1987 model 2554 scandinavian version. Considering changing OT for 16ohm ability

Discussion in 'The Workbench' started by Marshall Arts, Feb 24, 2021.

  1. Marshall Arts

    Marshall Arts Active Member

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    I have a 1987 Silver Jubilee 2554 combo. I am very fond of the tones I get.

    The only problem is that this model is the scandinavian SEM version without the 16ohm tap on the OT. 4 and 8 only. Applying to the danish, norwegian and swedish markets.

    The same thing as in many canadian models from that era from what I've read.
    That limits the versatility of this amp as I can't "borrow" 16ohm cabs from other amps.

    What separates these models from the international models electrically? Is it just the OT that needs changing? Or did they make more circuit changes to satisfy silly swedish regulations?

    I'm playing with the idea of converting it to international spec.
     
  2. tschrama

    tschrama Well-Known Member

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    Just plug the 16 Ohm cab in the 8 Ohm output. You might loose a tiny bit of maximum output but that’s all
     
  3. RickyLee

    RickyLee Well-Known Member VIP Member

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    I have always wondered about those output transformers, as in, is the 8 ohm secondary using the entire winding like the 16 ohm would be for the USA model? A completely different OT between them? Or is it the same exact OT, but the 16 ohm tap has been omitted? So then the 8 ohm tap is not using the entire winding?
     
  4. Marshall Arts

    Marshall Arts Active Member

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    Different transformer. Only common, 4 and 8 ohm wires on the secondary. I wondered the exact same thing to start with, if the 16 ohm tap was simply just unused. But unfortunately that was not the case ☹️.
     
    Last edited: Feb 25, 2021
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  5. RickyLee

    RickyLee Well-Known Member VIP Member

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    Actually, that OT would favor me in most situations as I am always running an 8 ohm load or cab. So I am never taking advantage of the full OT winding in my output transformers. So that has me wondering about the tonal nuances between those situations? Do you get a different audible tone when not using the entire winding? Obviously there is never any info stating it affects the max power output of the output xfmr.
     
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  6. Marshall Arts

    Marshall Arts Active Member

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    The max power would be pulled from the primary the same way on all secondary taps. The transformer is like a gearbox with multiple gear ratios. Low voltage - high current at 4 ohm and higher voltage - lower current operation at 16ohm. The end results in watt would be the same. So you will get full power at any impedance as long as the speaker impedance is matched accordingly.

    As far as tone differences go: I don't know.

    For me it's about making the amp more practical. Within the confines of what's reasonable. If it's just a matter of changing the OT, I might order one. I was just wondering if anyone else has done it before.
    Or if Marshall did other changes to the amps I have to take into account.

    I'm equally happy just keeping it stock and instead reconfigure the impedance on an eventual cab.
     
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  7. XTRXTR

    XTRXTR Active Member

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    You mean to factually state the Drake OT 784-139 is not in the 2554 that was marketed and sold in Scandinavian stores? What OT transformer model is in that amp? I am interested in any other changes you may be aware of as well, PT, fusing, etc.
    Interesting info.
     
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  8. Marshall Arts

    Marshall Arts Active Member

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    783-257 on mine.

    The one you mention is the one I want.
     
    Last edited: Mar 19, 2021
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  9. RickyLee

    RickyLee Well-Known Member VIP Member

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    Yeah, I have a few JCM 800 early -139 OT's in my stash. Glad I picked them up along with a few early 80's 50W power xfmrs many many moons ago. I am figuring they have to be worth much more than the cheap price I paid all those years ago.
     
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  10. XTRXTR

    XTRXTR Active Member

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    Is the Drake PT 1202-324/7 in your amp? An educated guess it is the 1202-304 and you have fuses on the Heater and the HT before the standby as well as a fuse after the standby, perhaps no lighted power indicator or at least different than the rest of the globe?

    Semko is about safety in the home as well saving the consumer some dollars via fusing to save your PT if something goes wrong like spilling a beer in your amp.

    I don''t understand the OT thing unless there is a standard in that area of the globe to use 8 and 4 speakers for importing reasons maybe. Less impedance means greater efficiency of energy transfer to the spkr coil which could translate to better frequency response - in general. Just my physics kicking in - I don't know this for fact but it makes sense to me. V=IR, If R is less, then V is less. Power = VI So less power is needed to drive with same current. It won't change the power needed to operate the amplifier circuit but the transfer of energy via the transformer to the speaker is more efficient.

    Hmm just rambling
     
  11. Marshall Arts

    Marshall Arts Active Member

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    I don't think it has to do with the dangers of spilling drinks into the amp. As that would be disastrous for any amp anyway.
    Being an electrician licenced in Norway, I know that the mantra in scandinavia has been to focus on "berøringsspenning". (touch voltage) Voltage available to touch. And to keep this below 50VAC by different means. Proper grounding, insulation classes, RCD's etc.
    Maybe that's where they got it from? Okay, but what about a 100W amp that has to double the signal voltage? 50W at 50V would also require 50 ohms to get a 1A load. Nah, can't be that either.

    Now, I'm rambling!
    They probably had some plausible reason for it, and I'm not going to speculate on that. It doesn't apply anymore, so I don't care.

    edit: typo
     
    Last edited: Mar 3, 2021
  12. Marshall Arts

    Marshall Arts Active Member

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    I'll check up on my PT later. I'll be sure to update. ;)
     
  13. XTRXTR

    XTRXTR Active Member

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    Well spilling a beer in a Semko certified amp has a better chance at surviving than a UL certified amp. The fuses in Semko should blow and save transformer damage at minimum. The UL certification in USA might as well have no regulation at all. Building electrical codes are decent but the things you plug in the wall...

    I just read about how CE is now the cert.

    I suppose find a OT 784-139 or a aftermarket replacement and if you change the PT, Weber has a good one W022798EU or the INT version. I have a 2204 clone with the EU. Plenty of options for mods.
     
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  14. johan.b

    johan.b Well-Known Member

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    Semko certification got sort of out maneuvered when we entered the EU back in mid nineties. From then on, things only needed CE certification, which manufacturers can do themselves... anyway, I believe the no 16 ohm output has to do with the connectors. The phone connectors we all use was originally designed for use in telephone switch boards. I guess they found a way to hurt yourself using those as output connectors (too high voltage diving 16ohm into power amp saturation.. as @Marshall Arts touched on.. in Sweden I belive its 48VAC/60VDC. But let's NOT make this gospel now.. it's only a semi educated guess from my part..)
    J
     
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  15. johan.b

    johan.b Well-Known Member

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    For those wondering why scandinavians got different power transformers with funky bias arrangement on late 80s amps... it allows much lower transformer voltages..
    .. here's a little exercise for anyone wondering how that is... calculate B+ with the switch in position 1 and 2
    16148812267395088297048141035289.jpg
     
  16. 2L man

    2L man Member

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    There are impedance matching transformers for up to at least 100W which price should be not more the new OPT is. But it is new small box to carry which also weighs few kilos.
     
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