Should you REALLY pull 2 of 4 power tubes?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by mbruffey, Jan 22, 2006.

  1. mbruffey

    mbruffey Supporting Member

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    Another question from Mister Question:

    If I have a Fender Custom Twin 15 and pull output tubes 1 and 3 or 2 and 4, will I hurt the amp?

    Why would I want to do that? Well, that amp has 4 output tubes, but I want to try other brands of 6l6gc's. I happen to have matched pairs in two other brands. Can I remove all 4 fender original tubes and substitute only two(2) in their place as indicated above--assuming a rebias?

    Thanks,

    Mark
     
  2. bailnout

    bailnout Member

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    I'm sure someone with WAY more knowledge than I have will jump on this thread to help guide you, but for starters, I don't think you wand to pull 1 and 3 or 2 and 4. I think you would want to pull 1 and 4 or 2 and 3. Like and inside or outside kind of thing. Like I said though, I'm no expert. I know I can do it with my OD-100. It says so in the manual. The only thing it really warns is that you have to drop your ohm setting at the amp to half of what the speaker is. So like if you have an 8 ohm speaker, you need to set the amp to 4 ohms.

    Good luck!
     
  3. mbruffey

    mbruffey Supporting Member

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    You certainly could be right, bail. I got the information over on HC, but it _could_ be misinformation, which is one reason I asked here . . . .

    " If you take out either the 1st and 3rd together, or the 2nd and 4th together, you end up cutting it's power rating in half." (HC, under regular Twin RI Evals)

    Mark
     
  4. bailnout

    bailnout Member

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    yea, I don't know. it could be a different circuit layout for that amp than with mine. But with most of the amps that I've heard that you can do that with it was an inside or outside thing. Do your homework before you do anything though. Would hate to see you f-up your amp. This is a good place to ask though. Lots of perpeller-heads hang out here. :)
     
  5. John Phillips

    John Phillips Member

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    You simply need to pull one from the left side and one from the right side. It doesn't matter which ones. So you can pull (or use) 1&3, 1&4, 2&3 or 2&4. But not 1&2 or 3&4.

    Set the impedance of the amp to half that of the speaker, if it has the option.

    If the amp has bias test points like the older Twins, which meter both tubes on the same side at once, you need to HALVE the readings when you're only using two tubes.

    If you do these three things it's perfectly safe for both the amp and the tubes.
     
  6. darkbluemurder

    darkbluemurder Member

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    I would not worry too much about reducing the speaker impedance in Fenders. Neither my Concert, Bassman and Deluxe II care whether they are played through a 4 or 8 Ohm speaker. But I do not turn the amps up to more than 4 on the volume control.
     
  7. Reactionjackson

    Reactionjackson Member

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    Well, the impedance doubles in 1/2 power (2 tubes) no? So if your running an 8 ohm out into an 8 ohm speaker you would wind up with a 16 ohm impedance running into the 8ohm speaker which could pave the way for some funky burnt rubben smells!
     
  8. GuitslingerTim

    GuitslingerTim Member

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    Marshall warns against pulling tubes period, and I wouldn't be surprised if Fender does as well. If you need less volume or headroom buy an overdrive or distortion pedal.
     
  9. hasserl

    hasserl Member

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    You've got this a little backward. It's not that you will end up with a 16ohm impedance running to an 8ohm speaker. It's that you will have an 8ohm speaker reflecting an approximately 2k load onto a pair of 6L6's that would ideally like to see around 4k. BUt this is not so great a difference as to cause any funky burnt rubber smells. As JP said, it will be fine.
     
  10. mbruffey

    mbruffey Supporting Member

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    You know, there's something I don't understand about this speaker load stuff. Suffer me . . . .

    It's B-A-D, I understand, to unplug the speaker completely from the amp. Why? I would THINK that merely opening a circuit would cause no harm.

    BTW, the Speaker in the Custom Twin 15 is labeled "4 ohm" FWIW.
     
  11. tjs

    tjs Senior Member

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    I have a related question: I've got a 4xEL84, cathode-biased amp with a half-power switch. It has an 8 ohm speaker and is switchable between 8 and 16 ohms. Since I can't switch it to 4 ohms, I just leave it set at 8 regardless of what mode it's in. Is there any danger to my amp, speaker, and/or tubes when using the half-power switch in this situation?
     
  12. mbruffey

    mbruffey Supporting Member

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    But if you already have a properly biased quad in the amp and simply pull two, you do not, at that point, need to rebias. Am I understanding correctly?

    You would only need to rebias if you pulled all four of the properly matched quad and put a PAIR of different tubes in, right?

    What I would do would be to measure the bias readings with the original quad, record it, pull two tubes, record the bias readings for that condition, and use those readings as a benchmark for my "experiments."

    Does that sound logical?
     
  13. mbruffey

    mbruffey Supporting Member

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    For the Twin Reverb RI, if you do pull the chassis is there ANYTHING else you have to do, like turn all controls to zero or plug the speaker into jack 2 before you take bias readings? Just checking.
     
  14. VaughnC

    VaughnC Supporting Member

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    If it weren't for an output transformer in the circuit you'd be right....as in most solid state amps. However, in a tube amp, if the signal from the output tubes is applied to an unterminated output transformer, a magnetic field builds up inside the transformer itself and it has nowhere to go. So, when the magnetic field collapses, it induces a voltage (called flyback) back into the transformer which is stepped up by the transformer windings and sent backwards toward the amp. This reflected voltage can then cause insulation breakdown in the transformer itself or other componants such as tubes and tube sockets. And this is why most tube amps have shorting type speaker jacks.....it protects the amp from flyback voltage if there's no plug in the jack by shunting it to ground. However, a speaker cord dangling from the amp will provide no flyback protection.

    There's also a misconception that a tube amp will be damaged with no load on the amp even if your not playing through the amp. If an amp is functioning normally, and your not playing through the amp, no flyback voltage can be generated so the amp can't be damaged if its unterminated. However, you never know when an amp is going to malfunction (and start generating its own signal) so it's best to play it safe and not take an amp off standby without a load connected.
     
  15. mbruffey

    mbruffey Supporting Member

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    OK, so if you took out ALL your Output Tubes, you COULD have an unterminated output with no harm, since the tubes supply the juice [signal?] to the OT, am I right?

    Similarly, by removing 1/2 of the output tubes, you are actually (potentially) reducing the energy sent to the output transformer, hence the output transformer is in no danger.

    BUT, the remaining output tubes still try to supply the energy the transformer/speaker combination expects, so they could get a little hotter? Something like that?
     
  16. VaughnC

    VaughnC Supporting Member

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    Right....with no output tubes, no flyback can be generated so an amp won't need terminated in that scenario.

    However, pulling two of the output tubes won't protect an unterminated amp from flyback damage....the reflected signal from 2 tubes still has nowhere to go except backwards toward the amp.

    The output tubes may or may not get "hotter" depending on how much mismatch you have between the amp & speaker and how hard (loud) you're pushing the amp.
     
  17. neve1073

    neve1073 Member

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    the only prob i've ever heard of when pulling two tubes out correctly is that a matched quad will wear at uneven rates. correct me if im wrong...
     
  18. Dickie Fredericks

    Dickie Fredericks Member

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    OK now Im totally confused... sorry to butt in.

    I just pulled the 1 and 4 tubes from my Marshall and replaced the 2 and 3 with YJ's. The speakers are 16ohm and the amp is set for 8 ohms. Should I now be switching it to the 16 (4) setting?

    Maybe this is the cause of my problem listed in the thread above?

    Richard
     

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