Bottlehead Forum

Bottlehead Kits => Mainline => Topic started by: MyAlpaca on January 07, 2024, 09:28:08 PM

Title: [resolved] 12AU7 tube start up very bright
Post by: MyAlpaca on January 07, 2024, 09:28:08 PM

Please check the attached gif, in summary, when I turn on amp, the 12AU7 glows very bright for a moment then comes back to normal, as if it receives a very high power for a second.

Here is my resistance and voltage check that is not exactly matched what mentioned in the manual but within reasonable range:
Terminal 1 and 5: 36.1K
Terminal 19 and 22: 221k
Terminal 20 and 30: 684k
+275vDC on the Power Supply Board: 270v
IA on both C4S board: 270v

Also, I have finished the biasing step and have both voltages read at 145v

Title: Re: 12AU7 tube start up very bright
Post by: MyAlpaca on January 07, 2024, 09:31:31 PM

Looks like the attached gif doesn't work, here are screenshots. #1 is immediately after power is turned on, #3 is 2 second later.

Title: Re: 12AU7 tube start up very bright
Post by: Deluk on January 08, 2024, 01:38:15 AM

It's can be described as normal or not unusual. European tubes apparently more prone to it. Nothing to worry about.
Google - 12AU7 flash on start up

Title: Re: 12AU7 tube start up very bright
Post by: Paul Birkeland on January 08, 2024, 05:50:26 AM

That is likely a Mullard or an Amperex 12AU7, and the heaters flash a bit during startup for that particular tube. 

Title: Re: 12AU7 tube start up very bright
Post by: Mucker on January 08, 2024, 12:38:55 PM

I have Tusgram 12AT7's in my Stereomour II that sure look like Mullards and they do the same brief flash on startup.