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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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    VTA-ST120 - Low bias on all four power tubes

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    alexbramford


    Posts : 17
    Join date : 2021-03-07
    Age : 56

    VTA-ST120 - Low bias on all four power tubes Empty VTA-ST120 - Low bias on all four power tubes

    Post by alexbramford Wed Oct 11, 2023 3:21 pm

    Introduction:
    • Under normal conditions, with bias pots at the 11 o'clock position, bias is at recommended 0.5 volts
    • Bias is suddenly very low on all four power tubes

    Issue:
    • With pots at 11 0'clock position, bias on all four power tubes is now very low, at 0.19 volts, instead of the 0.5 v recommended.
    • I now have to turn the bias pots to 4 o'clock to achieve recommended bias.
    • There is also a hum from both speakers

    Question:

    • I presume I have to replace the quad cap?
    • What else must be done?





    Observations:
    • It appears that the amplifier was turned on and off multiple times in quick succession during the brownout - see timeline, Monday
    • Quad cap looks damaged

    Quad cap:   Expected  Actual
    • Semi-circle: 80          87.4
    • Square:       40          4.66
    • Triangle:      30          130.04
    • Dash:          20          87.6

    Bias resistors:
    • Not blown. All at 10.1,   10.1,   10.3,   10.7 ohms

    Timeline:

    Monday 10/9 @ 19:00 :
    • Electricity supply brown out in my home: twice, 2 second duration, at two seconds apart.
    • Lights flickering, loud buzz from speakers.
    • GFCI did not disconnect power, amp remained on after brownouts stopped

    Tuesday 10/10:
    • Noticed the issue with the low bias
    • I replaced the Weber WZ68 rectifier with the original solid state rectifier ("Dynaco VTA ST 70 120 Mono Block Tube Amp Rectifier Kit")
    • Issue persists: low bias and hum from speakers

    Configuration:


    [/list]
    Bob Latino
    Bob Latino
    Admin


    Posts : 3271
    Join date : 2008-11-26
    Location : Massachusetts

    VTA-ST120 - Low bias on all four power tubes Empty Re: VTA-ST120 - Low bias on all four power tubes

    Post by Bob Latino Thu Oct 12, 2023 4:19 pm

    Hi Alex,

    You can't really get the actual capacitance of the four sections of the quad cap unless you remove all the wires and then measure the capacitance of each section by itself to chassis ground. I am pretty sure as you have found that the 40 uF section of the quad cap has been taken out by the power issues with your AC on 10/9/23. Replace the quad cap. I recommend the 550 volt Authenticap 80, 40, 30, 20 uF quad cap. Sorry about the power issue you had ? This happened to me once here years ago but I was lucky and my amp did not sustain any damage. You can get one at the link below with FREE SHIPPNG. I know the owner of this business and he is very reliable and ships quickly.

    550 volt Authenticap from Vacuum Tube Values

    Bob
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    Dale Stevens


    Posts : 203
    Join date : 2014-07-06
    Age : 75
    Location : Loris, SC

    VTA-ST120 - Low bias on all four power tubes Empty Re: VTA-ST120 - Low bias on all four power tubes

    Post by Dale Stevens Thu Oct 12, 2023 5:36 pm

    Alex, your GFCI does not appear to latch down (off) on a power flick. Look at TRC14650006-6 on Amazon. This has a manual reset. Dale

    DavidR likes this post

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    alexbramford


    Posts : 17
    Join date : 2021-03-07
    Age : 56

    VTA-ST120 - Low bias on all four power tubes Empty Re: VTA-ST120 - Low bias on all four power tubes

    Post by alexbramford Thu Oct 12, 2023 6:50 pm

    Hi Bob,

    Thanks for the confirmation. I've ordered a replacement quad cap and I'll be doing quad-cap surgery as soon as it arrives...

    @Dale Stevens - as it happens, I do have one of the TRC 14650006-6 bought off Amazon in my spare parts collection, so I'll be using that going forward.

    DavidR likes this post

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    alexbramford


    Posts : 17
    Join date : 2021-03-07
    Age : 56

    VTA-ST120 - Low bias on all four power tubes Empty Re: VTA-ST120 - Low bias on all four power tubes

    Post by alexbramford Tue Oct 17, 2023 8:07 pm

    I repaired my VTA-ST120 amp  Saturday 10/14 : replaced the quad cap and had glorious use of my amplifier for 48 hours.

    Now there is a more severe problem.
    Details in a new post at:

    VTA-ST120: variac fuse blows when the TDR switches on the "power circuits".

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