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    ST70 VTA Mod Question about Startup & Tube Glow...

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    vintess

    Posts : 6
    Join date : 2013-06-10
    Location : San Diego, CA

    ST70 VTA Mod Question about Startup & Tube Glow...

    Post by vintess on Fri Jun 28, 2013 5:27 pm

    Hi all,

    Happy TGIF! Hope everyone is ready for a relaxing weekend.

    I have a question that may have been asked and answered somewhere on this forum, but I can't seem to find it or something close to it. I have a ST70 which I modded with the VTA boards from Roy up in the Evergreen State. The power supply is stock, although I did add the upgraded, recommended multi-section 80/40/30/20 electrolytic capacitor from Kevin at dynakitparts.com. I have also added the triode/pentode switch assembly from Roy, as well. It has a stock choke and power supply. The tubes I am using are all vintage stock Dynaco/Mullard. No other modifications have been made. As I recall, prior to the VTA mod, the amp was doing the same thing on initial startup. So either this is normal or something that I haven't changed/added is the problem--if, there is a problem.

    My question is this. On initial startup, ALL tubes glow brightly and then dim back down as they enter their warm up phase. After the brrght glow they settle down to a faint glow. Is this normal? It seems like there is a surge of initial power on startup. What I am concerned about is the obvious: shorter tube life because of this startup surge to the tubes.

    Any help from the resident gurus would be greatly appreciated, as I am relatively new to the valve arena.

    skriefal

    Posts : 116
    Join date : 2011-09-20
    Location : Utah, USA

    Re: ST70 VTA Mod Question about Startup & Tube Glow...

    Post by skriefal on Fri Jun 28, 2013 6:15 pm

    That is normal for some European tubes. But usually only the small stuff -- 12AX7, 12AU7, 6DJ8, etc.

    Are your EL34s doing the same thing?

    vintess

    Posts : 6
    Join date : 2013-06-10
    Location : San Diego, CA

    Re: ST70 VTA Mod Question about Startup & Tube Glow...

    Post by vintess on Fri Jun 28, 2013 8:17 pm

    Thanks for the quick reply skriefal.

    I checked the startup on my amp just to be sure and the EL's are NOT doing the bright glow. It is the 3 tubes (12AU7's) on the VTA board that are brightly lit for an instant before dimming back down. If I turn the amp back off (after a "cold start") and then start it up, then there is no "bright startup." I surmise that this is because capacitors are charged at this point?

    Again, thanks for the quick reply.

    skriefal

    Posts : 116
    Join date : 2011-09-20
    Location : Utah, USA

    Re: ST70 VTA Mod Question about Startup & Tube Glow...

    Post by skriefal on Fri Jun 28, 2013 8:30 pm

    It's due to the construction of the tubes and how the filaments were made. And it may not happen every time. Your Dynaco-labeled 12AU7s are probably Mullards made in Great Britain. Head over to Google and search on "Mullard tube flash" for everything you'd ever want to know about this!

    vintess

    Posts : 6
    Join date : 2013-06-10
    Location : San Diego, CA

    Re: ST70 VTA Mod Question about Startup & Tube Glow...

    Post by vintess on Fri Jun 28, 2013 10:59 pm

    Thanks again skriefal! Yep! You are right on. The Dynaco/Mullards I have installed are indeed British made...I will take a look at what is out there regarding the Mullard tube flash...

    tubes4hifi
    Admin

    Posts : 1261
    Join date : 2008-11-30

    Re: ST70 VTA Mod Question about Startup & Tube Glow...

    Post by tubes4hifi on Sun Jun 30, 2013 5:36 pm

    very normal for most tubes made before the mid 70s, it's not a problem at all, yes, it's a turn on surge, what would YOU do if you had 2 amps of current applied to a short circuit?
    It takes a few seconds for the tube filaments to actually have some resistance as they warm up.

    Zimmer64

    Posts : 112
    Join date : 2013-01-29
    Age : 52
    Location : Switzerland

    Re: ST70 VTA Mod Question about Startup & Tube Glow...

    Post by Zimmer64 on Mon Jul 01, 2013 9:13 am

    I have that too and it seems to be normal for some small tubes. Mine run fine since I finished my build.

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