Noob output trans question.

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Mudder, Oct 14, 2006.

  1. Mudder

    Mudder Member

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    I've searched a bit but haven't found exactly the right answer. Thanks in advance for the lesson in basic electronics. :)

    I have a Valve Jr head and I'm considering upgrading the output transformer. I like the low output of the amp and I'm not really looking to increase that. I'm considering the Hammond 125 line and I see there's a difference in power ratings between the CSE, DSE, and ESE (8, 10, and 15w respectively). Does the output transformer actually change the output of the amp to it's rating (ie. a DSE equipped amp would change from 5w to 10w)? How do you determine which OT to use?

    On the same note, I've searched for the more popular mod kits for the Jr and found few actually describe what changes when the mod is done. I don't suppose someone would be kind enough to break down the common mods to layman terms? Or maybe just point the way to a site that does?
     
  2. 5992

    5992 Member

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    I can't really help you with the transformer theory, other than to say that it seems to me that the tube/valve is the limiting factor in wattage/output. The tube's anode has a rated maximum dissipation. In single ended Class A, the EL84 is rated for 12W maximum. Since Class A is inefficient, you can expect to get less than 50% of that in actual real world output power. A lot of the maximum output is wasted as heat. So it doesn't matter if the tube dissipates 5w into a 5w, 10w, or 15w transformer, you still only get 5W out. Generally the bigger the transformer the better the bass response.

    I've done some extensive mods to my Valve Junior. I've tried three OTs in it including a Fender Champ 12 OT, a Hammond 125CSE and a Hammond 125ESE. I liked the 125ESE the best. It has great bass response, and sounds fuller than the 125CSE or the Champ 12 OT. The 125ESE just barely fits and you have to drill a new mounting hole.

    There is a huge thread at the 18w site with all the various mods:

    http://www.18watt.com/

    Be prepared to read 80+ pages of posts. The mods start at about page 30, FWIW.

    Here is a doc that will explain what part does what in the circuit:

    http://www.s2amps.com/docs/vj_kit_inst.pdf

    Steve at S2 sells a few kits for the VJ if you are so inclined.

    HTH.



     
  3. hasserl

    hasserl Member

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    Careful, don't confuse plate dissipation wattage with power output. Just because a tube is dissipating 12 watts of dc power does not mean it is putting out 12 watts of ac power. There are a lot of variables that contribute to the power produced.

    Also, when you consider the Hammond trannies, keep in mind that they are way overated for guitar amp use. The power ratings they display are for full band width hifi use. Guitar amps bandwidth is much narrower, from appx 80 hz to 5kHz max. Anyway, a Hammond OT rated at 15 watts would probably be fine for 30 watts in a guitar amp.
     
  4. Mudder

    Mudder Member

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    Excellent, I really appreciate you guys dumbing it down for me.

    I'll do some more research but I'm pretty excited about the possibilities coming from such a cheap platform as the Epi.
     
  5. 5992

    5992 Member

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    That is true of Hammond's hifi OT series, but the 125ESE series (and the 125E series) are not rated for the full band width like the hifi series.

    The 125ESE is rated at 100 Hz. - 15 Khz. The 125E is rated at 150 Hz. - 15 Khz. These are not hifi transformers and are not overated for guitar use.
     

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