Is it possible to remove 3 tubes on a 100 watt to make it 25 watts?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by regotheamigo, May 17, 2006.

  1. regotheamigo

    regotheamigo Member

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    I am losing my rehersal space pretty soon, and just looking for an alternative, so I can play my 100 watt at my house. I know that I can either remove the inner or outer tubes to make it a 50 watter, but don't know about just leaving 1 for 25. If I can do this, which one do I keep in? the middle one? LOL I would rather not ruin my amp, so just making sure. Thanks
     
  2. brad347

    brad347 Member

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    if you do this expect poor tone and possibly expensive repairs.
     
  3. Sparky6string

    Sparky6string Member

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    I think you need at least 2.
     
  4. UfoPilot

    UfoPilot Member

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    There is no "middle tube" There are only four tubes. And no you can't do that.
     
  5. trdlasvegas

    trdlasvegas Member

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    I'm still laughing at this.

    While your at it why plug in a single 5881 and only get 20 watts instead of 25 with the EL34.

    -Tony
     
  6. VaughnC

    VaughnC Supporting Member

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    In a typical AB push-pull circuit you need at least 2 tubes to amplify both halves of the output signal. With only one tube, the output would be very distorted with half of the signal missing.
     
  7. regotheamigo

    regotheamigo Member

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    LOL I'm laughing at it too. I don't know why I even bothered asking about the one tube, because I really don't plan on ever doing it. I am sure down the road though I will try just two tubes.
     
  8. VacuumVoodoo

    VacuumVoodoo Member

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    It can be done but NOT by removing 1 or 2 tubes.

    What you have to do is leave all tubes in their sockets and remove the signal from one of them by removing the cap between PI and power tube grid. This will allow the OT to work under proper conditions i.e. power tubes' dc bias currents will cancel in the OT so it doesn't saturate. If you at the same time hotbias the tubes the tube still connected to the PI will work in a sort of "poorly designed" class A up to reasonable power level. Whether it will still sound good is a matter of taste.
     
  9. Matt H

    Matt H Guest

    Vaughn- have to *slightly* disagree with you there. depending on the operating point of the tube in question, one half of a push/pull circuit can be amplifying for much more than "half" the cycle... the PI does'nt cut the signal in half, it sends two out of phase (well, reverse polarity) signals to two tubes. This just ensures that when one tube is off, *at least* the other one is on- not necessarily both aren't amplifying at the same time.

    in short- you'd still get a distorted signal, but you realistically wouldn't *only* be getting hal of your signal.

    depending on operating point, the tube in question, and the desired tone- this doesn't always sound *bad* either. I believe kevin o'connor wrote something up about this for a "variable SE" mode or something in one of the TUTs (never read any of them- but remember a few parts being discussed on teh webz0rz)
     
  10. TubeAmpNut

    TubeAmpNut Member

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    All the aformentioned comments are pure poppycock. You CAN remove 3 tubes in a 4 output tube amp without blowing things up. The amp will run and function fine. Voltages might be slightly higher and you will have an annoying buzz from ripple in the plate supply normally cancelled when operating two or four tubes.

    It won't be 25W, and it might sound like crap or it might sound good. It's up to you.

    BK
     
  11. Rich M

    Rich M Member

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    I've been using a simple mod I came up with to cut B+ in half and compensate on the bias and preamp voltage. It basically cuts the power by a factor of four (25W). If you then pull two output tubes, your down to ~12 W. I use this to allow me to crank my PPIMV way up. Sounds good to my ears, but I'd be interested in feedback.

    http://webpages.charter.net/rmweb/Quarter watter 1959.pdf
     
  12. donnyjaguar

    donnyjaguar Member

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    I have to disagree with BKamps. Its never a good idea to off-set the magnetics. Also, depending upon how the amplifier is designed, the plate voltage could very well creep up too high once the power supply is unloaded. It might work for a while, but its not a good thing to do. I've seen a mod that allows you to take half your output tubes out of the circuit (someone talked about this) and in my opinion this is the way to go. I think it sounds like sh*t myself, but of course YMMV.
     
  13. Shea

    Shea Member

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    Good advice - this will also keep the plate voltage from rising. I wonder if the Kevin O'Connor single-ended mod works on this principle.

    Shea
     
  14. Shea

    Shea Member

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    http://webpages.charter.net/rmweb/Quarter%20watter%201959.pdf[/quote]

    Looks like a great idea, because it's so simple. It's the new Rich Mod! :D

    Have you tried the following?

    1. Use a DPDT switch wired so that one pole will select whether the plates are fed from the full voltage or half voltage, and the other pole will will switch that parallel 47k resistor in or out of the bias circuit.

    2. Instead of bypassing that 22k resistor at the top of the schematic, disconnect the right side (on the schematic) from the screen supply and instead connect it to the high voltage at the top of the first 100uf cap. That way the preamp voltage will stay the same regardless of the position of the switch.

    Shea
     
  15. Rich M

    Rich M Member

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    Thanks for the FB. I have the switches, but haven't installed it in an amp yet. I thought of a separate feed for the preamp, but I suspect that would affect the dynamics of the amp substantially.

    Incidentally, I though I was real frickin' smart when this idea hit me and I realized some time later, this was implemented way back in the Fender 75. Now that shouldn't discourage folks from trying it, that amp had a lot of other problems. THe Fender 75 had a separate preamp feed to make switching easier.

    Let me know if you try it. It has a much more pronounced affect on volume than pulling two tubes and can be combined to make an antenuator unnecessary. ANd my Mullard OT's are going to last for ever!
     
  16. Amit

    Amit Member

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    I ran my 50 watt amp with a single tube without ANY issue and with GREAT tone for a longgggg time. though not in gig levels, but enough to feel and hear any funny issues. plate and bias were fine.

    Read the faq at Kevin's site, he explains this in length.

    If you want to feel *safe*, install a PPIMV just on one side of the PI.
    I went back to use two power tubes just because I enjoy the sound of mixed tubes (EL34/6550, KT88/6CA7 etc.).
     
  17. John Phillips

    John Phillips Member

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    It's nothing new. This is exactly how the quarter-power switch on the Fender red-knob 'The Twin' works. It was AFAIK the first commercial amp to use it, c.1987, and Fender have been using it ever since in several of their amps.
     
  18. VacuumVoodoo

    VacuumVoodoo Member

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    Quote:
    Originally Posted by VacuumVoodoo
    It can be done but NOT by removing 1 or 2 tubes.

    What you have to do is leave all tubes in their sockets and remove the signal from one of them by removing the cap between PI and power tube grid. This will allow the OT to work under proper conditions i.e. power tubes' dc bias currents will cancel in the OT so it doesn't saturate. If you at the same time hotbias the tubes the tube still connected to the PI will work in a sort of "poorly designed" class A up to reasonable power level. Whether it will still sound good is a matter of taste.


    This way of running a SE class A has been used quite o lot in the past. One tube running idle is still cheaper than a proper air-gapped SE OT.
    Kevin has a pot connected between one PI output (coupling cap in place before the pot and another coupling cap from pot wiper to power tube grid) and corresponding power tube. This way you can vary the level of drive to that tube continuosly from full to zero. No bias readjustment though.
     
  19. VaughnC

    VaughnC Supporting Member

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    Yup, I agree and know better....I was just trying to keep the discussion from going too deep for an apparent non-techie player. If someone doesn't know why you generally wouldn't want to remove one tube from a dual tube AB push/pull circuit, chances are that things like operating points, phase inverters, amplifier classes, cycles, etc. won't mean much to them....that's all. However, even though it's not technically correct, I think almost anyone could grasp the fact that removing one output tube in some amps causes distortion because one half of the original input signal is missing.
     
  20. Rich M

    Rich M Member

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    The Fender 75 predates "The Twin". I have seen heads with "half-power" switches added as well. Don't know if those are triode-mode mods though.
     

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