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    New Build Questions

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    audiobill

    Posts : 357
    Join date : 2014-03-13
    Location : Philadelphia

    Re: New Build Questions

    Post by audiobill on Wed May 02, 2018 2:46 pm

    Are the f marks on the chassis indicating front? Those bias points are for the rear tubes,

    When all is well, all bias pots will be at about the same point, usually between 10 and 2 depending on tubes.
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    corndog71

    Posts : 673
    Join date : 2013-03-19
    Location : It can get windy here

    Re: New Build Questions

    Post by corndog71 on Wed May 02, 2018 3:15 pm

    WireNut wrote:Bill, I will do as you suggest... but why would it take three days of continuous play to get to the point of blowing a fuse? What would that noise have been?
    Thanks!

    Could have been a short in the tube. Debris can and does get inside tubes from time to time and sometimes it gets zapped. Sometimes it'll kill the tube and sometimes it won't.
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    WireNut

    Posts : 67
    Join date : 2017-02-21
    Location : Twin Cities

    Re: New Build Questions

    Post by WireNut on Thu May 03, 2018 10:46 am

    Seems I found the issue:

    Once the resistors are replaced, do I need to check anything else?

    audiobill

    Posts : 357
    Join date : 2014-03-13
    Location : Philadelphia

    Re: New Build Questions

    Post by audiobill on Thu May 03, 2018 10:59 am

    Not so fast- you need to find out what caused that. Either a tube or wiring problem, check all wiring as well as connections with a good light and magnifying glass from all directions. Be sure no shorts from switch pins to chassis. Good luck.
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    WireNut

    Posts : 67
    Join date : 2017-02-21
    Location : Twin Cities

    Re: New Build Questions

    Post by WireNut on Thu May 03, 2018 11:05 am

    I put on my jewelers visor and no wires were touching around there or to where the wires came from.
    It could have been a bad resistor... but what can I check to see if there was another issue?
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    Bob Latino
    Admin

    Posts : 2690
    Join date : 2008-11-26
    Location : Massachusetts

    Re: New Build Questions

    Post by Bob Latino on Thu May 03, 2018 11:37 am

    WireNut wrote:Seems I found the issue:
    Once the resistors are replaced, do I need to check anything else?



    Dave,

    Above is the photo of the wiring of your VTA ST-70. There are many differences between your amp and a standard VTA ST-70 amp. You added the bias meters, moved the inputs to the rear, and create your own (unnecessary) grounding buss bar. You also admitted to me that you had your bias meters incorrectly wired at first. When you go off the beaten path to create your own version of the VTA ST-70, you increase the risk of having playing and/or reliability problems. The VTA ST-70 is a VERY reliable amp if properly assembled. I still have the original fuse in my own VTA ST-70 which is now (2007 - 2018) 11 years old. This amp has thousands of hours on it and the amp is now on its third set of tubes.

    Since those two resistors only blew on the RIGHT channel, there must be some right channel issue (wiring, bad solder connection, bad output tube ?) You had said in an Email to me that the amp played correctly for three days before this happened which usually means the wiring is correct. You did not get your tubes from me. > It could be just a tube that went bad prematurely ? I will ship you out a new triode/ultralinear switch and two 100 ohm 1 watt resistors later today. If you have any more issues like this with your amp, I am going to suggest you send the amp to one of our VTA amp technicians to straighten out the amp for you.

    Bob
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    WireNut

    Posts : 67
    Join date : 2017-02-21
    Location : Twin Cities

    Re: New Build Questions

    Post by WireNut on Thu May 03, 2018 11:39 am

    I am going to put the tubes on my tube tester.
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    peterh

    Posts : 1040
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: New Build Questions

    Post by peterh on Thu May 03, 2018 3:59 pm

    If this is the screen stoppers ( connected in series with the g2 ) then i suggest that both tubes are shot and should be replaced.
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    WireNut

    Posts : 67
    Join date : 2017-02-21
    Location : Twin Cities

    Re: New Build Questions

    Post by WireNut on Thu May 03, 2018 5:28 pm

    Both tubes on that side tested good... am I the first person in the US that may have gotten a bad resistor?
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    peterh

    Posts : 1040
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: New Build Questions

    Post by peterh on Thu May 03, 2018 6:02 pm

    A tubetester won't detect tubes that are prone to flash.
    Junk them ! They will most likley flash again.
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    tubes4hifi
    Admin

    Posts : 1478
    Join date : 2008-11-30

    Re: New Build Questions

    Post by tubes4hifi on Thu May 03, 2018 7:14 pm

    also, did it EVER work in that config, or was this the first time you powered it up with the triode-pentode switching in place?
    First time up, yeah, wiring error somewhere. If it worked many many times before, probably a tube(s) went bad.
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    WireNut

    Posts : 67
    Join date : 2017-02-21
    Location : Twin Cities

    Re: New Build Questions

    Post by WireNut on Sat May 05, 2018 11:17 am

    It was on for over 30 hours straight, and then a loud cracking noise.
    All of the tubes test good for emission, grid, and shorts.
    I am not about to toss a Philips 12au7 "For Medicine" tube unless it can be proven it is bad. They are going for $300 a pair now.
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    peterh

    Posts : 1040
    Join date : 2012-12-25
    Location : gothenburg, sweden

    Re: New Build Questions

    Post by peterh on Sat May 05, 2018 11:21 am

    WireNut wrote:It was on for over 30 hours straight, and then a loud cracking noise.
    All of the tubes test good for emission, grid, and shorts.
    I am not about to toss a Philips 12au7 "For Medicine" tube unless it can be proven it is bad. They are going for $300 a pair now.
    It cannot be the pretubes ( 12au7), it must been the power tubes that has experienced
    a flash-over. When the flash is over, a tubetester won't detect it, but the tube have a
    much larger risk of another flash-over. Thats why you should replace them as a pair.
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    deepee99

    Posts : 2019
    Join date : 2012-05-23
    Location : Wallace, Idaho

    Agree with peterh

    Post by deepee99 on Sat May 05, 2018 7:07 pm

    Get some decent and fully-tested output tubes from Jim McShane.
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    WireNut

    Posts : 67
    Join date : 2017-02-21
    Location : Twin Cities

    Re: New Build Questions

    Post by WireNut on Sun May 06, 2018 12:10 pm

    Does he have a test that can check for arcing?
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    deepee99

    Posts : 2019
    Join date : 2012-05-23
    Location : Wallace, Idaho

    Re: New Build Questions

    Post by deepee99 on Sun May 06, 2018 1:12 pm

    WireNut wrote:Does he have a test that can check for arcing?
    You'd have to ask Jim about that.
    jimmcshane@mcshanedesign.net
    He's very responsive to emails and very patient with tube newbies, plus he warrants his tubes after putting them through some serious torture tests. A major re-seller for New Sensor and Gold Lion, when he's not restoring H-K Citations. Good guy to know.
    -d-



    Dogstar

    Posts : 341
    Join date : 2014-06-23

    Re: New Build Questions

    Post by Dogstar on Mon May 07, 2018 8:20 pm

    Can you post a photo of the amp from the outside?
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    WireNut

    Posts : 67
    Join date : 2017-02-21
    Location : Twin Cities

    Re: New Build Questions

    Post by WireNut on Tue May 08, 2018 10:05 am

    Dogstar, I have the replacement parts and will post a photo soon- I need to put in the parts and go through the protocol to fire it up first.
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    WireNut

    Posts : 67
    Join date : 2017-02-21
    Location : Twin Cities

    Re: New Build Questions

    Post by WireNut on Thu May 10, 2018 11:44 am

    Bill and I worked on getting the unit going by bringing it up with my variac- it is running now and has about four hours on it. I owe a HUGE thanks to Bill!!!

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