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  • #1

    Question about PT's

    I've built a 5F1 & a 5E3 & I'm having an annoying issue in both amps.
    Basically when they are cranked or close to it, they really kak out in a bad way.
    I'm not sure if it's sag to the extreme or what. I'm wondering if the PT
    affects this in any way. Both amps have cheaper, Asian PTs in them. I was looking at the voltage on the plate of my champ while this was happening & I see the voltage drops hugely. At idle it's at about 395, when dimed & playing hard it drops to 370ish and if I'm playing continuously like that it stays down. Should it be dropping that much? I've heard some clips of other 5F1 builds & originals that don't do that at all while dimmed. I haven't checked the 5E3 voltage under load but it does the same thing soundwise. I'm thinking of ordering a high quality US made PT & see if that changes it but would like
    some opinions on the matter. Both amps are stock circuits btw.

    Thanks
    Sean
    Tags: None
  • #2
    Cold bias might be the issue.
    WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personnel.
    REMEMBER: Everybody knows that smokin' ain't allowed in school !

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    • #3
      That's not much sag really. Even though class A amps aren't supposed to sag, the classes only apply up to clipping. When it's being driven to square waving, the operating conditions at quiescent are immaterial.
      Read up on blocking distortion, see link, maybe that's what you're not liking. Might need to reduce coupling cap values and / or put some grid stopper resistors in. Peter.
      http://www.aikenamps.com/BlockingDistortion.html
      My band:- http://www.youtube.com/user/RedwingBand

      Comment

      • #4
        Re: "I was looking at the voltage on the plate of my champ while this was happening & I see the voltage drops hugely. At idle it's at about 395, when dimed & playing hard it drops to 370ish"

        I agree with pdf64. The voltage drop you measured is not the cause of your problem.
        The 15V drop is not "huge" either. If you were to monitor the plate voltage in an amp that you thought sounding great, you would see similar voltage variations under the same conditions.

        Regards,
        Tom

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        • #5
          Hmm interesting stuff. I read that article on Blocking distortion & it sounds like what I'm experiencing. It's definitely killing the transients. I'll post some clips so you guys can hear. I grew up using Marshalls so maybe it's just a Tweed/Fender thing I'm not digging. I've experimented with the coupling values on my 5E3 & although I got the bass response more reasonable, there's still this issue going on that I don't like.

          Cheers
          Sean

          Comment

          • #6
            Funny thing...
            I recorded my 5F1 & the recording doesn't sound as bad as I hear it live!
            Maybe it's a feel thing. Also, my local music store just got in the Fender Tweed Deluxe RI & it sounds pretty close to what I built. It's a bit tighter but I definitely get the same buzzsaw experience while cranked. I think this must be the Tweed character everyone talks about. It's just pretty foreign to me. Still going to try & tighten things up a bit but I just need to learn their limits & work within them.

            Cheers
            Sean

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