Finished BUT octal tube POPS when touched/jiggled [resolved]

[Tom-s]

Cold solder joints can make the fuse blow.

Do as PB said.
Heat every solder joint till it's flowing (this can take a while), add just a touch of solder (to get the flux) and then keep the iron on that joint for 5-10 sec extra.
Let it cool slowly (don't blow at it).

When soldering, always wear safety glasses.

Redo resistance checks, post the out of line ones here. If all ok. Do voltage checks and post here.

Edit: What solder are you using?

[Jsin]

Thanks! I was obviously being much too timid about heat etc.
Am using 60/40 Rosin Core Solder (flux 1.8%)

[Jsin]

Okay - just made best attempt at reheating and flowing the solder joints everywhere.
Resistance test was all good EXCEPT FOR 20 - instead of 0 I get rapidly moving and fluctuating readings - seems to shift and change when I touch different areas of the terminal - but never reach my 0.2 or 0.3 as hoped/expected.

Might I have blown one of those tiny 0047 resistors that links into 20L?

All help so appreciated. Close doesn’t give me the cigar, and yet - junk I’m close.

[Jsin]

Wait - weird but now I can get my 0 reading on 20, but not if I move the lead around at all on the tab - is that the sign of a cold/bad solder? If so, on a couple of them the ohm reading changes a bit depending on where I place the lead on the tab (maybe clean read on tab and not so clean if touching a solder point)

[Paul Birkeland]

There could be a few things going on here.  When you use solder, there is flux inside it that flows out when the solder melts with the intention of cleaning the terminal so it can be well soldered.  This flux is a poor conductor, so if you touch your probe on flux, you may not get a proper 0 ohm resistance reading.  If this is the case, you may find that the tinned lead of the capacitor that hooks to that terminal is an easier place to measure a resistance reliably.

The other option is that the solder hasn't flowed all the way.  When this happens, that flux flows out to clean everything, but without enough heat the flux will stay put around all the leads and the terminal strip hole, so you'll get a flaky joint.  It sounds like you're getting the hang of all of this, so if there's any doubt about this, some extra heat will help flow the solder all the way and move the flux to the outside of the joint.

[Jsin]

Thanks!!!
So, should I take one more run at re-heating a few of these that act a little funky? Or, since I do get the right readings when I am a little more intentional about where I place the lead - should I go ahead with the voltage testing?
J

[Paul Birkeland]

If the cap lead at terminal 20 gives you a consistent 0 and that's the only joint doing that, you could try a voltage check.

[Jsin]

Ok. All was good with the ohms. Tubes glowed (though one half of smaller tube seemed dimmer than the other) but no leds were lit underneath. The voltage test did not give any expected numbers.

I went back and reheated some joints - passed the ohm test - plugged it in to test voltage, no tubes glowing and a blown fuse just from turning on.

Think I keep moving myself backwards somehow.

— high maintenance

[Paul Birkeland]

What numbers did the voltage checks give?  Actually knowing what they are could point us in the direction of your problem.

What value are you using for the fuse?  In the absence of a more solid understanding of why your amp is blowing fuses, you may want to go to a 3A fuse.  This will allow whatever is ailing your amp to get a bit hotter and make itself obvious.  I don't normally recommend this procedure right off the bat, but other than the soldering issues from your first set of photos, everything is in its place and it's quite obvious that you followed the directions well, so this may be a necessary risk to take.

The one thing I can see is one of the UF4007 diodes on the outside of the 6 lug strip.  If one diode is backwards, that would cause just about all of these issues. 

If one half of the 12AU7 isn't glowing at all, that would be the solder joint on A4/A5 not functioning well.  One side not glowing as brightly is more than likely just a production variation in the 12AU7 itself.

[Jsin]

Thanks Paul.
I gave it one more cycle of searching and re-soldering. Ensured the 4007s were all banded where they should be, checked ohms, and then turning it on blew the fuse again.

I can grab some heftier fuses later or tomorrow, but what exactly will I be looking for if those keep it running - how will it show me the problem?

Thanks again for all the attention. Am determined to get this back to where I started at least...

J

[Paul Birkeland]

Components that get hot will discolor, and that will tell you where to look.  In the meantime, can you post some fresh build photos for us to look over?

[Jsin]

Sounds like I'll be looking to break something - that I know I can do...
Here's some pics of the build with some - hopefully - better attention paid to soldering.

 -- might feel better about where I'm at if I hadn't been listening to music through this on Tuesday night --

[Paul Birkeland]

Could I see the terminal strip with terminals 17-22?  Terminal 19 looks like it has a lead sticking through it that needs to be trimmed off.

[Jsin]

Here are both sides. I think you are seeing the wire needing trimming on 20L... but I can't figure how to get in there. Could that be causing an issue of this magnitude?

BTW - ran and got some 3A fuses. What's the process? plug it in - see what discolors - unplug - post?

-- J

[Paul Birkeland]

I would say that 20L is not soldered.