Question re removing power tubes to reduce power

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by sfarnell, Aug 7, 2008.

  1. sfarnell

    sfarnell Gold Supporting Member

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    I'm thinking of buying a used Bruno Cowtipper 90. I was wondering if I could reduce the power by 1/2 by removing two of the power tubes without having to re-bias the amp???

    Any help is appreciated.
     
  2. scottl

    scottl Member

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    You can do this, however you must plug your speaker cab into 1 lower impedence speaker tap. For example, plug your 8 ohm cab into 4 ohm tap. This because the tranny is matched to the plate to plate impedence of four tubes and now you have two. Also, the bias may change subtely as the two removed power tubes are no longer drawing current. I have seen bias drift up 3 to 4 ma in my experiences. You may also experience higher B+ voltages and other effects. The feel may be different as well since the PS will effectively be stiffer and the speaker dampening may change.

    Congrats on the Bruno CT. They are great sounding amps.

     
  3. WaltC

    WaltC Member

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    Yes and No and Why?

    to expand:

    - yup you can remove two of the power tubes to reduce the amps power
    - no you can't do it responsibly w/o at least checking the bias after removal and probably adjusting it
    - why? Dropping the amp from a nominal 100watts to approximately 50 watts will make it sound *a little* quieter at full volume, but no where near 1/2 as loud. The way amps (sound to the human ear in fact) work is that to get a noticeable increase in volume you need to double the power. So to cut the overall volume of the amp in half (to your ears) you'd need to drop the power by over a factor of 10 (down to 10 watts or so). Not something you can do by pulling two of the tubes.

    Pulling two tubes and re-biasing (if needed) will make the amp behave differently as you turn the volume up, will probably break up sooner on the volume knob by some amount and may change the overall tone to some degree, but it won't make it half as loud. To do that you need to buy another (15 watt or less <G>) amp.
     
  4. scottl

    scottl Member

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    Btw, I agree 100% with Walt. I have always preferred the sound of my 100 watt amps in full power mode. I had a coupel with 1/2 power switches and the overall amp exoerience was always in favor of leaving things full powered. Plus, there was not much attenuation at all running in half mode.

    Get a CT 45 if you are really worried. It will be lighter and a tiny bit more manageable volume-wise.
     
  5. sfarnell

    sfarnell Gold Supporting Member

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    I just missed one on eBay that went for $1350. I'm still kicking myself. There's a CT90 for sale right now, but I think that you're right: I'll wait for a CT45 or maybe even a Super 100.

    Thanks for all of the help. I recently purchased my first Bruno (CT35 PRo II) and am obsessed with the cleans. I'm looking for a little more power though.
     
  6. scottl

    scottl Member

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    S100 is a nice amp but is no CT45 on cleans. I'd look for the CT45.

     

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