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Peavey PT for plex-ish build: Some design challanges.

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  • Peavey PT for plex-ish build: Some design challanges.

    Hi Yíall

    Iím designing a plexi-sh build around a gifted Peavey PT
    -HT is ~375AC with no CT
    -Secondary Auxilliary Winding is 40VAC
    -Filament is rated to 8 amps.
    All this leaves me with some design challenges so Iím hoping for a bit of a sanity check on my ideas:

    HT will produce ~525VDC B+ which is too high, but I propose to drop this with a 68V 75 watt zener, reverse biased between the 0 volt side of the rectifier and gnd.
    (Iíve previously posted on this and am pretty happy with the required implementation)

    Bias Voltage
    Due to the lack of the centretap on the HT winding, I donít believe I can use the old fender/marshall method of generating a bias voltage (1/2 wave rectifier), so Iím considering using the 40VAC winding.
    Rectified & filtered, this would yield ~54VDC
    I have one of those fender 10K bias pots, so Iím thinking of using this with a filter capacitor (wiper to earth) to generate an adjustable bias supply?
    I believe the pot is rated for 500mW.
    Output valves (2) are likely to be EL34, but I would like to be able to adjust for other types like KT66 & KT77
    My concerns are: having enough adjustment range for a variety of OP valves and reliability of supply (donít want to fry the OP stage if I lose bias supply)
    Is this ok ? or should I use a voltage doubler on the 40VAC and then step it down through a voltage divider network?
    What are your thoughts?

    5VDC Switching Relays
    In addition to using the filamant supply for the heaters, Iím thinking of rectifying and filtering it and using it to generate 5VDC.
    There will be 4 x 5VDC relays (P.U = 4.5V max voltage = 10V, 100mW rated coils)
    I could do this with shottky rectifiers and an LDO regulator or just keep it unregulated and maybe use the Vf of a couple of diodes to bring
    The volts a bit closer to 5V.

    Cheers in advance for your comments
    Attached Files

  • #2
    Generally when using the zener trick in the B+ supply, a lot of people will mount a string of 10 volt 5 watt zeners in series zig zag on a perf board where they can breath. You might also want to consider using a small separate transformer for your relays. They are super cheap and the totally isolated power supply cuts down on pops, clicks and such.
    Last edited by olddawg; 12-19-2011, 05:10 AM.


    • #3
      Thanks Oldawg.
      I'll consider a seperate PS for the relays but space is an issue.

      I've been looking at the Ampeg style capacitor bias circuit and it would work for my application (ie no CT on PT secondary)
      If I use this method, it will free up the 20V windings for the relay circuits, but in terms of dissipation 20V is a bit high for a 7805 regulator.
      I've seen these switching regulators which would resolve the dissipation issue, Switching voltage regulators - 7805 replacements
      Has anyone had any issues / experiences with these devices?

      Otherwise, I'm still curious to get opinions on using the 2x 20V windings to generate my bias voltage, as I'm worried that maximum neg voltage (54V) may not get low enough for KT66 & KT77


      • #4
        I'm going to say something I never thought I'd say (since I have asked similar questions more than once). Why not just buy the proper transformer for the Marshall style amp? I personally favor the ones sold be Triode Electronics.

        The PT you have would be excellent for a high powered channel switching amp. You could easily use the low voltage winding to provide bias and switch LED's or relays or whatever. Truthfully, I've been looking for something similar. What model is it from?



        • #5
          oops- just realized I dragged up an old thread. Hope no one minds!



          • #6
            Ta Jamie
            The amp I'm building is a switcher: 50Watt Bogner Blue channel/Marshall plexi but tweaked to my taste.
            Its nearly done.
            I'm using a 68V 75Watt zener on the HT which gives me ~460VDC B+...right in the 50watt plexi zone
            I'm rectifying the 40VAC winding, gives me ~-57 for the bias and I'm taking a rectified circuit of the filament for the relays.
            These PT transformers are beasties: also perfect for 100watt soldano build

            It came from a Peavey MXVTX (circa 1981-ish), there were a whole series of these hybrid amps (SS front end 130watt UL O/P stage)


            • #7
              That same transformer (or at least the CSA part number) was used in the Triumph 60 which derived a -56V bias and that CSA part number also shows up in the Classic VTX, I believe.


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