Voltage High on 1 and 7, one LED does not light [resolved]

[penmoid]

Hello,
Just wrapping up my Crack build (pre-speedball) and things went relatively smoothly. Resistance check passed (after relocating a wire I installed in the wrong terminal), but I ran into a couple of anomalies with the voltage check:

T01: 147 (expected 50-100)
T02: 162
T03: 0
T04: 162
T05: 75
T06: 0
T07: 147 (expected 50-100)
T08: 0
T09: 102
T10: 0

The LED between A3 and the 9-pin center pin lights, but the LED between A8 and the center pin does not. Likewise, the front portion of the 12AU7 tube glows but the rear portion does not. The 6080 tube glows as well. During the glow test, both portions of the 12AU7 were glowing. The amp outputs sound, but only from the left channel.

I dug around on these forums looking for similar issues but wasn't able to find anything that solved my problem. One of the suggested steps that I saw on here was to solder a jumper wire between A3 and A8, and when I did that, both LEDs lit up but voltage on 1 and 7 was still high. Unfortunately I also damaged the center->A8 LED. For now, I have the A3->A8 jumper in place until I can order a replacement LED.

I reflowed a variety of solder joints without any change to the voltage, and I've been over the docs a couple of times and can't see what I'm missing.

Has anyone experienced something like this before? I've attached pics and can provide additional angles.

[Paul Birkeland]

If one half of your 12AU7 doesn't light, the solder joints at A4/A5 aren't doing their jobs.

The A3 to A8 jumper is installed when we have become absolutely certain that an LED has been damaged.  At this point you would be best off purchasing some extras and replacing them both, resoldering A4/A5, then rechecking your voltages.

I would recommend turning up your 888 to 899 degrees.  Double check how to do this in the manual, as you can accidentally go into calibration mode rather than raising the temp.

[penmoid]

Thanks a lot for the quick reply. I have some desoldering wick coming in the morning so I will clean up the terminals on the 9-pin then. In the meantime I’ll do some work on A4/A5.

Will I be able to at least get it working with the A3/A8 jumper? The LED was ruined by my troubleshooting (connection broke directly at the body). Ideally I’d suss out anything else I might have made a mistake on in case I need to order more than just the LEDs.

One other question.. I have the 888 set at 755 which is a non-insignificant jump from what’s suggested in the manual but it’s what I’ve been comfortable with in general. I’m fine to take it up to the max if it will get me better joints but it’s so much higher than what the manual says. Anything I have to worry about going up that hot? Or am I misunderstanding?

Again thank you so much for the quick reply and all of the suggestions.

[Paul Birkeland]

More heat will let your solder flow out more quickly.  Really you just need to reflow the joints on A4 and A5 and verify that both halves of the 12AU7 are lighting, then if the jumper is in place recheck the voltage at terminal 1.

Solder braid isn't really necessary here.

[penmoid]

I reflowed the joints for the bridge between A4/A5 and despite open connectivity and essentially zero resistance between those two pins the behavior/voltage has not changed. In fact, it looks like there's pretty much no resistance between any of the terminals connected with green wire to other terminals connected with green wire. A3/A8 bridge has no resistance either. Possible the tube or socket were damaged?

I forgot to mention that I noticed this tube had pretty dull leads earlier so I cleaned them with Iso and a wire brush, but it didn't make a difference.

[Paul Birkeland]

I would just ignore the A3/A8 bridge and diodes for now.  If both halves of the 12AU7 won't light up, that isn't something that's even worth addressing. 

If you take the tube out of the socket, flip it over, then measure the DC resistance between pins 9 and pins 4 on the tube itself, what do you get?  What about pin 9 and pin 5?

[penmoid]

Good morning,

9->4 is reading 9.7Ω
9->5 is reading 0.809kΩ

[Paul Birkeland]

It may well be worth trying a different 12AU7.  It's possible that one of the heater connections popped loose in shipping.

[penmoid]

I can do that. Where would you recommend sourcing one from? I am looking at the Bottlehead store and I see the LEDs but I can't find replacement tubes listed.

Is there anything else worth trying until that comes in?

[Paul Birkeland]

eBay is full of inexpensive used 12AU7s, and you may find that a local guitar store has a 12AU7 or two available as well. 

[Doc B.]

Contact Eileen at [email protected] and ask for a replacement 12AU7. Tell her Doc says OK as it sounds like yours was damaged in shipping.

[penmoid]

Email sent, thanks! As it happens, there's a guy just up the road from me that sells tubes  and I was able to pick up a couple of used 12AU7 over lunch. I popped one of these in and both sides lit up, so it seems as though the problem was in fact the tube. The amp fires up and sounds great even with the used 12AU7.

Here are the new voltages:
T01: 74
T02: 168
T03: 0
T04: 168
T05: 72
T06: 0
T07: 100.6
T08: 0
T09: 102
T10: 0

Everything seems up to spec, except for T07 which is a nose hair out of range. Is this an acceptable voltage? Does it have something to do with the A3/A8 bridge?

Regarding that bridge, are there any major downsides to having it in place for several days while I wait for replacement LEDs to arrive? Is it safe to use in this condition? I have the speedball kit which appears to have 8 of the same LEDs, which I have left unsoldered. I could install a couple of these (the Center->A3 LED is pretty beat up from the troubleshooting) and wait on assembling the speedball until replacements arrive if I shouldn't be using the amp without the LEDs.

[Paul Birkeland]

That's great that you were able to source a spare easily.  The A3 to A8 connection should be considered a temporary remedy and replacing the damage LED is a good idea.

[penmoid]

Makes sense to me. LEDs are shipping out tomorrow with the replacement tube. I can't thank you enough for your help in diagnosing this issue!