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September 24, 2022, 09:53:40 AM

Author Topic: One LED not burning [resolved]  (Read 426 times)

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Offline Skipperrik

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Re: One LED not burning
« Reply #15 on: April 21, 2022, 02:56:04 AM »
Good morning, Paul.

I disconnected the leg of the LED attached to A3. The resistance now shows 0.L between A3 and terminal 3 with the tube in and out.

I also checked the continuity across the "bad" LED and there is continuity. There is no continuity across the "good" LED. Does that have any bearing on the problem?

Offline Paul Birkeland

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Re: One LED not burning
« Reply #16 on: April 21, 2022, 04:22:33 AM »
Yes, it looked like there was possibly a crack in that LED as well.  To temporarily get your amp up and running, you can add a wire between A3 and A8 and you should find that all your voltages settle to appropriate values.  You can then replace that LED down the road as time permits.
Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man

Offline photoduo

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Re: One LED not burning
« Reply #17 on: April 21, 2022, 04:52:42 AM »
To temporarily get your amp up and running, you can add a wire between A3 and A8 and you should find that all your voltages settle to appropriate values.  You can then replace that LED down the road as time permits.

I am curious as to what the purpose of the LED's is. If you can substitute a wire between A3 and A8, will that work just as well long term in place of the LED's or are they working as some kind of safety fuse? I tried searching, but can't find any info on the function of the LED's.

Offline Paul Birkeland

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Re: One LED not burning
« Reply #18 on: April 21, 2022, 04:59:50 AM »
The LEDs provide a constant 1.57V of drop from each 12AU7 cathode to ground.  If you tie both cathodes together, the amp will work but you may lose a bit of channel separation in the process. 
Paul "PB" Birkeland

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Offline Skipperrik

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Re: One LED not burning
« Reply #19 on: April 21, 2022, 05:06:41 AM »
Thanks for all your help with this. I really appreciate it. I'll contact Eileen today about a replacement.

Offline photoduo

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Re: One LED not burning
« Reply #20 on: April 21, 2022, 05:34:49 AM »
The LEDs provide a constant 1.57V of drop from each 12AU7 cathode to ground.

Again for curiosity sake, why are resistors not used in this case?

Offline Paul Birkeland

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Re: One LED not burning
« Reply #21 on: April 21, 2022, 05:40:42 AM »
Resistors would function but have higher AC impedance than the LED does.  You'd end up losing a fair bit of gain by using them instead of LEDs.
Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man

Offline Skipperrik

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Re: One LED not burning
« Reply #22 on: April 28, 2022, 09:55:32 AM »
Hi Paul. Just a follow up on the LED that wasn't burning. I received the replacement and installed it with no problem. The LED is burning bright.  8)

I rechecked the voltages and everything is within spec except for KregA. The specs call for 3-6V and I'm getting 7.34. Is that something that needs to be addressed?

Offline Paul Birkeland

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Re: One LED not burning
« Reply #23 on: April 28, 2022, 10:05:03 AM »
No, that's OK.  A higher voltage there would tend to suggest a rather potent regulator tube on that side, so nothing to be alarmed about.
Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man

Offline Skipperrik

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Re: One LED not burning [resolved]
« Reply #24 on: April 28, 2022, 10:06:11 AM »
Great. Thanks for all your help!