Divided by 13 half-power options

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Stratogatto, May 8, 2015.

  1. Stratogatto

    Stratogatto Member

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    I own a Divided by 13 FTR37 that has an "half-power" switch which put the power down to 18w changing the power amp class operation from AB to A (cathode bias) AND according to some circuit scheme found around the web shutting off 2 of the 4 6V6 power tubes.

    In smaller 2 x 6V6 models as the SJT 10/20 where (I guess) it is no possible to shut off any tube it seems that just changing the power amp class operation to class A the power cuts down to 10w from full 20w.

    Just curious to know if in fact what we have here are different ways to achieve the same result (half power) or maye be there is something wrong in what I know about those amps (also I noticed on the FTR the switch is clearly labeled full/half-power while in the SJT the switch shows class AB/A instead).

    Thanks for any help on making this matter more clear to me.

    Ciao! E.
     
  2. diagrammatiks

    diagrammatiks Member

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    A/B to A isn't quite half power.

    Are you sure the half-power in the bigger amp changes the bias? I don't see that on the schematic. All I see is that it changes two tubes.
     
  3. Jericfos

    Jericfos Member

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    I have an FTR37 as well. I didn't think the half power switch changed it to class A. I've always thought it was just going down to 2-6v6.
    As a side note- one way i've tamed the volume is by only using one speaker, that seems to help alot.
     
  4. Stratogatto

    Stratogatto Member

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    Oh... I'm looking once again at the FTR37 specs on the D13 site.. I see "37 watt class AB1/A" and always considered from the start AB1/A to be an option linked to the full/half power switch while may be it means just that the power amp is a sort of mix of fixed/chatode bias (heard it somewere..) that work the same way in both full/half power setting??

    In fact looking at the SJT specs is clearly explained how it works in class AB1/A at 20w and Hard Class A at 10w..... (so I got it wrong from the first minute)
     
  5. Stratogatto

    Stratogatto Member

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    You're right... see my previous post, maybe I made a bad assumption from the start.

    Switching on half power have (as usual) a minor impact on overall volume but what I clearly hear is a big change in character: less bass, less headroom, more "sag" (sort of vintage feel) and earlier breakup.. (it could driven my mind as the amp was working in a different class operation...) anyway I like it a lot!

    Using one speaker have a major impact on volume taming for sure (also if you loose some airness and that sort of 3D projection..) Are you using the blue/G12H combo? which speaker do you tend to use alone?
     
  6. Jericfos

    Jericfos Member

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    I know it's sounds crazy but i sit the head on a 1x12 custom cab with a Vintage 30 in it and it sounds amazing.
     
  7. Stratogatto

    Stratogatto Member

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    Any ideas on how in the SJT changing the power amp class from AB/A to A (as the switch says) the power goes down from 20w to 10w?
     
  8. pdf64

    pdf64 Member

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    Maybe the magnitude of the voltages (B+ and C-) from power supplies are reduced, eg by switching between taps on the PT B+ winding.
     
  9. diagrammatiks

    diagrammatiks Member

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    They could very well be changing the supply voltage. They could also be going to triode mode which would cut the power a little bit more. The most likely explanation is that amp makers can really say whatever they want in terms of wattage.
     

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