[resolved] 12AU7 tube start up very bright

MyAlpaca · 301

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Offline MyAlpaca

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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
« Last Edit: January 13, 2024, 11:03:56 AM by MyAlpaca »



Offline MyAlpaca

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Reply #1 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.
« Last Edit: January 07, 2024, 09:41:35 PM by MyAlpaca »



Offline Deluk

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Reply #2 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



Offline Paul Birkeland

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Reply #3 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. 

Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man


Offline Mucker

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Reply #4 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.