Arcing in Rectifier then blown fuse?

[alexg]

Hey there, i'm about to go through the debugging process from Geofex...but thought i'd lob this out there to see if anyone has seen this before and might be able to save me some headaches.

I'm working on a Matchless Lightning build, using the west labs trannies...no mods. When I power up, my pilot lamp goes on, the tubes glow orange...everything seems ok....then I hear (and see) what appears to be arcing in the rectifier after about 10-20 seconds. It zapped a little bit, the lamp starts to dim with the zapping sound then blew a fuse.

I replaced the fuse (same type), and powered it up again in standby....seemed fine again, waited for a little bit, then it did the same thing (I turned it off as soon as I saw the blue "arc" in the rectifier.

Filter caps are all installed the right way and measure ok...i'm assuming I have a short somewhere (or a bad rectifier?), but any clues on why it wouldn't blow the fuse right away?

thanks in advance,
- alex

[Tom Phillips]

Alex,
Sounds like a bad rectifier. They don't always fail as a dead short. It's just intermittent. I think you should put in a new rectifier and try the amp again.
Regards,
Tom

[Tom Phillips]

Alex,
After I thought about this over night I realized that I should have pointed out that you did some really good troubleshooting already. You used your power of observation. By seeing the arc in the rectifier you isolated the problem to that tube. Usually an arc like that means a fault in the tube rather than being caused by something down circuit pulling too much current. Especially since it happened in standby mode.
Tom

[alexg]

Hey Tom,

Thanks for the quick reply! I'm new to this so I'm quick to assume it must be something *I* did, so thanks for your insight.

I'll pick up a new rectifier and see how it goes.

thanks again!
- alex

[SlidePicker]

There shouldn't be anything in that rectifier to make a blue arc. Sounds like it's gotten gassy and is arcing over in reverse current.

[stokes]

Some rectifiers have a "slow" startup.With this type of rectifier it takes a few seconds to warm up before dc starts to flow that is why it doesnt blow immediately.

[alexg]

Well I bought a new rectifier tube....that didn't help, then I got busy at work so I shelved this for awhile. Came back to this recently with fresh eyes and found a grounding error (it was really a boneheaded mistake!!).

Anyways, I fixed that....and it now powers on without problems. I flip the standby switch, strum the guitar and....nothing but hiss! Is it sad that at this point i'm excited about getting hiss?

Here's a few more details, i'm hoping someone here might be able to point me in the right direction. I have the guitar plugged in when I power on...and I get nothing...when I unplug the guitar from the input...then I hear hiss. If I half insert the 1/4 inch plug...I get "noise" in the speaker like I would expect. The hiss is controllable via the volume knob, the tone controls affect it as does the master volume (volume knobs raise and lower volume, tone controls affect the frequencies). If I turn the volume knob all the way up it starts to squeal. I would think this means that there's something wrong in the input stage, but I don't have a scope to check where signal is even getting to.

I'm using known good tubes (tested in another amp). My next step was going to be going through from the input through the first few stages looking for wiring errors or cold solder joints....anyone have any ideas that might help here?

If it makes a difference, i'm using the ceriatone layout with west lab trannies.

Any advice appreciated, thanks!
- alex

[Enzo]

I don't want to sound unkind, but if you made a bone head mistake in the power supply, what prevents you from having done it in the input stage as well?

Go back over the first stage. And make sure all the tube socket wires are on the right pins.

[alexg]

Hey Enzo,

Unkind? For helping people out...for free?! I actually meant to post a follow up to this. I saw one of your posts helping someone else troubleshoot something....so I did a search through all of your posts and spent some time reading advice you had given others (and happened across a few of RG's posts as well). Your advice to others was incredibly useful in helping me start looking at the right types of things (I was ready to order a new OT!). At around 2am on Sunday, I discovered a couple tiny strands from the shielded wire touching the signal wire (yup another boneheaded mistake!). I fixed that and the amp worked! It sounds absolutely awesome!

I was riding quite high the next day and meant to post a HUGE thank you here...but later that day my wife went into labor and we had our first baby sunday night! In fact i'm pecking this out on an iPhone from my hospital cot. It was incredible to me that the day I didn't think could get any better...did!

Anyways, sorry for being long winded...but this proud papa couldn't be happier! Thanks again for the help...I wouldnt even have dreamed of attempting a project like this if it wasnt for the gracious help from people like you, RG and others in these forums!

Time to get some sleep!
- alex

[Jag]

Congrats Alex!!!