Different tube glow on start-up

AndrewA · « **on:** 19 Sep 2021, 04:53 pm »

I just got a matched pair 1960s 12AU7's from Brent Jessee. On start-up, one of them emits a quite visible orange glow for a second or two which then immediately dissipates and disappears. The other one emits no visible light on start-up. I can hear no channel imbalance (it's one tube each per side) or any other negative sonic impacts. Is this degree of variation normal? I should add that the tube that glows looks "dirtier" on the inside (there's some blackening of surfaces) whereas the non-glow tube looks "cleaner" (and a light shade of grey).

thorman · « **Reply #1 on:** 19 Sep 2021, 07:20 pm »

My Take is if it is supposed to be a matched Pair, they should both react, sound and look the same. Your pair does not do that. I would contact the seller and ask for another pair or money return..Any trusting Tube seller will happily do that......

bacobits1 · « **Reply #2 on:** 19 Sep 2021, 07:49 pm »

I had a pair of Tungsram 12AU7's. One from Kevin Deal and one from Tube Store in Canada. One did exactly as you said from Kevin Deal UpScale Audio the other did not. Tungsram in this case particularly are noted to do this.
No problem.
Brent Jessee is very good. Contact him if worried about the purchase. I just got some 12AU7 and 2 12AX7 in from him
all Telefunkin. Aint cheap. The 12AX7's were marked "The Fisher" so must be from the 60's. He tests everything very well. Trusted after many purchases here.

More info>>
https://dynacotubeaudio.forumotion.com/t1746-st70-vta-mod-question-about-startup-tube-glow
https://forums.stevehoffman.tv/threads/flash-on-12au7-tubes-when-turned-on.919102/
https://upscaleaudio.com/pages/tube-basics-and-frequently-asked-questions (down the page)

My Cronus MAG 2 has soft start and the Tungsgram still did it, never a problem. My Tele's none.

FullRangeMan · « **Reply #3 on:** 19 Sep 2021, 09:02 pm »

Quote from: AndrewA on 19 Sep 2021, 04:53 pm

I just got a matched pair 1960s 12AU7's from Brent Jessee. On start-up, one of them emits a quite visible orange glow for a second or two which then immediately dissipates and disappears. The other one emits no visible light on start-up. I can hear no channel imbalance (it's one tube each per side) or any other negative sonic impacts. Is this degree of variation normal? I should add that the tube that glows looks "dirtier" on the inside (there's some blackening of surfaces) whereas the non-glow tube looks "cleaner" (and a light shade of grey).

Dear fellow tube lover certainly your amp dont have soft start, so some tube are more sensitive when they are cold, to the huge voltage shock that the power ~~source~~ supply releases all at once to the Cathode + Grid + Plate, so the shock is so violent that this glow light release happens in a flash.

I would say this valve will not last long in this amp, if it is a cheap valve leave the amp this way anyway. If your amp have a soft start circuit the tube is defective.

rotarius · « **Reply #4 on:** 20 Sep 2021, 02:48 am »

Filament flash happens with some Mullard and Tungsram tubes. Which one did you get? Nothing to worry about, I never had a failure with the ones that flash.

RPM123 · « **Reply #5 on:** 20 Sep 2021, 04:11 pm »

I have bought tubes from Brent for years and he always stands by what he sells. The only thing that matters for tubes is how they measure and sound. I would not worry too much about how they look...at least for input tubes.

From tubeaudiostore.com:

"Mu (voltage gain), Gm (transconductance) and Rp (plate resistance) are the three electrical characteristics that make up the overall electrical characteristics for a vacuum tube. . There is much confusion in the audiophile community about these factors as they affect tubes and their performance in components. Written by world-renowned amplifier designer, Roger Modjeski, this article, will look at what each of these characteristics mean, how they are measured and what each of them means to the sound of a component. We will also look at tube testing, tube matching and which characteristics are most important when matching tubes for a given circuit.

How these measurements are applied and their relevance differs for triodes and pentodes. Triodes and pentodes both possess the characteristics Gm (transconductance) and Mu (voltage gain). However what is important in triodes is Mu and what is important in pentodes is Gm as these parameters are the dominant characteristics affecting performance when each is used in a typical circuit."

http://tubeaudiostore.com/tubmatdem.html

SteveFord · « **Reply #6 on:** 20 Sep 2021, 04:26 pm »

The input tubes on my VTLs flash, I wouldn't worry about it.

AndrewA · « **Reply #7 on:** 20 Sep 2021, 06:38 pm »

Thanks to all who responded. They're Blackburn Mullards, in a preamp (one per channel).

FYI: BJ got back to me and this is the information he provided: That is normal. Mullard, Amperex, Siemens, and even Ei Yugoslavia sometimes would flash dimly on turn on, sometimes brightly, and some of these tubes not at all. It all depends on how much filament is exposed at the bottom of the tube, and more importantly, the thickness of the coating on the filament. Some of these tubes will never flash, others will flash like a flash bulb!