Eico HF-60 Bias problem

Discussion in 'Eico Place' started by DraKnight13, Feb 11, 2019.

  1. DraKnight13

    DraKnight13 New Member

    Hello, I have a pair of Eico HF-60 that I have problem biasing them.
    I have follow the instruction as close as I can:
    1. Bias to -47 VDC
    2. match the meter to zero
    3. Bias to 0.65 VDC from the meter probe.
    No matter what I do, the tube in the back always have a little red plating going on. I have a very good multimeter that very precise, and I have get the value as close as possible. I know all of my tube are good (tested with calibrated TV-7/U tester).
    If anyone can please point me out to a right direction.
    Everything in the amp is original, I have tested all the cap and they tested over the rated value (50 mFD tested around 60 mFD)? I don't know if that affect anything.


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  2. tubeactive

    tubeactive AK Subscriber Subscriber

    Idling bias is one thing. This is a "static" test. A dynamic test would be playing music. Passing music through the amp, means some coupling caps, especially 60 plus year old caps, can "leak" DC onto the opt tubes' grids. It sounds like you are lucky with the power supply caps, considering the age of these great amps.

    Coupling caps should pass AC ( the music signal ) but block any DC. Even 1.5 VDC positive, or less negative, on an opt tube's grid can cause enough change in bias to cause a bit of red plating. Check those final cplg caps while passing some low level music...especially that red tube's cplg cap...

    While coupling caps pass the AC signal and block DC, power supply caps should pass DC and block the AC ( hum ).
  3. DraKnight13

    DraKnight13 New Member

    Sorry for the late reply, I have been busy.
    I did the bias test with signal, still a little bit of glow. Signal is fine, no hum, no noise. Sound ok to me. I did come back and test all the capacitor again, nothing out of spec. I will move on to the resistor next. I am waiting for my brother to decide (the pair is his). If he decide to keep it, we will do a full recap of the amps.

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