Output Transformers & Power

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Jammin'John, Mar 13, 2006.


  1. Jammin'John

    Jammin'John Member

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    I was recently asking a builder some questions about an amp he makes.

    He makes a 5F1 champ with either an 8w,4 ohm O.T.

    OR
    15w,8 ohm O.T.
    He says :
    "My 15 watt Champ runs an 8 ohm OT so with a 16 ohm cabinet it makes 7-8 watts,"plug another 16 ohm cabinet in and you get the full 15 watts plus twice the coverage.

    JJ: "How can one tube,single ended, make 15 watts...........no matter what the tranny ?

    Him:"According to the RCA Receiving Tube Manual a 6V6GT has a maximum ouput of 5.5 watts and a 6L6GC is just 6.5 watts. How is it possible for a deluxe reverb, bassman, or a twin to produce the output wattage they do if not for the OT?

    JJ: "Tube amps don't ramp up the power,with different loads, like transistor ones do.

    Speaking of mismatches,
    you like to mismatch UP 100%

    Most fender type amps will take a 100% mismatch,but they don't like it.

    If you have to mismatch,
    an 8 ohm tranny would like to see,and be happier with 4 ohms rather than 16."

    Him: "An 8 ohm OT wants to see an 8 ohm load, and 16 is fine with it and safe for the amp.
    4 ohms wouldn't control the amp from pushing too hard, creating too much heat, and self destructing in short order.

    A tube amp does react to the load and the difference can be quite noticeable from 4 to 8 ohms, or 8 to 16 ohms, or vice versa. The mismatch that tube amps can handle is somewhere in between like 6 ohms on a 4 ohm OT, but never lower than the output impedance as in 6 ohms against a 8 ohm OT as you can damage the amp. I also use a meter to check the output and can see the increase with a full load as opposed to a half load, so it's possible to verify the output, and the load does make a difference in the amp's output."

    JJ:
    "I don't think that putting a larger O.T. on a given amp gives the amp more power,it just allows ALL of the power to get to the speaker !

    I'm sure that the champ with the larger O.T. sounds louder and cleaner but it doesn't jump up to 15 watts."

    JJ
     
  2. John Phillips

    John Phillips Member

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    You're absolutely correct and this guy doesn't know what he's talking about.

    You can't get more power out of a single 6V6 than about 6W, no matter what combination of voltages, OT ratios or load impedances you use - since you would have to exceed the tube dissipation rating. You could get up to about 15W from a 6L6 type (if it was a GC or a 7581A), but not with the same voltages and ratios. Using a larger-rated transformer (of the same ratio) does not increase the power output, unless the smaller one was actually saturating and limiting it. The power capability of the circuit is more determined by the PT voltages, and a 5F1 PT does not supply enough to get more than about 4W from the 6V6 - the Champ only got to 6W when Fender raised the B+ voltage to 420V, which is about the limit for a 6V6.

    Higher-powered amps such as the Deluxe and Twin achieve their greater power by the use of push-pull topology. This can give up to almost four times the power in Class AB, since double the voltage swing can be achieved (nearly) and power is proportional to the square of voltage.

    Running into too high a load is not too dangerous provided the flyback voltages don't get stepped up to above what the insulation on the primary side (in the OT, power tubes and sockets) will take. In theory... but since most amps are designed quite close to the limit, twice the matching load is about as high as it's safe to go normally.

    Running into too low a load will draw a bit more current, but the amp will not 'self destruct in short order' at all. It will wear the tubes faster, and actually the power output will be reduced (unlike with a solid-state amp) but you can go down to half the matching load without any trouble in most cases. That's exactly why Fender designed their BF and SF amps with the extension speaker jacks in parallel with the main one, with the main speaker already the matching load - and these amps are very reliable. Perhaps he could explain that one...
     
  3. Matt H

    Matt H Guest

    dude- out this "builder"
     
  4. Jammin'John

    Jammin'John Member

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    This "builder" seems like a nice guy and a lot of folks on this board seem to like his stuff !

    Matt H,I ride my motorcycle around Seneca lake a lot !

    I don't go around your lake much 'cause I don't like goin' through Ithaca.

    I do go to the state park with the realy tall falls though.
     
  5. Jammin'John

    Jammin'John Member

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    How about this one from same builder :
    [​IMG]
    The OTs I use are all 6k - 6.5k, no more & no less. At 8 watts I use a single ended OT (a direct replacement for Tweed Champs), & at 15 watts I use a push/pull runnning single ended (a direct replacement for a Princeton or Tweed Deluxe). I have no lack for tone with either set-up and everyone that plays them is amazed at the volume and full-bodied tone that the amp produces and yes it can get very bright also depending on the tubes that you use as that's the only way to set the tone on this circuit. I've also run all the different amps on 6L6GC/5U4G tube sets for 4-6 hours at a time during jams and the trannies are just a bit warm to the touch, so between the customer's comments and the real life test rides it appears that the amps are just fine.[​IMG]

    I dunno,what do you think ?

    Is a tranny for push-pull way different than one for single ended ?

    JJ
     
  6. Wakarusa

    Wakarusa Member

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    I think there's a fundamental difference between what you should do and what you can do. I'm all for breaking rules to find new tones, but it seems to me that this guy has a strong disconnect with electrical theory. Also, as others have alluded to, some of the statements attributed to this builder are just flat wrong -- not a matter of opinion, but empirically wrong. So either you're misquoting him, or he really doesn't understand what he's doing (even if he keeps getting lucky and producing things that don't catch fire).

    Yes.
     
  7. Mickey_C

    Mickey_C The Original Racketeer Gold Supporting Member

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    I disagree with the part I put in bold.

    The 6V6 can take up to 500V... I run them myself at 450V without issue. Kevin O'Connor swears to them by even higher voltages, to name another source, and apparently there's some television applications that hit them with 1KV. Just FYI. Who knows what this guy is doing...

    One other point - there's modifications widely know for making a push pull amp run SE with a switch... so the pp transformer in se usage isn't that odd either. It's all over AX84, and even at 18watt.com. Where the builder may be confused, is just because he is using a PP transformer, he is not running the tubes at higher wattage (not saying he isn't running these 6V6 at some ungodly high and dangerous voltage, like 750V).

    Other than the rest of it... who exactly is this builder being discussed? It does sound odd, over all, so maybe he can speak to what he is really doing, first-hand.
     
  8. Jammin'John

    Jammin'John Member

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    I was gonna buy a champ from this guy so I asked him some questions.

    I did not think that the answers made any sense.

    I hate to *rat him out* 'cause he does seem to be sincere as to what he is building and people seem to like his stuff.

    I just don't think that he *gets* how this stuff works.

    JJ
     
  9. mbratch

    mbratch Member

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    I wouldn't care if he really knew how it all worked in theory; but the bigger concern (related to that) is whether he is therefore misrepresenting what you are buying? If he says you are getting a 15W amp that works, it had better be both.
     
  10. TheAmpNerd

    TheAmpNerd Member

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    I'm wondering which brand he is using? I've gone through a batch
    of 6V6s where the b+ was 420v and had a 50 percent failure rate.
    There was not screen protection either.
    I'm not sure of the exact thoery, but to be "true" to
    single ended design the output tranny needs to have an air gap.

    Of course, just be cause it works doesn't mean it is "right"
    Just because it is "right" doesn't mean it will sound good.


    Lest we forget:

    Monkey see monkey do.
     
  11. scottosan

    scottosan Supporting Member

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    The JJ 6V6's can handle 500volts, but I trust nothing else in 6V6's. I have ran some JJ6V6's at 475v for months with no issues
     
  12. VacuumVoodoo

    VacuumVoodoo Member

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    SE amps (in audio) are by definition and neccesity class A and as such the transformer must handle full power dc current without saturating the core. This in itself does not require an air gap only a large enough core that can handle the magnitude of magnetic field induced by the current in the windings. The air gap provides a high "resistance" to the magnetic field thus allowing for a much smaller and therefore cheaper transformer.

    If it sounds good it's "right", knowing "why" is what separates designers from assemblers.

    Just my lame attempt to say something profound..

     
  13. Shea

    Shea Member

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    Well, that's important. If original (i.e., not JJ) 6V6s really can be run at a plate voltage of 500, I have a hunch that the secret is to keep the screen voltage much lower.

    But I would want to read exactly what O'Connor said before I would try such a thing.

    And BTW, if those were Electroharmonix 6V6s that were crapping out on you, it wouldn't surprise me.

    Shea
     
  14. chuby galoso

    chuby galoso Member

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    of corse you can use a push pull trany se. you just have to keep in mind that if you use it anode to anode that you are using the full winding so you will have the full impeadance and the full current of the winding. or you can use half the trany if you are looking for that plate load and the current capability is ok.
     
  15. TheAmpNerd

    TheAmpNerd Member

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    Yes, they were EHs.
    They are a bummer of a tube.

    I asked about the air gap because a buddy of mine hand builds
    a lot of single ended hi fi stuff. All his trannies are very high
    quality and they all have air gaps. He specs all his SE trannies
    with them.


    Yep, it goes without saying SE has to be class A. I guess it wouldn't
    be much of an SE amp with cut off. I can hear it now. The lead guitarist
    telling the audience, "Sorry folks, my amp is drifting into class B, those
    licks should be searing leads...NOT a weak Tremelo effect!"

    Anyone know how much drift from class A you can do in a Music Instrument, SE amp
    without it being audible? How much of the duty cycle?

     
  16. Matt H

    Matt H Guest

    seymour duncan ran an el84 based amp with somewhere near 500V on the plate (i forget the exact number).

    anyway- they kept the screens at half of the plate voltage... definitely key to slamming power tubes.
     
  17. donnyjaguar

    donnyjaguar Member

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    That's not true chuby. The core will saturate and it won't work very well. What you can do is run some idle current through one half to offset this, but if you're going to do that you might as well just use a 2nd tube and go push-pull. SE transformers have an air-gap to get around this problem.

    Also, if you go from half the primary to using the full primary you will multiply the impedance by 4, not 2 (doubling).

    DJ
     
  18. John Phillips

    John Phillips Member

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    The amps actually look very nice - a little untidy in some places but fundamentally well-built and using good materials and parts. No reason at all why they shouldn't sound good, and with over-spec transformers and an efficient 12" speaker in a deep cabinet they will certainly be a LOT louder and fuller-sounding than an original 5F1 Champ.

    He's wrong about the theory, but since the circuit is already thoroughly proven that really doesn't matter...


    (FWIW, I've played an original 5F1 through a 4-ohm wired Marshall 4x12", and you could gig with it. You'd have a hard time convincing anyone who doesn't know how important speaker efficiency is that it was only 3.5W, but it really is.)
     
  19. Jammin'John

    Jammin'John Member

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    I like the idea of a deeper cab with a 3/4 back for sound. :AOK
    I don't like it in practice though.
    Look at the size of the chassis.
    I don't know what it's made out of but I would think that it would vibrate a lot. :eek:
    I would think that microphonics would be an issue. :confused:
    I also don't like the tubes sticking out of the bottom where I would be stuffin' the cord in at the end of the night. :BITCH
    I would also miss the angled front of a real tweed.
    It helps when you are tryin' to hear it and doesn't look so *boxey*. :YinYang

    JJ
     

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