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KT88 UL, phase inverter

Discussion in 'Amps and Cabs [BG]' started by BillnShell, Aug 9, 2007.


  1. I want to build a a 2xKT88 bass amp, and am trying to decide on traditional or UL output. Hi power isn't necessary ( small jazz venues ), clear low frequency extension is. Looking at available schematics I see a pattern of ultra linear output being used with the KT88. I also see what I'd call a "Short" tailed pair used for the phase inverter ( short because of the cathode resistor going directly to ground, vs. the LTP's additional resistor ). In the case of Marshall's KT88 PI's, the resistor values for the PI in general are much lower than the the traditional LTP, making me wonder why? Is the circuit optimized for the KT88's input sensitivity?

    Any circuit wizards out there want to enlighten me?

    Thanx,

    Bill
     
  2. It's more a function the plate impedance of the PI and its current handling ability. KT88's (and 6550's) need pretty significant drive power to stay clean in the lows. That's one of the reasons the SVT uses a cathode follower drive stage AFTER the PI.
     
  3. "It's more a function the plate impedance of the PI and its current handling ability. KT88's (and 6550's) need pretty significant drive power to stay clean in the lows. That's one of the reasons the SVT uses a cathode follower drive stage AFTER the PI."

    Thanx PBG,

    I was looking at the KT88's load lines, and see that they need a good bit of voltage swing, I'm guessing your referring to their grids placing a demanding impedance load on the output of the PI, thus the advantage of a cathode follower stage after the PI. I'd lke to avoid grid current/class B operation, but I could include a 12AU7 CF stage after the PI as I have room for an extra 3/4' hole in the chassis.

    My original plan was so simple, build a 5F6A circuit, subbing KT88's( @ 500vdc B+ ) for 5881's, use a solid state recto, and bump up the filtering.. It's not that simple, eh..
     
  4. A9X

    A9X

    Dec 27, 2003
    Australia
    As maxing the power isn't the issue, and you only want to drive a pair, I'd be tempted simply to build a Genelec 100W using a Hammond 1650R OPT or better still, a Lundahl LL1620 (tetrode only). Even in UL the grid drive requirements are hardly severe for a pair of output tubes. There are lots of ways to improve this, and I have better circuits, but they're paper and I have no way to scan and post.
    http://www.triodeel.com/gec100w.gif

    If you want 100W, honest, reliable watts, I'd run a quad of output tubes, with a lower PSU (say 500V, g2=350V, Ia = 50mA, RL = 2k5 - 3k p-p) which will give you circa 150W tetrode and 125-ish UL at 43% taps. Drive this with an input stage of 6SN7/ECC99 etc LTP (using a CCS in the tail) driving a pair of EL84 trioded, each RL= 9k, Ia=30mA driving the KT's from the plates and feedback returned to grid of the input LTP. Use a clean g2 supply.

    This is just how I'd do it based on my experience (20+ years of designing, building and repairig since Uni). I'm definitely not a classicist.

    However, if you really like, say an Ampeg, clone the power stage just using one set of output tubes.
     
  5. Thanx for the Genelec schematic. My chassis is pre-punched for two power tubes and small bottles for the pre-section. The Lundahl iron sounds good, as I'm leaning away from UL now.. I sent emails to Bran Wallace, who built a beautiful plexi OT for me a few years ago, but haven't heard back from him, or MM for that matter. In their defence, it's summer, they both probably need a break, and what I'm after isn't exactly off the shelf.. The Lundahl certainly fits the bill though.

    Thanx again,

    Bill
     
  6. gerryjazzman

    gerryjazzman Supporting Member

    Dec 31, 2006
    New Jersey
    Sounds like if you just want one pair of output tubes, I would start with cloning an Ampeg schematic, at least the output section. Take a look here: http://users.aol.com/bluemuse/schematics.html and you should be able to find something that would work for you. I would suggest looking at the power amp section of a V2 or a B15S. They show 7027A output tubes, but you should be able to use KT88's or 6550's if you want with no modification except for adjusting the bias. You should be able to get 50 -75 watts out of something like this easily. Going for 100 watts from one pair of output tubes is going to require KT88's or 6550s running at 600 volts of plate voltage (look at the SVT) and you'll be pushing them to the limit.
     

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