Pulling power tubes for reduced wattage?

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

  1. DreamTheaterRules

    DreamTheaterRules Former Lyricist for Calhoun Tubbs Silver Supporting Member

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    I've heard that in most amps that have 2-4 output tubes, you can pull a pair, or even 3 of the power tubes to reduce wattage of the amp. I am a fairly advanced amateur DIY guy, but I can't get my head around how this can be ok. But the person who told me you can do it is in EE and has off the charts knowledge on most things.

    So, on my classic 30, or other normal push/pull type amps, IS IT OK to just pull tubes to reduce power? And, I know there have to be some side effects, so what are they?

    I just keep thinking impedance will be screwed up somewhere, voltage flowing around in open circuits.... something bad has to be involved. :confused:
     
  2. Structo

    Structo Member

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    The output impedance changes when you do this as well.
    [FONT=Verdana, Arial, Helvetica, sans-serif]Then you need to adjust the load that the amp is driving. If you have a two speaker cabinet, like a Twin Reverb, disconnect one speaker. This will change the load from 4 ohms to 8 ohms, and the amp will be happy. If you have an impedance selector on the back of your amp, set it to half of the speaker impedance. If you use an 8 ohm cabinet, set the selector to 4 ohms.[/FONT]
    [FONT=Verdana, Arial, Helvetica, sans-serif]If you intend to run the amp this way for a while, consider rebiasing because now only 2 tubes are pulling current where 4 tubes where before, and the bias may be off a bit. [/FONT]


    But the fact is, when you half the power, you are only dropping 3 db of the sound level.
    So it isn't that big of a volume drop.
    The tone seems to change as well and most of the time in a negative way.

    You are much better off using an attenuator to lessen the volume.
     
  3. FrankieSixxxgun

    FrankieSixxxgun Member

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    Structo is right. The drop from 100 watts to 50 watts is 3-4db. It's not really a whole lot, and the tone loss you'll get definitely outweighs the volume drop. Get an attenuator.
     
  4. DreamTheaterRules

    DreamTheaterRules Former Lyricist for Calhoun Tubbs Silver Supporting Member

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    right guys, I know the wattage-db equations. But the thought is, it's easier to get the amp into the range it sounds better. Example, my Classic 30 is WAY too loud to play in it's good sound range, when my wife is home, but my High Octane and Valve Jr are not. If I pull 2 tubes from the C30 I'm at half power. But if I pull 3, I half it again.

    I haven't tried it, but I knew from all my reading up on building and modding amps, that it had to change impedance AT LEAST and I wondered about bias drift, etc...
     
  5. thesteve

    thesteve Member

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    From what I've read, pulling a pair in concert isn't a huge deal. Pulling three is going to give you issues on a push/pull tube configuration. When you pull the right pair you still retain the push/pull configuration. When you pull three you have a single tube left that is either "push" or "pull" thus leaving you with some definitely circuit issues.
     
  6. FrankieSixxxgun

    FrankieSixxxgun Member

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    You can't pull 3 power tubes, only one push/pull pair. 3 tubes missing = no sound. You'll also find driving two tubes into breakup is not going to give you the same tonal satisfaction, fullness, and depth of driving 4 tubes into breakup. Feel free to try it, but I'm sure you'll immediately put the other 2 tubes back. Typically you yank either the two outside tubes or the two inside tubes then half the impedance on the selector. Kinda like this: .||. or |..| in the back of your amp if you get what I'm saying.
     
  7. mooreamps

    mooreamps Senior Member

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    Just more reason for people to start looking at electronic power brakeing...

    -g
     
  8. FrankieSixxxgun

    FrankieSixxxgun Member

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    Wait, isn't the Classic 30 EL84-based? Don't the heater leads run in series? You might not be able to pull any tubes out of that amp as the heaters will stop working.
     
  9. TopBooster

    TopBooster Member

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    Yup, according to this thread...
     
  10. davemccarthy707

    davemccarthy707 Member

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    This works well for amps like Fender super twin or evil twin.
     
  11. FrankieSixxxgun

    FrankieSixxxgun Member

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    Now, the question looming over my head is what is this classic 30 too loud for? 4xEL84 amps aren't notorious for being ear shatteringly loud, so are you trying to get bedroom levels with this guy? Just stick him off to the side for gigs and buy a smaller amp for your house. Beats breaking him or spending good money on an attenutaor.
     
  12. DreamTheaterRules

    DreamTheaterRules Former Lyricist for Calhoun Tubbs Silver Supporting Member

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    Well, for starters, clean channel on 3, with say the OCD on and it is so loud that with all windows and doors closed, my neighbors 2 doors down (120 ft wide lots, not houses right on top of each other) can hear it from inside their house.

    At an annual jam weekend we have in another forum I'm in, about 4 guys have classic 30s (and there are Marshall, Bogner, Orange etc there as well) and we had to agree after the first year, to not play as loud as the C30s are on 3 with an OD pedal, because after an hour, everyone ears were getting burned out badly.
     
  13. FrankieSixxxgun

    FrankieSixxxgun Member

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    Sounds like you'd benefit more from new speakers then. There's good loud and then there's bad loud. You might be in the bad loud category. Try some inefficient, low wattage speakers and I bet you'll be surprised.
     
  14. DreamTheaterRules

    DreamTheaterRules Former Lyricist for Calhoun Tubbs Silver Supporting Member

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    I have a Governor it it, and other cabs with a Greenback, Wizard and Governor.

    Keep in mind, using the clean channel on 3 with on OD set above unity, is probably like the amp being up around half way.

    Heck I see you have a Valve Jr. You know how loud those can be in a home. My music room at home is the spare bed room. I bet the Jr can push 110db in that room.
     
  15. DreamTheaterRules

    DreamTheaterRules Former Lyricist for Calhoun Tubbs Silver Supporting Member

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    I'm bad at any volume, so what's your point? :D

    Seriously, I've looked into the "less efficient" thing before. What quality speaker is really that much less efficient than the majority? Most of them these days seem to be around 100dB 1W/1M Don't see many that rate below 97 or so. It would be nice if there was a 85-90db 1M/1W speaker out there... and if it sounded like the "regular" speaker of your choice (greenback, V30 or whatever). I'd buy one right now.

    And lest you think we're just a bunch of old dudes, in that jam weekend I spoke about a couple guys with C30s can easily drown out the drums. So that's pretty loud.
     
  16. FrankieSixxxgun

    FrankieSixxxgun Member

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    No no, don't get me wrong! I wasn't putting down your playing, I was putting down the amp. The stock speakers are likely very tinny, icepicky, brittle, etc... An amp with too much bite will drown out most things in the mix. Anyway, you wanted some speaker suggestions:

    Celestion G12M = 96db
    Celestion G12-65 = 97db
    Scumback M75 = 97db
    Scumback M55 = 96db

    They don't get much more Greenback, save for the 65, than those, sir. I personally run two 4x12's loaded with G12C-25 Greenbacks and I can manage the volume out of a 100 watt Marshall head with some tricks, but no tubes pulled.
     
  17. FrankieSixxxgun

    FrankieSixxxgun Member

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    Bingo! My 100 watter is plain out unusable with G12T-75's like it came with, but on some 25 watt Greenbacks I can get a little nuts with the volume knob :D
     
  18. DreamTheaterRules

    DreamTheaterRules Former Lyricist for Calhoun Tubbs Silver Supporting Member

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    nah man, I knew you weren't putting down my playing. I WAS! :)

    My Blue Marvel went by-by right after I got the amp. It has an Eminence Governor in it (which as you know is a V30 copy that is preferred over the original by many here). I also have external 1x12 Avatar cabs, one with a Greenback, one with a Wizard, one with a Governor (all closed back). So I have some good speakers and a good range of speakers, tone-wise. But they are all LOUD.

    I can peel paint off the walls with the Wizard and my Valve Jr or my High Octane (also around 5-7 watts).

    Too bad there isn't something that sounds like a greeny or V30 that's down around 90dB. Then you could actually open the amps up a bit at home, and use your more efficient speakers at the gig.
     
  19. FrankieSixxxgun

    FrankieSixxxgun Member

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    Well, if you like the tone, why not just get an attenuator? It's probably exactly what the doctor ordered. I run a Hot Plate on my Marshall in order to keep it manageable while still staying in power tube compression. You can't run a Marshall MV at 8 like I do an expect to not get thrown out of a club! :D
     
  20. DreamTheaterRules

    DreamTheaterRules Former Lyricist for Calhoun Tubbs Silver Supporting Member

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    Frankie,

    Yeah, but you can run your Marshall at 8 and put a BIG smile on your face, can't you! :AOK I know how good those things can sound when you open them up. I don't have the space to do that.

    No less than Robin Trower has been quoted as saying he couldn't open up his Marshall stacks at even big clubs these days, (In a comment he made about his OCD settings at the time).

    My spare bedroom aka music room is NOT a large club! So, I'm looking into attenuation vs. power scaling. But it would be nice if you could get 90dB V30! (Eminence will make you one, BTW. Minimum order, 5000 copies...)
     
    Last edited: Oct 9, 2008

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