Reducing OT = low watt amp?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by jsnwhite619, Apr 23, 2015.

  1. jsnwhite619

    jsnwhite619 Member

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    I asked this over at TDPRI.com also, so if you frequent there, yes, this is the same question. I'm still trying to find an answer though. I've been reading up on the smaller, low watt amps in general. I found this post at Lil' Dawg amps, and it seems to me that they simply changed to a smaller OT to achieve the goal of a low powered Deluxe. In the picture, it looks like a small, Hammond OT. Is it really that simple? Does anything else need to be changed? Could you just get one of these Hammond 125 Series OT, match up the required primary, and have a 3, 5, 8, or 10 watt Deluxe? Thanks in advance
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    Last edited: Apr 23, 2015
  2. mark norwine

    mark norwine Member

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    Not, it's not "really that simple".

    Output power comes from the VA curve of the power transformer, not the OT.

    Think of it like a car: engine + transmission. A smaller transmission doesn't reduce power; indeed, a smaller transformer might fail with the existing engine because it's incorrectly matched.

    A smaller OT might reduce your headroom & mess up your frequency response, but that's not really "lowering your power".
     
  3. pdf64

    pdf64 Member

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    At face value, the idea seems to be nonsensical.
    My understanding is that the wattage rating of an OT is a balance of various parameters.
    Additionally, maybe the p-p impedance is also being altered, along with the power rating, to limit the power output.
     
  4. Blue Strat

    Blue Strat Member

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    Varying the output power of an amp begins in the power supply.

    Maybe a better approach is to tell us what you're trying to accomplish.
     
  5. axpro

    axpro Member

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    Definitely not that easy, and of course the frequency response would suffer.
     
  6. jsnwhite619

    jsnwhite619 Member

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    So, what is he doing at Lil Dawg? If he's slapping a 25 watt on a 35 watt Super, and a 3 watt on a 15 watt Deluxe, what is the difference? Just getting really hot OT's?
     
  7. axpro

    axpro Member

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    I would think there is more at work than just switching out the transformer, and yeah, all that power HAS to go somewhere otherwise. So you'd get some very hot transformers.

    Makes me think of putting tiny wheels on a truck to slow it down..... YES it would work, but there are going to be some drawbacks, some serious, some not.
     
  8. axpro

    axpro Member

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    Also to point out, the undersized transformer i am replacing on my Weber amp, it was undersized, suited more to 6v6 than the 6l6 the amp shipped with, it wasn't noticeably quieter, but it certainly lacked any "balls" from any circuit i put in there. Depends on the type of tone you like. If you are a low frequency fan, you might have issues.
     
  9. donnyjaguar

    donnyjaguar Member

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    I made an 18Watt 6V6 amp out of a 10Watt Hammond universal transformer.
    I get 10Watts out of it so unlike other very conservatively rated Hammonds, this series would appear to be more realistic. Also, pay attention to the 150Hz limit. It will produce below that, but the roll-off is so noticeable I ended up modifying my 18Watt circuit to provide extra amplification below 150Hz.

    It does sound alright though.
     
  10. swiveltung

    swiveltung Member

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    it's complicated for sure. But I have created Princeton reverbs with 6L6's and a small but fat stack PT, bigger OT and they were a LOT louder.
     
  11. J M Fahey

    J M Fahey Member

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    It sounds somewhat awkward even to me, but I see it this way:

    a) you can build a 10W amp, with matchingtransformers, expecting EL84/6V6 and wire 6L6/EL34/KT88 there ... it will still be 10W and last forever

    b) you can build a "large amp" with "large tubes".... and every thing else, but use small undersized 10W transformers.
    They will suffer, overheat and eventually die.

    Maybe even what's idle current for the large one is full current for the small one.

    So both situations are not the same.
     
  12. axpro

    axpro Member

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    I think the bigger OT allows the full voice of the tube to come through. If i compare my Badcate dual 6v6 (BIG iron) to one with small iron, there is a massive difference in power, especially low frequency. an overspeck transformer will give you a bigger beefier sound, than stock, and an underwound WILL be quieter, for sure, also less bass, and is acting as resistance against the tubes, Not saying it won't work, it will of course, it will just have other issues as well. like possible overheating and lower bass response. If you design the amp to have lots of room around the transformer to dissipate heat, and don't mind the lowered bass response, then you are all good.
     
  13. axpro

    axpro Member

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    Theoretically at least..... :D
     
  14. Diablo1

    Diablo1 Member

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    If you want to lower the output power, there are quite a few methods that are more practical than replacing the OT. Attenuators between the OT and the speaker is one method. An adjustable plate voltage like the Variwatt is another.

    http://www.skipzcircuits.com/VariWatt.html
     
  15. Carol-AnnAmps

    Carol-AnnAmps Gold Supporting Member

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    Totally !!

    There's a difference between magnetic saturation and over-current.
    It is quite possible of course to build an lower power OT that will safely run a pair of 6L6's, but its part of a designed system, not a single component. The Tweed Super is fine example of using a smaller OT with pair of 6L6's, but the anode voltages are lower and consequently the primary impedance higher. TW Express is another example. 35W fixed bias 2xEL34, again lower anode voltage, 6.6K primary.
     

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