Flash seen from bottom of 12AT7 PI socket

[pjd3]

Hello,

I sure could use a little sanity check on this if you can.

I was once again trying my rebuild which had been looking fine, except, when I hit the power switch this time, I witnessed a whiteish flash through the base of the Phase inverter socket that had a NOS 12AT7 in it. I have not seen this happen before and it alarmed me. But, I went forward to look at some things. I always look at the highest B+ at the first main filter cap to see that it is where it usually is. It has consistently rose to 465 to 468vdc every time after 15 seconds or so when the GZ34 cathode has warmed up. Only this time, the B+ was down to about 440vdc which was very unusual. I turned the amp off right away so to come back when there was more time to see what was going on.

I'm thinking that something bad happened internally inside the 12AT7 and is continuing to exhibit a short or something that would pull down the B+ that much. Maybe a cathode to filament short?

Thank you for your input on this. Ahhhh,

Best,
Phil

[xtian]

I've had some preamp tubes (vintage USA) that flash on power up. Worked fine.

[pjd3]

Ok Xtian, thanks.

I will keep it in mind that that may not have been an issue. It was seeing my B+ 20-25 volts lower than usual that got my attention. Must see about that.

Thanks again,

Phil D.

[maxkracht]

https://www.youtube.com/watch?v=bX0o3jAzSBs

This guy claims it's a manufacturing problem affecting tube lifespan. I think his background is more in transmitting tubes. Probably more critical with the high power stuff. I've seen heater flash in some pretty old tubes that were still running fine. Probably nothing to worry about.

[pjd3]

I'm quite sure this PI tube is a NOS Mullard CV4024.

I've only ever purchased 3 of them but this one and another showed unexpected early symptoms. Actually, this one I referring to here may be one that started whistling at some point. It still passed signal adequately but began high pitched whistle that you could manipulate by flicking the tube.

Maybe just time for a couple of new 12AT7's.

The journey continues on the Blackvibe 6L6 rebuild. And yes, as RG has commented, it is much more of a cleaner venture to build from scratch than to do a rebuild. Yup.

Thank you,
Best,

Phil Dl

[didit]

Do a search on “mullard triode flash” and read up about the wide observations that this is typical and believed benign.

..

[pjd3]

Thanks for didit,

I see that there is a lot talked about, the Mullards power on flash, that it tends to happen when cold and not when the amp gets turned on and off in close time.

Very interesting.

thank you,

Phil D

[TUBEDUDE]

I have Mullards and an odd Telefunken that flash. Decades of use with no issues. The plate voltage issue may be a small brown out or dip in the line voltage. A small line voltage change percentage wise shifts B+ up or down a fair amount.

[pjd3]

Thanks Tubedude,

I do try to remember that my HV coil is about a 3:1 ratio to the line coil and do expect some variation, its just going from a solid 466 +/- 3vdc all the way down to 440vdc or so, is such an unusually big dip, but, maybe the line voltage did do something crazy low at that time. And it was dark out. I suppose my brains and eyes may have darkened too!

A really good tech I used to work with would always say, "Simultaneous events are not proof of cause-and-effect relationship". So, when the flash happened and my meter read 440vdc I did take it then as a cause-and-effect relationship but, that will need a nice close look when I can get to my bench for a while!

thanks again,

Phil D

[Reeltarded]

I have Mullards in most of all amps V1. The 12ax7s ALL flash.

[bepone]

Allmost all vintage tubes flashes when powered on, it is coming from heaters when they are powered on, this is a good sign (you have good old tube)

[pjd3]

Thanks, Bepone,

This tube I believe is the Mullard CV4024 from KCA. Helped make my Deluxes and Fender type builds sound fine.

I'm glad I found this information out here or else, I may have forever turned my back on perfectly good functioning NOS tubes!

Thank you,

Phil D

[maxkracht]

Check out the 3:30 mark on the video I posted, it shows a very nice closeup of exactly what is happening.

[pjd3]

HI Max,

watched that video and Yip, I'm nearly certain that that's what happened. A nice brisk flash of white light.

a couple of the tubes that I acquired from the deceased engineers hoarding cellar were Mullard 12ax7 type tubes so, will keep an eye of them too. Literally.

Thanks Max,

Best,

Phil D

[Raja_Kentut]

Because of the flash I power up my amps through a current limiter on the primary side, between the wall socket and the amplifier power cord.
That also helps the input fuse and the input rectifiers and powers up everything including the anode voltages in a smooth manner…

I don‘t know if all values of the components are the end-status…

IMG_0788.jpeg

IMG_0784.jpeg

IMG_0783.jpeg

the power resistors are still missing in the 2nd picture…