Could this cause my issue?

[daz]

Some of you may recall i posted about my marshall JCM800 style homebrew saying I had 2 tubes pop fuses one soon after the other. I had a lot of winged C el34's and never had one do this then 2 within a few days, and in the same socket. But then after that no more issues. At least till last nite when i plugged in and the tone was buzzy and thin. I popped another tube in and all was fine again. Then it started again in a while and i ended up trying a lot of the SED's had and at that point they all did it. Thats when i figured there was a problem in the amp. I found the issue today....the wire from the main 100uf/100uf cap can to the screen resistors was thru the hole in the lug, but of the 3 wires at the caps lug that one was loose thru the hole. Last time i soldered it the wire apparently didn't allow solder to flow to it. So it makes sense that when i'd insert a tube it would move the wire in the lug because it's only a couple inches to the first screen resistor and it's stiff teflon wire.

So heres the question...while it caused the tone to suck as i tried to test it with various tubes last nite, no fuse blew. Could it be that when those 2 tubes blew fuses last time around it was because the wire had lost momentary connection with the cap long enough to lose screen voltage? Would that pop the fuse?

[Jazz P Bass]

If the screen is not connected, the tube is basically in an off state.
Now that (loss of screen connection) may cause dc offset on the OPT, I am not too sure.

[daz]

If the screen is not connected, the tube is basically in an off state.
Now that (loss of screen connection) may cause dc offset on the OPT, I am not too sure.

Well, i wonder if a quick intermittant on/off of the supply could cause it because i remember i tapped the tube s and thats what got them to pop the fuse. I have no doubt the loose wire was being moved when i tapped the tube because the socket pins move when you do that and that wire would certainly be moving in the lug. In any case, now i can check all those tubes and maybe the "bad" ones weren't bad after all. Especially important since they are all winged C's and my favorite and soon to be NOS only tube !

[Amp Kat]

Not likely. Thy only way it would blow the fuse is if it touched ground and you would here it loudly. Opens won't cause fuse blows even if they are arching back and forth. I doubt the imbalance through the transformer would cause it to blow it would just not work.

[Chuck H]

What Amp Kat said. UNLESS... Your wired in such a way that, as JPB eluded, the screens were just floating with no voltage on them. Ideally the wiring should be done so that if the filter cap fails (or is intermittently disconnected) all you lose is filtering, not voltage. But we can't see how it's wired.

[daz]

What Amp Kat said. UNLESS... Your wired in such a way that, as JPB eluded, the screens were just floating with no voltage on them. Ideally the wiring should be done so that if the filter cap fails (or is intermittently disconnected) all you lose is filtering, not voltage. But we can't see how it's wired.

Well, the voltage is supplied by the choke's output which is also in that same cap's terminal. So when the wire to the screens was loose in that terminal but the choke was not, it WAS sans voltage if it wasn't making contact for the fraction of a second it's being moved. The only way around that would have been to twist and solder the choke and screens together then solder them to the cap. And thats fine except i would have never thought this would happen.

[g1]

Slow-blow or fast-blow fuses? With slow-blow I'm with the others, doubtful the surge or even arcing would make them blow. With fast-blow, all bets are off.

[daz]

Slow-blow or fast-blow fuses? With slow-blow I'm with the others, doubtful the surge or even arcing would make them blow. With fast-blow, all bets are off.

fast 4A

[Chuck H]

Removing the voltage from the screens shouldn't do much other than stop the tube from conducting. I would think SOME electrons would still make their way to the plate without the screen to prompt them. But having the voltage pop on and off repeatedly in rapid succession?!? No idea what that would do. But I don't think it's good. If your unloaded voltage is high-ish then the tube jumping between conducting and not conducting with high voltage spikes would seem to me like a very bad scenario. Hopefully fixing the joint has solved the problem. AND...

I NEVER thought I would have a cold solder joint in one of my amps. Well... It's never happened in a customers amp, but I can get a little cavalier with my personal stuff. And I recently had a thread here about a very odd problem that ended up being a cold solder joint! Which is embarrassing. It happens. Especially if you get too relaxed and trusting of the solder. I mentioned this because I think it's a good idea at this point to go through your amp and retouch (with fresh melt flux core or added flux) any solder joints that don't look just right. That's what "I" would do.

[daz]

I NEVER thought I would have a cold solder joint in one of my amps. Well... It's never happened in a customers amp, but I can get a little cavalier with my personal stuff. And I recently had a thread here about a very odd problem that ended up being a cold solder joint! Which is embarrassing. It happens. Especially if you get too relaxed and trusting of the solder. I mentioned this because I think it's a good idea at this point to go through your amp and retouch (with fresh melt flux core or added flux) any solder joints that don't look just right. That's what "I" would do.

The problem with this joint is the damn cap terminals aren't big enough and fitting three wires in it is hard to do cleanly. Plus i hate flowing solder too long on a cap terminal and with all that wire it takes a bit of time to get it all to flow fully. It looked ok but it was hard to see with all the wire in it.

[Enzo]

If you are worried about solder iron heat travelling up the cap lead wire and damaging the cap, I doubt you have anything to worry about. But it is simple to add a heat sink. They make little clip-on soldering heat sinks, but really, all our shops SHOULD have clip wires around. Just clip a wire to the lead, and the clip acts as a heat sink. Certainly enough to keep the cap from evaporating.

[daz]

I do use alligator clips as sinks. But it still seems like i have to get it awful hot sometimes wen theres a lot o metal there.

[Chuck H]

Do you have an adjustable iron? If not, you should. IMHE the best way to overheat a component during the soldering process is by having the iron too cool!!! You just hold it there, and hold it there. Until EVERYTHING it hot enough to melt solder!?! A hotter iron will heat the contact and joint enough for flow and then you can get the iron off the joint. I usually solder at 710F for eyelets and pump it up to 800F for big lugs, jacks and contacts that have a bunch of leads together, like some power supply joints. My soldering skills are pretty mediocre. But if a joint seems to take too long to heat I will stop, wait for things to cool and then revisit the joint with a higher temp and a little fresh melt solder. The already melted solder helps to distribute the heat throughout the contacts and reduce the amount of time the iron needs to stay on the joint.

There are a few soldering tutorials around. I've never watched any of them carefully. But what I've seen seems like good advice. Free advice.

Hope this helps.

[km6xz]

Chuck is right, time is the enemy of the cap not point temperature. The sorter the time needed to solder the connection the less heat transferred to the internals of the cap. If you are having problems getting a good solder flow, increase temperature, use additional flux and a well tinned iron, start to heat the joint at a higher iron temperature, and add a little solder to the tip<>joint contact point. That will transfer heat more effectively to the joint, most of which does not directly contact the iron tip. Think of the solder as the same as "temperature lubricant" that wets more area, and voids of the joint so they all come up to temperature uniformly. Then add additional solder to flow into and around the joint.
Solder was never intended to add strength to the connection, just stabilize it and increase the contact area which lowers the resistance. The main strength and quality of the joint comes from mechanically creating a good tight connection, even before solder is added. Wires that are tacked on, are fine for pre-prototyping where you might be changing values during tests but not for functioning units. In fact, solder is not that great of a conductor so the mechanical connection is key to best solder joints. Crimped connections usually have lower resistance than soldered joints.

[Enzo]

We used to use 100/140 watt Weller soldering GUNs on point to point wiring. The reason you use a really low wattage iron on printed circuit boards is so as not to lift copper traces off the board. STaked in eyelets don't have that problem.

And I agree, one needs to solder with confidence. Putting timid heat on the joint leaving it there, melting it again, maybe I need to add just a little more solder. Maybe I need to melt it some more, in case part of it didn't flow, etc etc etc. is a great way to destroy a board or a part. A hot iron with enough power to heat the joint promptly, apply solder, then get out of there.