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    Diode on bias supply caps

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    colin86325

    Posts : 41
    Join date : 2014-04-01

    Diode on bias supply caps

    Post by colin86325 on Sat Nov 08, 2014 10:11 pm

    I have a modified Dynanco 70 that has SS rectification and a 30-second delay circuit using an AMPERITE 6N030T.
    I am wondering, why the diode? Is there some backrush current that might apply with the 6N030T? You can see that the diode is attached to the red/black power transformer lead.




    sKiZo

    Posts : 1310
    Join date : 2013-04-01
    Location : Michigan USA

    Re: Diode on bias supply caps

    Post by sKiZo on Sat Nov 08, 2014 10:31 pm

    Voltage regulation? Maybe the mod was a bit drifty ...




    peterh

    Posts : 679
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: Diode on bias supply caps

    Post by peterh on Sun Nov 09, 2014 4:15 am

    It still has "death-caps" connected to the power leads ....

    colin86325

    Posts : 41
    Join date : 2014-04-01

    Re: Diode on bias supply caps

    Post by colin86325 on Sun Nov 09, 2014 8:34 pm

    Hmm, that could be. Unfortunately the diode doesn't have a visible band on it, so I'm not sure which direction it should face.

    Peter, are the "death caps" the ceramic caps? Why the name?

    Bob Latino
    Admin

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    Join date : 2008-11-26
    Location : Massachusetts

    Re: Diode on bias supply caps

    Post by Bob Latino on Sun Nov 09, 2014 10:41 pm

    Those .022 uF caps are there to ground the center taps of the amp's two 6.3 volt AC lines. The GREEN/YELLOW wire grounds the left channel's two GREEN AC filament lines though one .022 cap. The BROWN/YELLOW wire grounds the right channel's two BROWN AC filament lines through the other .022 uF cap. On the original Dynaco ST-70, Dynaco use a "dual" (two caps in one body) .022 uF cap.

    Those two caps are not "death caps" ... "Death caps" were place on the main AC lines of some guitar amps. The voltage on these two brown caps in the photo is exactly 1/2 of the 6.3 volt AC of each AC line or about 3.15 volts AC - not really enough voltage to cause "death" ..

    Bob

    peterh

    Posts : 679
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: Diode on bias supply caps

    Post by peterh on Mon Nov 10, 2014 2:51 am

    Bob is right. My dim sight thought that i saw mains leads from the transformer. I stand corrected.
    About the diode, why don't you measure the voltage, it _might_ be a diode for bias ( replacing the
    selenium block).

    anbitet66

    Posts : 132
    Join date : 2009-12-23
    Location : Valley Stream, NY

    Re: Diode on bias supply caps

    Post by anbitet66 on Mon Nov 10, 2014 8:05 am

    Colin,

    To answer your original question, that is the bias supply diode.  It appears when the original selenium rectifier was eliminated the diode was soldered to the bias tap (red wire with black tracer) and to the terminal strip at position 4, as in the dynaco assembly manual.

    If it is oriented correctly, the band, if it has one, would face the wire and any voltage measured from terminal 4 to ground would be negative.  Usually, when the selenium rectifier is removed, a 2 or3 position terminal strip is installed with the original screw and nut.  A quick check with a volt meter will prove if the diode is oriented correctly.  It may look dicey, but electrically it should work the same.

    Tony




    Edited to add pictorial.

    colin86325

    Posts : 41
    Join date : 2014-04-01

    Re: Diode on bias supply caps

    Post by colin86325 on Mon Nov 10, 2014 10:28 am

    Tony, thanks for the clear explanation!  Yes, the selenium rectifier has been deleted. In my case it looks like the bias supply gets a single diode and the red wires from the power transformer are treated with 2 diodes.
    It would seem like the bias voltage would have a ripple, but I'm not sure if this is important.

    anbitet66

    Posts : 132
    Join date : 2009-12-23
    Location : Valley Stream, NY

    Re: Diode on bias supply caps

    Post by anbitet66 on Mon Nov 10, 2014 11:17 am

    Colin,

    In your photo, the diode connects to the resistor/capacitor bias network, and if the caps are good, there will be no perceptible ripple.  If there were, then there would be hum in your speakers.

    Look at the diagram above; instead of the selenium rectifier and the rectifier tube in the pictoral, your amp uses solid state diodes.  Not ideal, but it should not present a problem except the stress placed on the filter capacitor(s).  The bias circuit looks to have been modified; that is resistor values were changed to accept the diode to allow a wider range of voltages so the tubes can still be biased properly.

    Tony

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