Finished Build - Normal Resistance/Voltage - No A8 LED Light - Loud Noise

[Tommy Coyote]

Minor update: I managed to desolder A3, A7 (moved it out of the way a little), A8, A9, and the center pin of the A socket without damaging the LEDs as far as I can tell. No-clean flux braid is pretty sweet, thanks for the tip

I reconnected and soldered everything, swapping the LEDs on A3/A8 this time. Resistance measurements all within range, and the voltage measurements are pretty consistent with what I got originally... T1-10 are all within range, the A3 LED lights up, the A8 LED does not light up (the problem is staying on A8, even after swapping the LEDs).

Latest voltages:
T1: 78.1V
T2: 168.0V
T3: 0.2mV
T4: 167.8V
T5: 84.8V
T6: 0.3mV
T7: 116.7V
T8: 0.6mV
T9: 103.7V
T10: 0.4mV
A3: 1.7V
A7: -0.8V
A8: -14.1mV

I got shipping confirmation today so will try swapping out the 12AU7 when it arrives. I can try reflowing some more joints as well.

Any other ideas welcome of course... I'll share when I have updates as well.

Thanks!
-Tommy

[Tommy Coyote]

Hiya, I got my 12AU7 tube replacement and tried it out today, and could use some additional help.

So I disconnected the tubes and performed another resistance check first, and everything was within range. I then plugged in the 6080 and the new 12AU7, and turned it on... both tubes light up, although it's harder to tell on the new 12AU7 as the top isn't transparent like the original tube.

Bad news: neither LED is lightning up now. Earlier this week the A3 LED lit up, but not the A8. On to voltage measurements...

NEW TUBE VOLTS:
T1: 141V
T2: 156.9V
T3: 0.4mV
T4: 157.1V
T5: 138.3V
T6: 0.4mV
T7: 139.9V
T8: 0.7mV
T9: 139.2V
T10: 0.4mV
A3: 10.99V
A7: 0.6mV
A8: 11.38V

I noticed some of those are a tad high, although I have no idea what this indicates. I went ahead and reconnected the old 12AU7 to take more readings. Both LEDs remain dark now, even with the previous tube.

OLD TUBE VOLTS:
T1: 81.1V (slowly dropping, staying steady around 70V)
T2: 161.4V
T3: 0.2mV
T4: 161.8V
T5: 143.0V
T6: 0.4mV
T7: 116.1V
T8: 0.7mV
T9: 142.5V
T10: 0.4mV
A3: 12.08V
A7: 0.2mV
A8: 48.0mV

So I'm getting pretty significant changes with the newer tube, although T5/T9 are both much higher than before regardless of which tube. Also A3 was around 1.7V earlier this week, but seems much higher now regardless. And A8 varies greatly depending on which tube I use.

This is all still Greek to me, so any further advice is highly appreciated!

[Paul Birkeland]

This still sounds like bad solder joints, especially on the ground buss (the black wires in the amp).

A good tube running with open (blown) LEDs on the 9 pin socket will show about 12V or so on pins A3/A8 (whichever has the open LED), so this isn't manifesting as an LED issue.

[Tommy Coyote]

Hi PB, thanks for reading and replying again! I want to make sure I'm understanding what you're saying.

So a good tube with bad LEDs should read about 12V on A3 and A8, correct? I have 2 tubes now: with the new one I'm getting about 11V on A3 and A8, but with the old one I'm getting about 12V on A3 and about 0V on A8.

So it sounds like the old tube is probably bad and the new tube is probably good... can you confirm if that sounds correct, or why they could be performing so differently?

[Paul Birkeland]

Oh, I didn't see that, I thought you have mV there. 

You may indeed have a pair of damaged LED's, or the ground connection from the center lug of the socket working back is open somewhere.

What is the DC resistance between the center pin on the 9 pin socket and terminal 3?

[Tommy Coyote]

OK cool, glad I double-checked with you.

I'm getting 0 ohms between T3 and the center of the A socket.

[Paul Birkeland]

I would carefully replace the LEDs.  If you're having trouble with them, you can substitute a 470 ohm 1/8W resistor for each LED.

[Tommy Coyote]

Resolved! With the new LEDs and new 12AU7 all of my voltage measurements were spot on this time, and both LEDs lit up. Seeing both of those LEDs light up is probably my high point of 2020 so far haha.

And my Crack built sounds great by the way. I also have the power cord to build and the Speedball upgrade on standby, but I'll spend a few weeks or so getting familiar with this guy before starting on that.

Thanks for all of your help PB, I couldn't have finished this otherwise!
-Tommy

[Paul Birkeland]

You're welcome.  When you go back in there, 3L may benefit from a little extra solder too.