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  • Fender 400 Ps

    what is the best cab to use with a 400 ps head??

  • #2
    To get the full power out of a 400PS you're supposed to plug three cabinets into it. They have three speaker jacks each connected to its own winding on the output transformer.

    So I guess you could use three small cabinets (two 1x15"s and a 2x10"?) A stack of those green carpet Trace Elliot cabs would be awesome.

    Or try one of those big Ampeg "Coffin" cabs that let you drive the upper and lower sections separately, plug each section into one of the 400PS output jacks, and leave the third jack unused. I think they make a 6x10" in this style now, which should be a bit easier to lug around than the full size coffins.

    I don't know if it's possible to modify the amp to have a single output jack that delivers the full 435w, so you have more flexibility in speaker choice. If I owned a 400PS, I'd certainly try it. You might need to add low value resistors to even things up.

    Some people pull four of the six tubes and use it at one-third power with a single cabinet, but I'm not too sure about that.
    "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

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    • #3
      you might try asking the author of this page:

      http://www.timeelect.com/400-PS-IDX.htm

      he apparently hangs out at alt.guitar.amps.

      http://groups.google.com/group/alt.guitar.amps/topics

      Comment


      • #4
        One thing I never understood, and I'd challenge the author of that page to try and explain, is how the 400PS manages to connect each pair of output tubes to the corresponding speaker jack, when it only has a single output transformer.

        The manual says that if you're using a single speaker, you can plug it into a different jack each time you use the setup, for "triple the output tube life". But according to my understanding, the load should reflect onto all six power tubes just the same, no matter which of the three jacks you choose. And, with only one jack in use, the tubes should see three times the design load impedance, which is a pretty serious mismatch in the transformer trashing direction.

        So, I would be reluctant to use this amp unless it had at least two 4 ohm cabinets connected, which would show the tubes 3/2 of the design impedance. In fact, from what that guy writes, modern 6550s are only about 2/3 as good as the GE originals, so you probably shouldn't even try connecting three cabinets.
        "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

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        • #5
          I haven't worked on one of these in a couple years now... try finding a 1000v 680uf filter cap (or what ever it was) ... doesn't the speaker jacks have extra lugs to shut the unused tubes off?
          Bruce

          Mission Amps
          Denver, CO. 80022
          www.missionamps.com
          303-955-2412

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          • #6
            I thought that this comment by the author was pretty funny:
            its right above the photo that showed the melted envelope on the 6550.



            looking at the amp, its obvious that they didn't follow the recommended engineering practice of spacing the 6550 on 4-inch centers for heat dissipation. cramming them in there like sardines could explain why the amp melted its tubes.

            Last edited by bob p; 05-25-2007, 05:50 PM.
            "Stand back, I'm holding a calculator." - chinrest

            "I happen to have an original 1955 Stratocaster! The neck and body have been replaced with top quality Warmoth parts, I upgraded the hardware and put in custom, hand wound pickups. It's fabulous. There's nothing like that vintage tone or owning an original." - Chuck H

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            • #7
              Bruce, that makes perfect sense, if they put switches on the speaker jacks to break the cathode circuits or whatever. In the light of what Rich Koerner said about the wussiness of new production tubes though, I think I'd want all six tubes active even if I was just using two of the three jacks.

              I think they probably melted the tubes by just biasing the thing way too hot, or by cranking it with a 2 ohm load into one jack and the others open. Nobody ever seems to follow those tube spacing recommendations. If you look in the back of a SVT2, the tubes are packed like football players in a Geo Metro, but I guess it has a fan.
              Last edited by Steve Conner; 05-26-2007, 09:21 AM.
              "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

              Comment


              • #8
                its funny Steve, nobody seems to follow those tube spacing recommendations. even when you look at high end HiFi stuff like a Conrad Johnson or an Audio Research amp, they're crammed into the amp like circus clowns in a subcompact car. I guess that designers feel like they can get away with it when they use forced air cooling -- that is, until the fan fails.

                And lots of amps didn't fail even without forced air cooling. Even the McIntosh MC275 had 4x6550 spaced damned close together, with no fan. I guess that the limit of how long you could listen to cranked 6550 at home limited the possibility of damaging the tubes in Mac, but I'm thinking that a 400 PS could likely get cranked for a long time on stage. If it didn't have forced air cooling, it would be problem waiting to happen.

                You're probably right - looking at those photos it looks like one tube was melted, and it was shown from 3 different perspectives. I had originally thought that it was a photo of 3 tubes side by side. Either way, I think that those tubes were the result of a bench test burn-in experiment gone awry.
                "Stand back, I'm holding a calculator." - chinrest

                "I happen to have an original 1955 Stratocaster! The neck and body have been replaced with top quality Warmoth parts, I upgraded the hardware and put in custom, hand wound pickups. It's fabulous. There's nothing like that vintage tone or owning an original." - Chuck H

                Comment

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