Power tube surge

[rafe]

Hey All, When I turn on my vibroverb in standby mode one of the 6l6's surges pretty bright up by the base, it's the tube not the amp as I have switched sockets and it moves with the tube. It settles down quickly and I wait usually a minute (always have) and take it off standby....It works fine....My concern is if the tube fails or I should say when....will it just be the heater and no problem or am I playing russian roullette?

[eleventeen]

The short answer is....some tubes do this. Mullard 12AX7s light up like you think they are going to blow on initial turn on. In neighboring sockets, RCA and GE 12AX7s  just kind of gradually come up to orange, like you'd expect. The Mullards go bright yellow and quickly settle down. I don't really have anything else to say about the phenomenon.  I cannot tell you (from my own experience) that the Mullards burn out faster than other brands...I can tell you that it is pretty rare to find tubes with open filaments. Sure, it happens. Not often, usually the tube has lost useful levels of emission by the time that happens. I will confess I have very rarely seen this on fat power tubes. I guess you should go buy another set of tubes (or at least 1) in case it blows up, so you'll have it on hand.

[rafe]

Thanks ....I actually have a nice set of old RCA's actually two sets ready , these are the original tubes (GT's)that came with the amp and they have been in it for 7 years and have many hours on them ....They really sound good though and if it's just the heater surging I'm Ok with getting more time on it

[eleventeen]

"...sometimes heaters will ground out to cathodes."

Yes, that can happen too, but again, it's pretty rare. On the assumption that this is a stock Vibroverb (as far as filament wiring goes) the cathodes of the output tubes are at ground and the CT of the 6.3 volt winding is at ground either by virtue of a genuine CT or via the famous twin 100 ohm R's. In that case, the worst case is that some part of the filament is 3.15 volts away from ground, which is the way things are normally. Even if there is cathode bias and the K's are held off ground via a resistor, that resistor is not dropping any volts before the tube conducts...so if this happens on turn-on, it's probably not H-to-K short (causing el flasho) in that case either.

Some tubes do this. The only thing I might suggest is to measure ohms across the heater of the flasho tube with the tube cold and compare it to another tube of another mfr. Plug the tubes into the amp. Turn the amp on, let it heat up for a few minutes, then turn the amp off and again make the ohms comparison between the two heaters (you'd need to pull the tubes so as not to be measuring across the "zero" ohm filament winding. Just for your own curiosity. Not much you could do about it unless you're ready to install some kind of choke in the heater line.
 

[tubeswell]

Heater flash on startup is normal in some tubes. Does it look like its coming from the heater filament (at the tube base)?

[rafe]

Yes Tubewell, it glows at the base for only a second then it calms down....I'm thinking it's the heater and I'm assuming that it will fail, I just don't want it to take anything with it....so that is the question I am asking. . It's the 64 reissue that
i've had for a little over 7 years .... I've done nothing except drop in a few 12au7's which
i do like nice for reverb

[tubeswell]

Yes Tubewell, it glows at the base for only a second then it calms down....

Then its normal heaterflash.

[tubeswell]

What is causing the heater flash?    I've seen the slow motion pictures of an incandescent lamp starting up, and the filament wiggles in all directions.  Is the heater making temporary contact with the cathode?   

http://www.hifitubes.nl/weblog/index.php/electron-tube-heater-flash/

[rafe]

It's still doing it   

[Paul1453]

Almost all of my Mullard and Amporex Holland preamp tubes do this, and they sound fantastic.

One jerk on Ebay returned a pair of Amperex 6DJ8's to me over this.

He claimed they were gassy and didn't pass specs on his fancy tube tester.
He wouldn't even try them in his Hi-Fi amp to hear how good they sounded. 

I'm guessing these tubes have always done this as a result of the Mullard/Amperex manufacturing process.

I haven't noticed it on my output tubes, but I don't have as many vintage Mullard/Amperex output tubes.

If the tube sounds great, and is not red plating or having other indications of problems, I'd continue to run it. 

[rafe]

Absolutely, and thanks for the reply. I haven't been online here for a while...and the original post was three years ago that is why I reposted ....sounds great and hasn't changed . I have a 68 twin on the bench that I will start a thread on ....I'll probably need some help as I have not been doing much on amps lately and forget some things ....