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The Dynaco Tube Audio Forum

Dedicated to the restoration and preservation of all original Dynaco tube audio equipment - Customer support for Tubes4hifi VTA tube amp and preamp kits and all Dynakitparts.com products


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    ST-70 NO BIAS

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    joseramonnavarro


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    Join date : 2022-07-09

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    Post by joseramonnavarro Sat Jul 09, 2022 6:02 pm

    HELP! After installing a second output transformer from "laboratorio musicale " I started to obtain too high bias in put 1-8 and now I got "0" bias.
    Bob Latino
    Bob Latino
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    Post by Bob Latino Sat Jul 09, 2022 9:02 pm

    joseramonnavarro wrote:HELP! After installing a second output transformer from "laboratorio musicale " I started to obtain too high bias in put 1-8 and now I got "0" bias.

    If you are getting zero bias in your amp try another rectifier tube .. A bad (not working) rectifier tube is the most common cause of no bias on all output tubes. Note that just because the rectifier "lights up" is no guarantee that it is producing the high voltage that the amp needs.

    Bob

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    Gabriel


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    Join date : 2022-01-23

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    Post by Gabriel Sun Jul 10, 2022 6:44 am

    I purchased a Genalex GoldLion 5AR4 which lasted me approximately one month!
    Switched the amp on this afternoon and no sound!
    Checked the internal components, including the resistors. All good.
    I swapped it out for my trusty 5u4gb electro harmonic, which despite its lower price, was told is more reliable?
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    joseramonnavarro


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    Post by joseramonnavarro Sun Jul 10, 2022 7:33 am

    Thanks. I just replaced the previous rectifier and obtain the same results, zero bias.
    Bob Latino
    Bob Latino
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    Post by Bob Latino Sun Jul 10, 2022 7:57 am

    joseramonnavarro wrote:Thanks. I just replaced the previous rectifier and obtain the same results, zero bias.

    See if you can post a photo of the inside wiring of your amp after you have been a member 1 week. Read the sticky on how to post a photo on the Dynaco Tube Audio Forum.

    If a new rectifier doesn't fix a "no bias" issue then it could be a bad selenium rectifier (if the amp is an original Dynaco ST-70) or a bad diode if the selenium rectifier has been replaced by a diode.

    Bob
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    joseramonnavarro


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    Post by joseramonnavarro Sun Jul 10, 2022 8:33 am

    ST-70 NO BIAS My_ste10

    This is my st70, almost original. I do have negative dc voltage on the left side of the diode (-69).

    Bob Latino
    Bob Latino
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    Post by Bob Latino Sun Jul 10, 2022 8:56 am

    If you do have a negative DC voltage of -69 VDC at the start of the bias circuit then try this > With the amp OFF, check the value of the two resistors and two capacitors on the 7 lug terminal strip. The resistors should be 10K resistors and the capacitors should be 50 uF to 100 uF.

    Bob
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    joseramonnavarro


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    Post by joseramonnavarro Sun Jul 10, 2022 9:28 am

    Correct, both resistor are at 10K and capacitors 75uf 100vdc.
    ST-70 NO BIAS Img-0611
    Bob Latino
    Bob Latino
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    Post by Bob Latino Sun Jul 10, 2022 11:21 am

    In some cases people measuring bias accidentally don't have their meter set on the proper setting OR the meter is not working. Make sure that you have your meter set to the 0 to 2 volt DC scale to measure bias. If your meter is autoranging > I had a customer whose meter stopped autoranging for some reason and would not measure bias anymore.

    Try a different meter ? It is sometimes helpful to have an alternate meter in case your daily meter starts giving improper readings.

    Bob

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    joseramonnavarro


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    Post by joseramonnavarro Sun Jul 10, 2022 12:08 pm

    Thanks: I ordered another voltimeter with capacitance measure also. I also notice to have the correct resistance ck at pin 1-8 been 15.6, also the negative -69 volts is present on pins 5-6 of all output tubes, el34.
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    joseramonnavarro


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    Post by joseramonnavarro Sat Jul 16, 2022 7:12 pm

    I just received my new meter wih Capacitance Reading and attemptedto measure the capacitance inside the circuit, but, also just read that i need to remove all wires in the capacitor to test each section to ground. i am about to do this but first. Its anyway my no bias at pin 1-8 could be related to the quad capacitor? All sectuions are producing over 350vdc, please explain. than
    thks.
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    joseramonnavarro


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    Post by joseramonnavarro Sun Jul 17, 2022 3:22 pm

    I desolder all points and measure capacitance from ground to square, triangle, blank and semicircle and read this values 42uf 33uf 24pf and 86uf respectivelly.
    Any ideas?
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    joseramonnavarro


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    Post by joseramonnavarro Mon Jul 18, 2022 7:21 pm

    Correction.... my capacitor is 80/40/30/20 starting with the halfmoon.

    Could the faulty item be the choke? how can i test it?
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    joseramonnavarro


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    Post by joseramonnavarro Tue Jul 26, 2022 8:22 pm

    I AGREE 200% It does looks like a bad meter, not able to measure up to 2 volts. BUT...
    I used another meter and there is no DC voltage on pins 1-8, and there is -69 vdc on pin 5, where does the voltage escaping to?
    tubes4hifi
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    Post by tubes4hifi Thu Jul 28, 2022 6:21 pm

    follow the wires from the -69vdc voltage divider, they go to the two bias pots. Make sure you have good connections on the two bias pots.
    The center connection of each pot goes to the PCB and out to pins 5 & 6 of each output tube.
    Under normal operating conditions, with the bias pots set near midrange, the voltage on each output tube pin 5 & 6 will be around -40vdc.
    If you don't have that, you've got a bad connection somewhere. Once you have that, check the bias across the 15.6 ohm resistor, should be set to around 1.30vdc (not 1.56vdc as manual says)

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