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Hammond OT/PT pairing question

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  • Hammond OT/PT pairing question

    hey there.
    I have a hammond 278cx that needs a project. I am considering using it with 6 KT88's and a Hammond 1650W to build a bass amp somewhere close to the 300W range.

    The two things I am aware of with this PT (400-0-400, 465ma) are:
    a. the B+ will be a bit lower (~100V) than your typical 300W head, which will eat some clean headroom.
    --this is not necessarily bad as I like a nice overdriven bass tone that I cannot get out of my SVT without drowning out the rest of the band/world.

    b. the current rating on the PT is 465 ma,

    I was wondering if anyone has ran a sextet of kt88's with this combo, and with what degree of success?
    I know Hammond is legendary for their conservative ratings, and in theory, this should fly current wise, but any opinions/tips from experience would be nice...
    looking to see if I can avoid having to use/buy/lift the 717 for this --a PT named after a boeing jet

    any thoughts/comments/suggestions would be much appreciated!
    thanks!
    (and thanks in general for the many contributions to this wonderful resource I only stumbled upon this week!)

  • #2
    your talking about 550 volts plate voltage vs 600. maybe 250 watts?

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    • #3
      I can't help you on the PT/OT specifics, but the member black_labb has had a couple of treads going for a similar amp with similar specs regarding power supply design. They are worth checking out because when you're at that high of voltage, some things are a little more interesting to design around that others.
      -Mike

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      • #4
        have a look at those threads defaced mantioned. they have alot of good info.
        i found that www.antekinc.com seemed to have great deals on big power trannies. torroidial is also nice to keep the weight a bit more manageable.
        465ma is alot, but 6 kt88's use about 300ma at idle, and 900ma at full power. you would have to use a dropping resistor or zeners to drop the voltages for the screens and the preamp. there would be alot to drop, and the voltage would sag alot if you use a big resistor. otherwise a big zener/zener chain.

        i've decided using 2 power trannies, one for the screens and pre and a second for the plates. this keeps it simpler to get the right voltages without a custom wind or sagging supplies.
        i havent done anything other than drawing and pondering for a while as i have been traveling canada and norther usa for a month, and am now visiting family in quebec for a few more weeks. after that the beast is going to be on the way.

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