i put the ohm selector on the amp 8 or 4?
     
  6. IvanMike

    IvanMike Player Characters fear me... Staff Member Supporting Member

    Nov 10, 2002
    Middletown CT, USA
    i would get info on the specific model 1st before doing it. also, you can't just remove any 2 tubes, they have to be the right ones.
     
  7. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    We don't recommend it. We don't warrant it. We don't support it.

    But, this has come up, so it is as well to go through it.

    The transformer causes the tubes to "see" a higher impedance than the speaker, due to the winding ratio of the transformer.

    This may for instance be 3600 ohms "plate-to-plate", which is 900 ohms load on either plate lead. That is what the tube plates "see". (I am pulling these numbers out of the air, don't go by them in a specific case)

    In the original system, with two tubes, each tube "saw" 1800 ohms, because the other of the pair was driving the other 1800 ohms equivalent. The two parallel 1800s equal 900, so it works Ok if that is what the tubes should be loaded with.

    Now, if you pull two tubes, one from each side, of an amp that has 4 output tubes, you don't change what the tube "sees". it is still loaded with 900 ohms. But only one tube is present per "side" , and it "sees" the whole 900 ohms.

    If it was properly designed originally, that is too heavy a load, and the gain and power output change. The tube will be overstressed as far as current, and will probably "wear out" faster due to excess cathode emission.

    So, assume you had an 8 ohm load on the unit's 8 ohm tap with all 4 tubes in place, then you pull out two of the 4. (You need to pull one from each "side" for this to work at all.)

    To correct the impedance, you can move the 8 ohm speaker to the 4 ohm tap. By doing that you change the effective tube plate load back to 1800 ohms. Since just one tube "sees" that, things are are reasonably "ok" again, at half power.

    Voltages are likely higher due to less loading. And the spike voltages from clipping etc will go up. Tone will probably change.

    If the unit is cathode biased, you are maybe in deep trouble, because the bias will now be way wrong. The remaining tubes may be turning red hot from excess current even at no load. If that is what you have, don't do it.

    I cannot guarantee what any of that will do in your particular case. And you may not have the proper tap to re-connect to anyhow.

    I am not recommending this. I am just explaining what happens and what can be done about it IN SOME CASES. Might be useful on a gig sometime if some yahoo breaks a power tube moving the amp.