Half Power in a cathode biased amp

Discussion in 'Amps and Cabs' started by re-animator, Feb 17, 2009.

  1. re-animator

    re-animator Senior Member

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    hey all. in the never ending quest to coax sweet tone out of my ac30 at lower volumes, i've stumbled upon ted weber's Hi-Lo Power Switch mod kit for cathode biased amps.

    I know that cathode biased amps are special and you can't just pull a pair of output tubes from them and adjust ohmage to decrease the wattage. I assumed it wasn't possible to do a half-power sort of deal on a cathode biased amp like an ac30, but apparently this kit allows you to do that:

    [​IMG]

    $15
    Hi-Lo Power Switch

    This kit allows you to go to half power in a cathode biased amp by changing the cathode resistor and ungrounding two of the output tubes. It can also be used for fixed biased amps by just ungrounding two of the output tube cathodes.

    Build info:
    Suggested Implementations
    Qty
    Cathode Resistor
    50 ohms 100 ohms 150 ohms 250 ohms 300 ohms

    For this kit, there are two resistors in the output tubes cathode circuit. If you calculate that you need 50 ohms for 4 EL84's, for instance, you would order 50 Ohms. Then, when you flip the switch to the low power mode, it puts two 50 ohm resistors in series for 100 ohms, and also disconnects two of the cathodes.




    does that seem legit?? I'm not that techie, but i guess it all checks out. I know a lot of cathode biased amps have pentode/triode switches to adjust the wattage. Is this a different beast or in the same ballpark?? I have an ac30cc2 which has a bias selector that lets the amp operate at 22 watts instead of 33. I always have it on the lower wattage setting. If I could install this half-power dealy then I could presumably bring the wattage down to 10-12 watts which would make it much more friendly to my playing as i crave low headroom.

    so is it that simple?
     
  2. Tone_Terrific

    Tone_Terrific Member

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    That is what you need to do.
     
  3. re-animator

    re-animator Senior Member

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    so what goes on with the bias? is it no longer a cathode biased amp?
     
  4. re-animator

    re-animator Senior Member

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    volume's not the issue for me. Headroom is the major concern. For me, less is better. If I could even drop the ac30 down to 5 watts, I would try it. Its still plenty loud with efficient 2x12s.
     
  5. asintoras

    asintoras Member

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    Another problem is that you'll have to modify your amp (install that piece inside, make a hole in the chassis, etc).
    I was thinking about do that in my GA30RV, but I decided not to do it.
    In addition, the difference in volume is not that much.
     
  6. Masco

    Masco Member

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    Get a London Power SB-2 power scaling kit. For the tone you've described, this is the correct option for you. Get someone to install it for you if you're not up to it.
    No Drive Compensation control needed, you can use the stock MV for this.

    Go to the London Power website to read more.
     
  7. Deadduck

    Deadduck Member

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    Does your AC30 have a master volume? Maybe you need an AC15CC with a half power switch and master volume.

    My Tiny Terror is cathode biased and has a half power switch. I've read it does something with the bias, but it doesn't make that much difference in volume. It does give it a little less headroom, and a slightly crunchier sound.
     
  8. Steve Dallas

    Steve Dallas Supporting Member

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    Technically speaking, that will work to give you half power, but like all other solutions, it is a compromise. You would achieve half power by lifting the cathodes on 2 out of 4 power tubes and resetting the bias on the 2 remaining tubes. So far, so good. But one side effect of this is the remaining tubes would be running at half their optimal load in terms of primary impedance. The result will be more distortion and a spongy feel. You may like it or you might not.
     
  9. re-animator

    re-animator Senior Member

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    ac15 won't cut it.... vox doesn't make an ac15 with a normal channel (only top boost). Plus I like the depth of sound with 2 speakers.

    MV is..... not the same. I'm more about power tube distortion.
     
  10. nomadh

    nomadh Member

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    I've been thinking about this for my amp also. The concept is sound. It is only a minor diff in vol. Remember 3 watts is 1/2 as loud as 30 watts. The diff between 15 and 30 is very little. If you were close , like +- 1 or 2 on the vol it could do the trick it seems.
     
  11. Rod

    Rod Tone is Paramount Silver Supporting Member

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    What you really need is a Suhr Badger 18 with power scaling,,,,That really would be the ideal amp for you......
     
  12. re-animator

    re-animator Senior Member

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    nah i'm more than happy with the tone of my vox. The sound really speaks to me.... but as with virtually any good sounding tube amp, taming the beast isn't easy.
     
  13. Masco

    Masco Member

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    Power Scaling your amp's output tubes with a London Power SB-2 kit will yeild more output tube distortion than with your amp stock.
    I would find an authorized power scale installer in your area (Val King?) and have it done. Or DIY for $50. This is a no brainer, you'll thank me in the end.
     

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