Volume drop randomly with scratchy crackle

[Happyorange27]

2 weeks ago:  While playing for about 20 minutes, I noticed my volume drop about halfway.  When I would play a loud note it had some crackle to the sound as well.  I assumed it was a pedal loosing power or a loose cord.  I checked the whole rig.  Well the problem went away after a minute.  I figured no big deal.

Subsequently this began to happen a bit more frequently as I had been practicing over the last week.  The volume would drop and it had a nasty overdrive crackle when I strummed.  So I would strum with one hand and jiggle things with the other.  I did finally deduce it was from the amp since I plugged straight into it and the problem was still there.  I took the back panel of the Blues Jr. and poked around with a wooden chopstick, looking for loose parts.  I couldn't find anything disturbed, but I was only concentrating on the circuit board.  I was actually hoping that it was a loose power capacitor, like the 100uf which is the biggest.  Because that would make a lot of sense, but come to think of it, that would make all the volume drop, not just half of the volume.

Needless to say this was frustrating.  I researched online about this problem.  Most people recommended new tubes or cleaning the pots and sockets with Deoxit.  So I bought a can of Deoxit, but is has arrived yet. 

Solution found!:  Last night it only took 5 minutes for the problem to occur, so I tipped the amp on it's face and played while I could poke some more.  Well I did indeed see a loose solder joint on one of my new octal sockets.  I had recently modded the amp for larger power tubes (6v6).  So this one wire was hooked around the socket tab, but the solder was not bonded; A COLD SOLDER JOINT!  Shame on me.  So when I wiggled this wire (with much safety used) I could recreate the scratchy sounds.  Even if this wasn't the root cause, I surely needed to fix this right away.  Within minutes I had it soldered up good.

I played for the rest of the night with zero issues.  I'm pretty sure that was the problem.  I was seriously getting worried about the whole deal before I figured it out.  When your amp is sick and you don't know what to prescribe, you cry for your baby.

Well I hope this helps the next guy that has a similar issue.  Poking around with a wooden chopstick is actually a great and easy technique.  BE SAFE MY FRIENDS...AND JAM ON! 

[Heady Jam Fan]

Nice - I was thinking it sounded like power to a tube!

[Happyorange27]

Yeah I wanted to give all the details to make you think a bit before I revealed the solution.  But I gave you a hint by saying the volume dropped about half way.  Anyway I read about folks having problems but they are not always solved. 

[the_great_lemon]

How, specifically, would one go about 'being safe' when poking around inside of an amp?  I know it is extremely dangerous so I have not tried...you must drain the residual power from the transformers correct?

[Happyorange27]

Lemon, everyone is always putting disclaimers on such posts. I've always been cautious. There are some power supply caps, the big ones, that store that voltage. Usually if your amp is on for a good thirty seconds or more (tubes have to be up to temp), you can turn it off and the caps will drain in about 10 seconds. If you here your guitar fade accordingly, you can assume they are drained. Now me, I have put a voltmeter on my largest cap to measure the voltage and so I can watch it drain. Always a great way to ensure you are good. I watch it go from 430 to zero. Also unplug the amp after the drain.
Anyway it's always smart to poke or grab with insulated tools like wood or plastic handles.

But yeah I poke with a chopstick when the amp is on when I need some real time analysis. Also keep one hand away from the amp so you don't increase the risk of completing a circuit through your body.

[IamWILSON]

Good job Happy man!!!  Preach the safety... Don't poke with a chopstick, or anything for that matter, while holding the chassis with your other hand because if you do get a shock/jolt, that voltage will go right through your heart to your other amp and back to the chassis (since you've grounded yourself that way).  Hence why you keep one hand behind your back, in your pocket, or just at your side.  So if you do get some juice, there's a better chance it goes to ground, and not pass directly through your ticker! 

The best rule of thumb with poking around your amp, if you don't know 100% what you are doing, don't do it!

Knowing Happy's background, he is more than qualified, but I'm sure he's always very cautious about how we does it.  I need to finish building my 5e3 amp, as I did find a bad solder joint, and need to add a resistor because the voltage on the 6v6 tubes was like 430 when I measured it.  Tubes were glowing blue, and I was getting some phasey type pulsing when it was turned up loud.  Other than that, it sounds awesome!

[Happyorange27]

Wilson, i didn't know you were building an amp.  Cool!  Hey thanks for the extra words of safety.

Have you considered an adjustable pot for the bias of your power tubes instead of just a fixed resistor?  I'm assuming you are talking about adding to the grid; or are you talking about the plate voltage?  I think you need to control the bias at the grid, hence an adjustable bias.  Not entirely sure about that though.  GLOWING BLUE YOU SAY?  Wow, have fun with that.

[Happyorange27]

Ok so I claimed that I fixed the problem earlier but the problem came back.  I asked Billm for help.  He directed me to this page:
http://billmaudio.com/wp/?page_id=1254

I really like this tool you can make to probe the amp.  I made one and it helped me really get to know my circuit.  I learned a bunch by following the signal path.  Also Bill said this.

"It's unlikely that both tubes would fail sporadically at the same time, and if only one output tube fails, you get lower volume and a big increase in distortion.

If you can catch the amp in the act of going silent, I would to a quick series of voltage tests. But first, pull the black wire from the reverb tank and touch the center pin. If you get a buzz, your fault is somewhere in V1 or V2 circuitry. If you don't hear buzz, the problem is in the phase inverter or output stage."

I found some loose metal shavings in the amp from my drilling in the past and such.  Maybe those were shorting out some components.  Anyway I cleaned them out and I didn't have the problem all night so I'm hoping that was it.  But in the future if I have issues, I should be able to pinpoint the location much faster.  Good stuff.

[cactuskeeb]

That sounds exactly like what happened to me when I forgot to solder a heater wire - I just had it hooked onto the appropriate metal loop on the socket, dry.  If the problem comes back, you might try resoldering everything on the power tube sockets.

[Heady Jam Fan]

I had a somewhat similar issue on my Express Plus - Mesa sent me new power tubes and that fixed the issue. Volume would cut out, get fizzier and ultimately there would be no sound. It only happened in the 25w setting, so it was probably only one half of one of the power tubes.

[Happyorange27]

Well I replaced the output tubes and I still had just a shade of the issue, but could not confirm it.  I ended up biasing the output tubes afterwards (should have done it upfront, but I was just testing the general swap theory).  So after I bias the tubes my amp has been fucking singing!  Life is good!

[Heady Jam Fan]

Sweet! Glad to hear your back in business!