One LED not burning [resolved]

[Skipperrik]

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?

[Paul Birkeland]

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.

[photoduo]

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.

[Paul Birkeland]

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. 

[Skipperrik]

Thanks for all your help with this. I really appreciate it. I'll contact Eileen today about a replacement.

[photoduo]

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?

[Paul Birkeland]

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.

[Skipperrik]

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. 

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?

[Paul Birkeland]

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.

[Skipperrik]

Great. Thanks for all your help!