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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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Rbertalotto
James_Weir
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    Catastrophic failure of my VTA ST70

    James_Weir
    James_Weir


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    Post by James_Weir Wed Jul 26, 2023 6:27 pm

    The problem: Heard a loud whomp type sound and went in the other room to find my power amp dead and a distinct smell of something burning.

    What I can see: Fuse is blown and I wasn't able to see anything clearly burned/melted/scorched. There is some residue around the choke inside the unit that may be burn/heat related.

    History (Why I'm an idiot). Since the build in early 2020, there has been a loud electric zap sound through the speakers infrequently. I posted back then and Bob suggested replacing the rectifier which I did. The problem persisted. The zap would somehow result in a system shutdown and initially I'd wait 10 minutes before starting it back up. But being impatient and not so bright I eventually started just flipping the switch down and back up and I'd be back in the music in seconds. Some time in the last year I'd also checked the bias and found I was unable to get the bias low enough on any of the tubes. All of this went to the bottom of a to do list and lived there until now.

    Any and all suggestions are welcome of course. I'm assuming there are just some things worth replacing that will hopefully resolve the issue?

    Thanks in advance

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    Rbertalotto


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    Post by Rbertalotto Thu Jul 27, 2023 7:48 am

    OUCH!....Hope it all works out for you. Following to see what happened.
    Bob Latino
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    Post by Bob Latino Thu Jul 27, 2023 8:23 am

    The most common cause of problems with kit built tube amps are bad solder connections.

    Take off the bottom cover and check all the wiring. Resolder any connection that looks "suspicious".

    You might also post a photo of the inside wiring on the forum here. We can all take a look at the wiring and maybe help you out.

    Bob

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    Brap
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    Post by Brap Thu Jul 27, 2023 8:46 am

    On a side note and just as a suggestion from a very happy VTA ST-70 builder. I have my preamp managing all my inputs having the output going to my ST-70. in powering up or down, I always have the preamp muted soas no signal to the amp. Zero speaker "pop"
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    Wharfcreek


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    Post by Wharfcreek Thu Jul 27, 2023 11:08 am

    I’m quick to point my finger at rectifiers. ANY ST-70 ‘type’ amp is generally over-taxing the rectifier as originally designed. This can often lead to early rectifier failure, which can manifest in multiple forms. Today’s music is NOT the Lawrence Welk and Doris Day if yesteryear!! We push amps a LOT harder than most HiFi users did back in the ‘50s, ‘60s, and even into the early ‘70s. I’d take a good hard look at your rectifier, make sure you have a good quality unit in there, and do as Bob suggested and just look over all your build’s connection points. Beyond that, some deeper diagnosis may be necessary.
    Roy Mottram
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    Post by Roy Mottram Thu Jul 27, 2023 11:27 am

    of course ignoring the bias is bound to get to the point where the bias keeps creeping up enough that the tubes are drawing more current than the rectifier tube can handle,
    and then BOOM !! Simple common sense. And at least two ways to prevent this.
    One - fuse the rectifier output. (and also the output tubes) Two - switch to dual rectifiers or bite the bullet and use an SSR (and then you can worry about the instantaeous 500+v on all the tubes)
    Do yourself a favor and buy at least two meters to keep tabs on the bias EVERY WEEK !
    AND - do not turn on a hot amplifier, wait at least 3-4 minutes to turn on after it's turned off, or . . . POOF !!

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    James_Weir
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    Post by James_Weir Fri Jul 28, 2023 1:39 am

    Safe to assume I should replace the rectifier tube as part of the process? I'd already planned to order a new power switch, speaker posts, and those digital bias meters. Sounds like the fuse kit is a good idea as well.

    Anything else like the choke (original) or the quad cap (new upgraded in 2020) that make sense to replace? If I'm going to be under the hood meticulously checking every solder, I figure I might as well swap out any of the other more likely culprits.

    And one more thing? does the fact that all tubes had the same bias issue tell me anything about where the issue may be? Am I safe to focus less on the tube sockets?

    Any practical advice is greatly appreciated as I'm woefully ignorant of how exactly this piece of technology works. I'll try to get some photos up soon as Bob suggested.

    Thanks
    Bob Latino
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    Post by Bob Latino Fri Jul 28, 2023 7:41 am

    James_Weir wrote:

    And one more thing? does the fact that all tubes had the same bias issue tell me anything about where the issue may be? Am I safe to focus less on the tube sockets?


    Yes .. If ONE tube had a bias issue then I would look on that tube socket itself but if ALL tube sockets give the same bias problem, then usually the rectifier is at fault OR the diode on the back edge of the driver board is bad.

    You can also check the voltage on both sides of the diode on the driver board to see if the diode is good. (rarely do these diodes go bad)

    1. On the side where the RED/BLACK wire comes into the driver board, you should measure about 55 AC to chassis ground.

    2. On the other side of the diode, you should measure about -75 DC to chassis ground.

    Again > This is a test to see if the diode on the driver board is OK. The diode creates a negative DC voltage to help control the bias on all four output tubes.

    Bob
    Roy Mottram
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    Post by Roy Mottram Fri Jul 28, 2023 10:27 am

    many times when a rectifier tube blows out, it can short out the quad cap.
    With the power off and the cap discharged, you can check it with an ohmmeter.
    All sections should measure greater than 2M ohm
    James_Weir
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    Post by James_Weir Fri Sep 22, 2023 3:34 am

    Solved (at least so far). I replaced the switch, added digital bias meters, added fuses on each output tube, and probably most significantly installed the dual rectifier pcb setup[. All of this, maybe short of the switch, was from tubes4hifi. I needed some guidance on the dual rectifier install, but for someone who is generously called a hobbyist, this was very manageable.
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    Rbertalotto


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    Post by Rbertalotto Fri Sep 22, 2023 6:15 am

    Would the solid state rectifier prevent most of this issue?
    James_Weir
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    Post by James_Weir Fri Sep 22, 2023 1:10 pm

    I saw some posts about solid state rectifiers, but like I said, I'm a hobbyist and sorting out how specifically to replace the quad cap and single rectifier tube was a headache I wasn't interested in. The tubes4hifi upgrades are specific to the st-70 and there's great support, which I fully took advantage of.

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