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    Oroginal dynaco Stereo 70 high voltage problem

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    simonfck

    Posts : 15
    Join date : 2013-03-04

    Oroginal dynaco Stereo 70 high voltage problem

    Post by simonfck on Mon Sep 02, 2013 5:23 pm

    all parts oroginal
    Live Voltage 122Vac

    cannot adjust the bias voltage, bias voltage very high over 2.0vdc
    rectifier tube is new. What should i check
    many tanks

    gz34
    pin2 500vdc
    pin8 500vdc
    pin4 379vdc
    pin6 379vdc

    el34
    across pin 2 and pin 7 6.9vac
    pin3 500vdc
    pin4 500vdc
    pin5 32vdc
    pin6 32vdc

    quand cap
    half circle 459vdc
    no symbol   493vdc
    triangle    416vdc
    helf square 393vdc

    Bob Latino
    Admin

    Posts : 2415
    Join date : 2008-11-26
    Location : Massachusetts

    Re: Oroginal dynaco Stereo 70 high voltage problem

    Post by Bob Latino on Mon Sep 02, 2013 5:58 pm

    Yes - 122 VAC is a little high .. Pins 4 and 6 on the rectifier tube socket to chassis ground will be AC voltages - not DC.. All your voltages are high ...

    What I would do is use a 5U4 rectifier tube in place of the GZ34. This will get the important DC voltages on the quad cap and pins 3 and 4 of each output tube down somewhat. When these voltages drop, you should be able to bias the amp. If you still cannot bias the amp, then try dropping the value of the two 10K resistors on the 7 lug terminal strip to about 5K. You can also just parallel the two 10K resistors with two more 10K resistors (1/2 watt or higher).

    The other option would be to run this amp off a variac set to about 118 volts. This would drop ALL the voltages on the amp and you could run a GZ34 again. Understand that the original Dynaco ST-70 amps were made to run on 115 - 117 VAC which was the line voltage in the USA in the 1950's and 1960's. When you run these amps on 122 VAC all the voltages in the amp will be high as is the case in your amp.

    Bob

    simonfck

    Posts : 15
    Join date : 2013-03-04

    Re: Oroginal dynaco Stereo 70 high voltage problem

    Post by simonfck on Mon Sep 02, 2013 6:16 pm

    Thanks Bob.
    I have variac also will try 5u4.

    peterh

    Posts : 681
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: Oroginal dynaco Stereo 70 high voltage problem

    Post by peterh on Tue Sep 03, 2013 1:59 am

    simonfck wrote:all parts oroginal
    Live Voltage 122Vac

    cannot adjust the bias voltage, bias voltage very high over 2.0vdc
    rectifier tube is new. What should i check
    many tanks

    gz34
    pin2 500vdc
    pin8 500vdc
    pin4 379vdc
    pin6 379vdc

    el34
    across pin 2 and pin 7 6.9vac
    pin3 500vdc
    pin4 500vdc
    pin5 32vdc
    pin6 32vdc

    quand cap
    half circle 459vdc
    no symbol   493vdc
    triangle    416vdc
    helf square 393vdc
    If all parta are original and too much bias current ( you mention 2C and if it's original
    1.56V is the "target) then i would suspect the selenium rectifier.

    Replacing that with a single 1n4007 ( remove se-rect., mount a 2 point support and solder the
    1n4007 and the connections there). Then at power up, remove rectifier tube and measure the
    bias voltage , make shure its negative ( eyelet 6 or 21 on the board, same as pin 5 on the el-34)and about -30 to -40V .

    If negative is correct, remove power and reinstall rectifier tube and power up, adjust bias
    as per dynaco instructions.


    simonfck

    Posts : 15
    Join date : 2013-03-04

    Re: Oroginal dynaco Stereo 70 high voltage problem

    Post by simonfck on Tue Sep 03, 2013 2:14 am

    thank peterh,

    I don't have 1n4007, but do have 1n5408. Is it ok to replace the  selenium rectifier

    Thanks again

    peterh

    Posts : 681
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: Oroginal dynaco Stereo 70 high voltage problem

    Post by peterh on Tue Sep 03, 2013 3:24 am

    simonfck wrote:thank peterh,

    I don't have 1n4007, but do have 1n5408. Is it ok to replace the  selenium rectifier

    Thanks again
    yes. it will do. Make shure it's mounted the right way, making a negative bias voltage.


    poconoman

    Posts : 106
    Join date : 2013-01-17

    Re: Oroginal dynaco Stereo 70 high voltage problem

    Post by poconoman on Tue Sep 03, 2013 12:51 pm

    simonfck wrote:thank peterh,

    I don't have 1n4007, but do have 1n5408. Is it ok to replace the  selenium rectifier

    Thanks again
    What I learned, is if you have the original selenium rectifier, that's the first thing to replace FAST. BTW, peterh is very helpful. Knows his stuff. Smile

    GP49

    Posts : 735
    Join date : 2009-04-30
    Location : East of the sun and west of the moon

    Re: Oroginal dynaco Stereo 70 high voltage problem

    Post by GP49 on Tue Sep 03, 2013 1:18 pm

    poconoman wrote:
    simonfck wrote:thank peterh,

    I don't have 1n4007, but do have 1n5408. Is it ok to replace the  selenium rectifier

    Thanks again
    What I learned, is if you have the original selenium rectifier, that's the first thing to replace FAST. BTW, peterh is very helpful. Knows his stuff. Smile
    ABSOLUTELY.  The selenium rectifier almost always fails high-resistance, resulting in inadequate NEGATIVE bias voltage on the grid, which leads to excessive current in the output tubes.  

    I will admit: since its failure mode is high-resistance, I've sometimes been lazy and simply soldered in a new silicon diode, using the selenium rectifier as a terminal strip when I could not scrounge one up at 11pm on a Saturday night; and have had NO failures from doing this.  Makes sense, as the silicon diode would then be handling all the current.

    1N5408 (1000PIV, 3amp) is fine. Higher amperage rating than 1N4007 (1000PIV, 1 amp).

    simonfck

    Posts : 15
    Join date : 2013-03-04

    Re: Oroginal dynaco Stereo 70 high voltage problem

    Post by simonfck on Tue Sep 03, 2013 3:17 pm

    GP49 wrote:
    poconoman wrote:
    simonfck wrote:thank peterh,

    I don't have 1n4007, but do have 1n5408. Is it ok to replace the  selenium rectifier

    Thanks again
    What I learned, is if you have the original selenium rectifier, that's the first thing to replace FAST. BTW, peterh is very helpful. Knows his stuff. Smile
    ABSOLUTELY.  The selenium rectifier almost always fails high-resistance, resulting in inadequate NEGATIVE bias voltage on the grid, which leads to excessive current in the output tubes.  

    I will admit: since its failure mode is high-resistance, I've sometimes been lazy and simply soldered in a new silicon diode, using the selenium rectifier as a terminal strip when I could not scrounge one up at 11pm on a Saturday night; and have had NO failures from doing this.  Makes sense, as the silicon diode would then be handling all the current.

    1N5408 (1000PIV, 3amp) is fine.  Higher amperage rating than 1N4007 (1000PIV, 1 amp).
    Thanks you guys info.

    I'll replace  selenium rectifier with 1n5408

    replace stock 1k feedback resistors to 2.7k
    the negative feedback around -12db,8db gain than stock


    change 4 main coupling capacitors to 0.22uf

    thanks again

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