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    Desirable bias voltage for 6l6gc's on st-70

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    Westy56

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    Location : Albuquerque

    Desirable bias voltage for 6l6gc's on st-70

    Post by Westy56 on Fri May 13, 2011 11:55 am

    What should the bias be set on new SED 6L6GC's? I can't get it much lower than 1.63 DCV.
    I'm using a stock 70 with a diode replacement for the selenium rectifier.

    I would like to run them at there full potential.

    regards,


    Steve

    BTW, I had no problem biasing (1.56) worn (not bad) RCA 6L6GC's.

    Bob Latino
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    Re: Desirable bias voltage for 6l6gc's on st-70

    Post by Bob Latino on Fri May 13, 2011 12:35 pm

    Hi Steve,

    Different tubes even of the same type (6L6) but from different manufacturers will many times have different bias points. If you have gone from a selenium rectifier to a diode, sometimes tubes won't bias at the correct point because a diode has more forward voltage than a selenium rectifier.

    If you have a stock ST-70 with a stock or a stock replacement driver board then try dropping the value of the two 10K resistors on the 7 lug terminal strip. A common thing that is sometimes done is to parallel a 10K resistor with each of the 10K resistors that are there now. This will give you an effective resistance of 5K on each resistor and should put you into the correct bias range with the stock bias pots.

    Bob

    Westy56

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    Join date : 2011-05-09
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    Re: Desirable bias voltage for 6l6gc's on st-70

    Post by Westy56 on Fri May 13, 2011 1:41 pm

    Thanks Bob, I don't know how I missed your site this last year.
    BTW, how do I tell if my preamp is a 3 or 3x?


    Steve

    Bob Latino
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    Re: Desirable bias voltage for 6l6gc's on st-70

    Post by Bob Latino on Fri May 13, 2011 2:48 pm

    Steve,

    The "3X" has a very slight detent at the center of each bass and treble control. When the bass and treble controls are set at this detent they are effectively removed from the circuit. I am not entirely sure but I think that the treble control is removed from the circuit and the bass control is "shorted out" to render it ineffective. The "X" models also have a 1 uF cap capacitor in the circuit to prevent transient pops as the bass and treble controls are moved in and out of the circuit.

    Bob

    Westy56

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    Re: Desirable bias voltage for 6l6gc's on st-70

    Post by Westy56 on Fri May 13, 2011 3:55 pm

    Are these the caps? (yellow arrows)
    I could not feel any detent in the controls.




    Steve

    tubes4hifi
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    Re: Desirable bias voltage for 6l6gc's on st-70

    Post by tubes4hifi on Fri May 13, 2011 4:45 pm

    yes, those are caps, typically 1uF
    they are also commonly used in the PAS3 model, not just the X

    Westy56

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    Re: Desirable bias voltage for 6l6gc's on st-70

    Post by Westy56 on Fri May 13, 2011 6:21 pm

    tubes4hifi wrote:yes, those are caps, typically 1uF
    they are also commonly used in the PAS3 model, not just the X

    Thanks, its prolly a 3. I'll poke around and figure out how to remove the tone controls from the circuit. scratch
    Keep in mind, I don't have much experience with electronics. This is the reason I like the simplicity of the st70.


    Steve

    Sal

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    Re: Desirable bias voltage for 6l6gc's on st-70

    Post by Sal on Fri May 13, 2011 10:30 pm

    To tell the difference between the PAS-3 and PAS-3x, the PAS-3X tone controls has a rotation of approx 200 degrees vs 260 degrees of a PAS-3. If the rotation of the tone controls match the volume control, you have a PAS-3.

    Sal


    Last edited by Sal on Sat May 14, 2011 10:50 am; edited 1 time in total

    GP49

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    Re: Desirable bias voltage for 6l6gc's on st-70

    Post by GP49 on Sat May 14, 2011 4:04 am

    There is no physical detent in either the PAS-2/3 or the PAS-2x/3x. Sal's method, by the amount of rotation in the controls, is the correct way.

    Anything else (the 1μF capacitors and different resistors on the stereo/mono/blend switch) could be altered by the owner or a technician.

    Westy56

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    Join date : 2011-05-09
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    Re: Desirable bias voltage for 6l6gc's on st-70

    Post by Westy56 on Sat May 14, 2011 7:57 am

    Sal wrote:To tell the difference the PAS-3 and PAS-3x, the PAS-3X tone controls has a rotation of approx 200 degrees vs 260 degrees of a PAS-3. If the rotation of the tone controls match the volume control, you have a PAS-3.

    Sal

    Well SAl, I have a 3x. Thank you for the easy ID.
    Are there different models of the FM3 also ?


    Steve

    Sal

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    Re: Desirable bias voltage for 6l6gc's on st-70

    Post by Sal on Sat May 14, 2011 10:51 am

    Your welcome Steve.

    I am not sure about the FM-3's

    Sal

    GP49

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    Re: Desirable bias voltage for 6l6gc's on st-70

    Post by GP49 on Sat May 14, 2011 7:00 pm

    There were no OFFICIAL sub-models of the FM-3, though internal changes were made according to what parts were available at the time of production (brands of capacitors, for instance). However there are quite a number of FM-1 mono tuners which were upgraded with the FMX-3 multiplex stereo kit and the optional faceplate/knobs which were sold by Dynaco, changing the FM-1's appearance (brass/brown front plate and knobs) to that of the FM-3. The easy way to tell is to see if the volume knob push/pulls for stereo/mono switching. In factory trim, that was only found on the FM-3, though it is always possible that an owner bought the FM-3 volume control with its attached slide switch and retrofitted it to an FM-1/FMX-3.

    Sal

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    Re: Desirable bias voltage for 6l6gc's on st-70

    Post by Sal on Tue May 17, 2011 11:17 am

    On another note, I have a ST-70 with 6L6 tubes installed. I replaced the 5AR4 tube with a 5R4 tube that lowered the B+ a little and I was able to adjust the bias without a problem. I would bias the 6L6 tubes to 1.4 volts, that is what I did in my amp and it sounds fine to my ears.

    Westy56

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    Re: Desirable bias voltage for 6l6gc's on st-70

    Post by Westy56 on Tue May 17, 2011 12:53 pm

    Sal wrote:On another note, I have a ST-70 with 6L6 tubes installed. I replaced the 5AR4 tube with a 5R4 tube that lowered the B+ a little and I was able to adjust the bias without a problem. I would bias the 6L6 tubes to 1.4 volts, that is what I did in my amp and it sounds fine to my ears.

    Thanks again Sal, you got to the heart of my OP.

    Are you running just 6l6's or are they GC's?

    Regards,


    Steve

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