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Pulling Power Tubes to Reduce Level

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by stratovarius, Feb 20, 2006.

  1. stratovarius

    stratovarius Supporting Member

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    What are the factors that make it possible to do this for some amps but not for others?
     
  2. Mark Barratt

    Mark Barratt Supporting Member

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    I've only ever pulled out the power tubes on a Marshall JMP 100.
    The 2 "Outside Tubes" only --- or you can pull out the 2 "inside" power tubes. But NEVER tubes 1 and 3 (or 2 and 4). This is my understanding anyway.
    It will reduce the power/volume, but bear in mind that if you run the amp "real loud" it will burn out the 2 remaining power tubes quicker.
    At a medium/loud volume level it should be O.K.
    When i did it i noticed a earlier break-up of sound i.e. dirtier at a lower volume and a slightly richer/warmer tone.
    Hope this helps.
     
  3. hasserl

    hasserl Member

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    On most four tube amps you can pull two of the tubes, but not on all. For instance, on a Peavey Classic 30 the power tube heaters are connected in series, so you cannot pull two of the tubes on these amps. Other amps may have similiar or other resttrictions, so don't go and do it willy nilly. Know how your amp is constructed and if it is possible to remove two of the tubes before you do it. As far as which two to pull, it again depends on the specific amp, but usually you can pull either of the two tubes on each side of the output primary, either the inner or the outter pairs, or one of each. Not both from one side.

    On those amps that you can do this on, it is best to adjust the impedance of the load to compensate for the change in the requirements for the primary circuit of the output. If your amp has an selectable impedance switch, it is best to set it to the next lowest position when removing two of the power tubes. This will keep the reflected speaker load at the correct impedance for teh primary. If your amp doesn't have an impedance switch you can still remove the tubes, but there will be a slight impedance mismatch, i.e. the load will be a little lower than it should be and the amp will work a little harder. But it is not too severe and with most amps this should be fine; it should not cause any problems.
     
  4. aleclee

    aleclee TGP Tech Wrangler Staff Member

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    You can't generally do the tube pull thing on cathode biased amps.
     
  5. stratovarius

    stratovarius Supporting Member

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    Thanks for the replies. My Maven Peals can do this, but we were having a discussion about whether it could be done for a Bogner Metropolis. Some folks are saying the 30 Watter is way too loud.
     
  6. aleclee

    aleclee TGP Tech Wrangler Staff Member

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    I believe the Metro is cathode biased.
     
  7. electroid

    electroid Member

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    Another thing to consider, when you pull 2 tubes, the plate voltage will go up, that means the idle current and full power current of the remaining tubes will go up too. This might be a problem on some amps.
     
  8. jezzzz2003

    jezzzz2003 Member

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    I hear that pulling power tubes = losing juice and tone.
    I read this in the tube amp book,
    I can see the logic behind this,
    Anyone else agree?
     
  9. hasserl

    hasserl Member

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    No, running on just two tubes can allow you to turn the amp up a little more than you otherwise would be able to, improving the sound of the amp.
     
  10. John Phillips

    John Phillips Member

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    You can pull 1&3 or 2&4 perfectly safely... what you shouldn't do is pull 1&2 or 3&4. You must pull one from the left pair and one from the right pair, it doesn't matter which*.

    (*Usually. With some vintage Marshalls and possibly other amps it's better to pull the inner pair, because these don't have grid stopper resistors. The outer pair do, and this is less hard on them if you're going to really thrash it.)

    You also should reset the impedance - to half what the cabinet is. This is because by pulling half the tubes, you double their impedance; so you must double the impedance of the load (relative to the transformer) to keep the correct match. If you do this, the tubes won't burn out any quicker, at the same amp settings - the volume will simply be reduced (of course, you may then want to drive it harder too, which will increase wear). If you don't reset the impedance the tubes will definitely wear quicker though.

    Only very marginally. You're reducing the overall current draw by no more than about 80mA, which even assuming a tube rectifier with a forward resistance of 100 ohms gives only an 8V drop in plate voltage. That's equivalent to a supply voltage change of only 2V (in the US, 4V in Europe), and is well within normal variation. I wouldn't expect it to significantly increase stress or wear on the tubes.

    Only if by 'improve' you mean more distortion :). I actually agree that pulling tubes can often make the amp sound less 'juicy' and full... stiffer and more midrangy as well as more distorted. Pulling two tubes will gain you only a volume reduction of 3dB, which is the same as running a parallel dummy load or an attenuator set one click (on most types) down... which can IMO sound better.

    Pulling two tubes does double the life of the tubes though... because you're only wearing out one pair at once ;).
     
  11. jezzzz2003

    jezzzz2003 Member

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    In theory pulling two power tubes is all fine and good but what about the transformers that are made for use in a 100w amplifier thats been reduced to 50w?
    The transformer is without a doubt one of the most tone contributing aspects of an all valve amplifier, they are partly responsible for that flat out tone we all know and love
    and when were not running the amp as it was designed to do I do believe the results are not as pleasing.

    This is just my thought and I have heard this before also.
    Thanks for listening though.
     
  12. brad347

    brad347 Member

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    i might be old fashioned but I'm a believer in asking an amp to do what it was meant to do.

    If a 100 watt marshall is too loud (it almost always is) then don't buy a 100 watt marshall.

    Seems simple enough to me...
     
  13. electroid

    electroid Member

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    I still say it is important to re-check the bias after pulling 2 tubes. There is more taking place here than just the voltage drop across the rectifier. Power transformers are rated at for a specific output voltage at a specific current draw. When you reduce the current (pulling 2 tubes), the plate voltage will go up enough to affect the idle current in the tubes. If you have your tubes biased hot, pulling 2 tubes can be enough to damage the remaining tubes in in some amps. I've seen it happen. It really depends on the power transfomer.

    The final answer is as stated above by others, really a matter of personal taste, (and budget). Pulling 2 tubes is obviously less expensive than buying a high quality attenuator. It will sound a little different too. Some may prefer the sound of running on 2 tubes. Others may prefer the sound of an attenuator.
     
  14. Mark Barratt

    Mark Barratt Supporting Member

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    I agree.
    When i tried this with one of my Marshalls the fad only lasted for a few minutes or so and i put the other tubes back in.
    It did make a bit of a difference but i lost "something" in the overall sound.
    I like to play loud anyway.
     
  15. EADGBE

    EADGBE Member

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    Marshall told me I could do it. But that I should re-bias the remaining two tubes. And set the amp's impedance to 8 ohms to run into my 16 ohm cab.
     

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