half cracked

[nsorens]

Need some help diagnosing a Crack issue...
I have enjoyed one of the first Crack amps with the Speedball upgrade for a few years now. But in the last week I noticed my right channel crackling a bit and this morning the right channel had very low volume.  I checked under the hood and found that terminal 10 had come loose.  I have installed a pair of the giant Axon caps and even tho I velcro-ed them in place, the term 10 solder joint had worked loose.
I hoped this would fix the problem but I still had practically no right channel volume.  After doing the voltage checks with a 122 mains voltage I come up with these abnormal readings:
Term 1  -  174V (should be 90)
Term 7  -  173V (should be 100)
Term 21 -  0V  (should be 206)
Term B1 -  182V (should be 90)
Term B3 -  173V (should be 100)
Any ideas on what broke?  I switched tubes out to make sure that wasn't the cause.  What might the damage be from a bad joint at term 10 (the right channel output cap)...?

 

[Paul Birkeland]

If 21 is zero, you would have 0 at every other terminal downstream from it, so I would suggest remeasuring that.

Terminal 1 being 174V means that half of the 12AU7 isn't working properly.  Are both LED's glowing on the 9-pin socket?

 

[nsorens]

Yes your'e right Paul, I re-measured, 21 is fine (220V).  Sorry...
Both LED's are glowing on the 9-pin socket.

 

[Paul Birkeland]

If both LED's glow and terminals 1 and 2 are the same voltage, then the voltage is getting to the tube, but the tube isn't conducting well.

This could happen if the red wire leaving the volume pot and going to pin 7 on the 9-pin socket was disconnected.

If this connection looks good, I'd nab another 12AU7.

 

[nsorens]

Thanks Paul, damn you are good!  It was a bad 12AU7. I marked it with a sharpie so I don't make that mistake again...
So that failing Term. 10 solder joint to the output cap probably took out that tube?

My next question: Apparently I have a bunch of tubes that I have no idea of their health.  Is there an inexpensive way to test tubes out there?
Thanks again for your help.

Nels

 

[Grainger49]

Nels,

If the tubes are all 12AU7s plug them in and give them 15 minutes warmup.  Decide with your ears.

If you are worried about your headsets the capacitors should protect them.  Just wait a minute for warm up before plugging the headset in first.

 

[Paul Birkeland]

So that failing Term. 10 solder joint to the output cap probably took out that tube?

Nah, that shouldn't bother the 12AU7. 

 

[Doc B]

Thanks Paul, damn you are good!  It was a bad 12AU7. I marked it with a sharpie so I don't make that mistake again...

A light tap with a framing hammer is a much more certain way to avoid mixing bad tubes with good ones.

 

[nsorens]

Sounds good Grainger, I'll sort by ear...

Yes Doc, I trashed it.  No more use for it