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Hot to connect preamp to Allegro amp?

Discussion in 'Fisher' started by thornev, Sep 7, 2018.

  1. thornev

    thornev Well-Known Member

    Messages:
    515
    Location:
    Mid Hudson Valley, NY
    I've mounted a moving magnet (MM) cartridge (cart) on the Garrard Autoslim turntable in my Allegro. It sounds so much better than the ceramic cart that was in there. Of course a MM cart requires a preamp. I built a preamp from a kit that requires 8 tubes. I'm all ready to try it out, but there are connection requirements that have me stumped. How do I find these connection points in an Allegro (or any) Fisher amplifier?

    ... 12VDC heater power supply. Is this point a socket pin for the heater(s) of one of the tubes? Two 12VDC connections are required, one for each channel. Connect to 2 pins?
    ... B+ power supply. Anywhere between +200VDC to +300VDC is required.

    There is a spot on the schematic that displays 250V which is at pin 1 of one of the 12AX7 tubes and 300V at pins 3 and 8 on a 6HU8 tube just before going into a transformer that powers the speakers. The schematic PDF file is too large to post. Can I simply connect to those pins?

    There is also 330V and 280V early in the circuitry, at resistors attached to the pins of a can capacitor.

    Also, the tubes I'm using are twin cathode, dual triodes e.g. 12AT7. They can be wired in parallel or series. The preamp board has jumpers among the tubes so I can wire them in series. I guess parallel is the default. According to the 12AT7 data sheet the heater voltage is 12.6V when run in series, 6.3V when run in parallel. Are there advantages and disadvantages of each configuration?

    PS - Did I really title this thread "Hot to..." when I meant "How to"? Geez.
     
    Last edited: Sep 8, 2018

     

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  2. gadget73

    gadget73 junk junkie Subscriber

    Messages:
    36,764
    Location:
    Southern NJ
    there probably isn't going to be 12vdc available, most things run 6.3 vac heaters so you'll probably need an external supply for that. If you have the option to run it in 6.3vac mode that would be easier, but there probably isn't enough excess capacity in the power transformer to run that many tubes. Thats really the difference in 6v vs 12v operation, half the volts, double the current.

    may not have available B+ current to run 8 tubes either.

    Honestly I'd power this from an external power source rather than tapping out of the amplifier itself.


    Really for a phono stage you don't need more than two 12ax7 tubes. For this I'd either use the RCA phono preamp schematic from one of the tube manuals, copy a Fisher phono stage from say a 500b, or keep a solid state preamp in there.
     
  3. thornev

    thornev Well-Known Member

    Messages:
    515
    Location:
    Mid Hudson Valley, NY
    Thanks, gadget. I do have a Pro-Ject phono preamp hooked up now and it's nice and small, sounds great, if I wanted to mount it somewhere inside the Allegro.
     
  4. gadget73

    gadget73 junk junkie Subscriber

    Messages:
    36,764
    Location:
    Southern NJ
    My own console with the mag cart conversion runs a Radio Shack preamp that probably dates from the early 80s. It works fine, and with the limitations of the turntable and the speakers I can't justify anything better.
     
  5. Bob

    Bob AK Subscriber Subscriber

    Messages:
    2,269
    Location:
    West coast
    if you're new to tube diy I would not recommend tapping a power amp's power
    to supply a preamp. you could with one single slip, destroy both amps. or
    tap the wrong spot, not do the right voltage thing, and again, destroy things.

    you can design and/or buy a cheap Chinese tube power supply to give you raw 300v
    (which from that board you string a wire to this preamp), then the voltage regulator
    can supply a regulated 250v to the other part of the preamp.

    lastly, most modern implementations of heater power supply design in preamps
    allow a complete sharing of all heaters in the preamp (the old days, some
    heater supplies also provided bias, etc).

    so look up the tubes' 6 and 12 volt current requirements, typically 6v current draw
    is twice the requirement for 12v, then add up either the 6v totals or the 12v totals.
    then buy a 6 or 12v ac-to-dc power supply with at least the total you just calculated.

    if you are new at this level of integration, add fuses everywhere and do NOT get
    fancy with grounding. IOW research grounding requirements for the preamp, the
    power supplies, and the power amp and obey each requirements rules.

    for example, there are signal ground(s), power grounds, chassis grounds, AC,
    etc for each device.

    before you put anything into "production", put everything on a power strip and use the
    on/off switch (and hopefully it will also have a fuse or a circuit breaker).
     
    Last edited: Sep 8, 2018
  6. thornev

    thornev Well-Known Member

    Messages:
    515
    Location:
    Mid Hudson Valley, NY
    Thanks, Bob. I have done several mods to my 500-C, but I'm not comfortable with tapping into existing circuitry. I've decided to buy a compact phono preamp and find a place inside the Allegro to install it. Voila, problem solved.
     

     

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  7. Bob

    Bob AK Subscriber Subscriber

    Messages:
    2,269
    Location:
    West coast
    yes, much better solution.
     

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