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Eico HF-61 mono preamp question/schematic

Discussion in 'Eico Place' started by gkargreen, Jul 5, 2017.

  1. gkargreen

    gkargreen Active Member

    Messages:
    314
    Location:
    Fairfax, VA
    Anyone have a "proper" schematic for the 61? I see on google the listing are for an HF-65, not quite the same but close if I have to use it. Trying to figure out the output cap voltage and the coupling cap voltage to see if I can use lower ratings than what is in the original. Thanks!
     
  2. RWood

    RWood Sound hath no temperature Subscriber

    Messages:
    1,352
    Location:
    Richmond, VA
    HF61.JPG Here is the schematic from the Yahoo Eico groups site. I'm afraid it doesn't have voltages, though. Is it powered externally, from a power amplifier?
    The SAMS folder 487-4 might have more info.
     
  3. RWood

    RWood Sound hath no temperature Subscriber

    Messages:
    1,352
    Location:
    Richmond, VA
    If the power supply is coming from an Eico HF30, which appears what it was made for, the voltage starts out at 360V from the power amp, goes through a 680Ω resistor before going to the octal plug to the preamp. Then, at the preamp, it goes to the first filter cap there (probably around 340VDC there), then through a 10KΩ resistor before hitting the next cap, and 100K for the last cap.

    Bottom line, even though 350V filter caps might eek by, I would avoid the worry, use 450V filter caps and then either 400V or 630V coupling caps, whichever you have on hand.
     
  4. tubeactive

    tubeactive AK Subscriber Subscriber

    Messages:
    6
    Let me see if I can help, here...The Photofact for the HF-61/HF-61A, Sam's set # 487 Folder 4, does not indicate supply voltages or any tube voltages. There is a great article in the January 1957 issue of Radio & Television News, page 54 and 55(?). Again, voltages are not listed on the schematic, but the end of the article mentions an amp supplying 350 Volts (DC) at 10 mA and 6.3 VAC at 1 Amp is required, if you do not own the HF-61 with its' own power supply.

    Nevertheless, I would not hesitate in the least to use 300V DC rated coupling caps. In fact, I might even try 250V DC rated coupling caps. If you "step-up"(?) to 450V rated metalized caps, they will take quite some time of passing music through them before they "break-in" and settle to sound good. Consider finding vintage PIO coupling caps for that vintage heritage sound. That vintage of mil-spec grade PIO caps had metal to glass, hermetically sealed end seals, plus they were very underrated in voltage handling. 300V DC rated Vitamin Q type caps would be fine, and they will settle in quicker than any polypropylene cap.

    That HF-65 scheme you have on hand might come in handy, but the circuit layout differs a bit from the HF-61. The factory manual for the HF-61, usually available on the bay for under $15, shipped, includes a voltage chart. The highest DC voltage, besides at the 6X4 rectifier, is at the final stage, the 12AU7 cathode follower, with 275V DC at the plate, 53V at the grid and only 128Volts at the output cathode. There is an earlier stage 12AU7 with 120Volts on one plate and 95V on the other plate. The plate voltages on the 12AX7 are both at only 65V. These "working voltages" occur with the factory supplied power tranny supplying 290VAC to the 6X4 pins 1 and 6 (plates) and 350 Volts DC at the 6X4 cathode rectifier output. For units without the factory power supply, any amp or power supply supplying 300-350V DC at 10mA, plus the 6.3VAC at ~1A should work fine.

    You can link to the 1/1957 Radio and TV News article at www.americanradiohistory.com which is a fantastic site for gaining knowledge....

    Since the switch-on or plug-in "surge voltages" can rise above the 350V DC momentarily during warm-up, realize they occur at very low currents, only as the tubes begin warming up. This is why I suggest 300V DC rated caps as worthy. As you can see in the above mentioned working voltages, even lower DC Voltage rated plate coupling caps can be used, if you dare...Be careful and forewarned concerning high voltage safety, as nobody posting on these forums can be held responsible for any electrical or electronic hazards you might encounter. Best of Luck with your resto. These early preamps are very versatile performers and can sound delightful.
     
  5. gkargreen

    gkargreen Active Member

    Messages:
    314
    Location:
    Fairfax, VA
    Thanks. all for your replies, tube I will use some of your values to see if I am in the right area, voltagewise. This particular version has its own power supply, no jack/harness for supplying from an external amp.
     

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