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

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.

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

Are your EL34s doing the same thing?

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.

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!

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...

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.

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