Shorted LED, but amp still works

StudioDweller · 1599

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Offline StudioDweller

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on: February 23, 2015, 08:13:17 PM
I completed a very enjoyable build and all resistance values were good and initially, all voltage checks were good with the exception of the following:
B1 149 (90 expected)
B3 149 (90)
A6 149 (90)

If my memory serves, both LEDs were not illuminated during the initial power up. During subsequent checks, I accidentally shorted the A8 LED to A9 and I believe the ground probe of my meter shorted to one of the adjacent terminals when I removed it producing a spark. After both of those things happened, I also had higher than expected voltages at terminals 1 and 7 - both were 149.

After reading this thread about the LEDs I reflowed the LED connections and a suggestion was also made to connect a wire between A3 and A8 which I did. After adding that wire all voltages measure fine now and the only remaining issue is the A8 LED not lighting. However, the amp seems to work. I plan on giving it a more extensive test later, but if the A8 LED is not lit and/or is blown what effect would that have on the amp? -Thanks
« Last Edit: February 23, 2015, 08:59:03 PM by StudioDweller »



Offline Paul Birkeland

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Reply #1 on: February 24, 2015, 07:58:15 AM
if the A8 LED is not lit and/or is blown what effect would that have on the amp?
If you have an issue with one half of the 12AU7, but not the other, having the bias for each side done with two LED's can be a little more visually informative. 

Channel separation with the single LED shouldn't be compromised, as the dynamic impedance is very low.


Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man


Offline StudioDweller

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Reply #2 on: February 24, 2015, 08:09:42 AM
Thank you! Everything checks out and sounds great, but I will replace the LED so it's as it should be. Next time I will be bit more steady with my test probe!