Rectifier Arcing problem

[hughesp]

I'm a little stumped right now.

I have a princeton reverb clone that (mojo kit) that I built about a year ago. I've played it quite a bit, gigged it, recorded with it - it has worked great, until a few weeks ago, when it blew a fuse at a gig.

I like to check out why a fuse went rather than just popping in a new one, so I took all of the tubes out, and went in to check for any signs of trouble. Everything looked good, so I go through some boot up tests - first checking voltages with no tubes, then I add only the rectifier. As it warms up (with no other tubes) I get a loud hum, and then my rectifier starts arcing like crazy.

I figured the tube must have gone bad, so I go grab another, it arcs too. I grab a NOS one that I have in an amp that I know works well, and again, after it warms up I start getting arcing inside the tube. So I figure I've eliminated the tube as a suspect.

In one of the tests, something started smoking in the amp, but I haven't been able to track down what that was.

I disconnect the B+ connection from the rectifier to the first filter cap - no arcing at all, and voltages are exactly where they should be. I could be wrong, but this should at least point to it not being a bad tube socket (and I did clean it, and redid all of the connections just in case).

I reconnect the B+ connection to the first filter cap, and I get arcing again. So I'm thinking it has to be somewhere in the circuit that is causing the grief. Then I disconnect the red centre tap of the OT - suddenly the arcing goes away.

This all has me quite puzzled... I'm normally pretty good at troubleshooting (I did build the amp, after all!), but this is a new one!

Thanks!

[The Dude]

Do you have a schematic of the amp you could provide? Otherwise, we're shooting in the dark.

[hughesp]

Standard princeton reverb (as I discussed). The exact schematic for the kit is here:

And the layout is here:

[The Dude]

Well, according to the schematic, there are no flyback diodes and you pulled the tubes except rectifier. Arcing stops when disconnecting the OT center tap. That leaves only tube sockets and the OT itself. Maybe the OT is arcing or shorted. Have you checked for a short from primary to secondary? Also check the primary resistance center to each outside. To eliminate sockets, you can leave the center tap connected and disconnect the outsides. To eliminate an arcing primary to secondary, you can unhook the secondary to remove any ground path.

[hughesp]

That's what I was thinking... I'll try your advice and report back!

[Leo_Gnardo]

What The Dude says ^^^, also lets pull the output tubes and look for carbon tracks from arcing, both on the sockets and on the tube bases. Also closely examine output tube sockets inside the chassis for arc tracks. The areas between pins 2 and 3 are commonly where you would find them. You could disconnect OT primary leads one at a time to narrow the suspect tube socket on the very rare case of an internal arc path you can't see.

Consider putting on flyback quenching rectifiers once you resolve this problem, they limit the voltage at the plate connections to no more than B+. Surprise, you can have voltage spikes up to 3000V without them. Some folks don't like this solution but it sounds like you're playing your amp well into clipping and flybacks may be a good thing to have. And/or a spare amp.

[Enzo]

You said even with no tubes you got a loud hum? Hum from the speaker? Only way for speaker to make noise is if current flows through OT, and that suggests to me a short to ground on the primary side, or perhaps a short from p[rimary to secondary.

[hughesp]

okay, so here's where I'm at now...

With the primary side of the OT primary leads disconnected from the tube sockets, I still get arching, but I don't get the hum anymore.... so I think the problem is in the OT, but perhaps something is up with one of the tube sockets too?

I don't have the time to do it, but I have an old OT that would work okay in the amp that I can test out as a replacement. I might look into that tomorrow...

that said, all of this is strange to me, for an amp that worked flawlessly for a year.

If it is the OT, any idea what might have caused this problem in the first place? Thanks!

[Chuck H]

What is the voltage at the PT secondary. I'm thinking of a possible PT primary short (rare but no unheard of). That many rectifier tubes are arching without the OT connected is the tip I'm following here. The hum may be coming from the PT itself!?!

[hughesp]

What is the voltage at the PT secondary. I'm thinking of a possible PT primary short (rare but no unheard of). That many rectifier tubes are arching without the OT connected is the tip I'm following here. The hum may be coming from the PT itself!?!

Just to clarify - the rectifier tubes only arc with the OT connected. If I disconnect the OT, the arcing stops.

[Chuck H]

From post #8

With the primary side of the OT primary leads disconnected from the tube sockets, I still get arching

But ok.

Test the output transformer for shorts between the primary and secondary as suggested above.

Also, how are you disconnecting the OT primary? If you're simply lifting the lead from the rectifier socket that will also disconnect the HV filter circuits. If that's what you're doing then what happens if you leave the HV rail connected, but lift the OT secondary HV lead from the rail?

[hughesp]

From post #8



But ok.

Test the output transformer for shorts between the primary and secondary as suggested above.

Also, how are you disconnecting the OT primary? If you're simply lifting the lead from the rectifier socket that will also disconnect the HV filter circuits. If that's what you're doing then what happens if you leave the HV rail connected, but lift the OT secondary HV lead from the rail?

I've been disconnecting the OT primary rather than the lead from the rectifier socket. I initially thought it might be the filter cap can, so I wanted to make sure the HV filter circuits were still connected in my testing.

[Enzo]

The OT primary has three leads, the two ends and the center tap. What happens when you disconnect the center tap. Disconnecting that should not do anything to the cap circuit, but will reveal if the OT has a short.

[The Dude]

The OT primary has three leads, the two ends and the center tap. What happens when you disconnect the center tap. Disconnecting that should not do anything to the cap circuit, but will reveal if the OT has a short.

The way I read it, that's exactly what he did and the rectifier did not arc when the OT CT was disconnected.

[hughesp]

The way I read it, that's exactly what he did and the rectifier did not arc when the OT CT was disconnected.

Yep - that's what is going on.

I used my voltmeter to look for any shorts, but it hasn't revealed anything obviously wrong.

As it is, I just so happen to have an extra OT around (a hammond 1650e), so I might install and see if that cures my problem. It is a much bigger OT, so it will certainly change the sound of the amp, but the impedance is right, as are the wattage and current ratings, so it will at least give me insight as to whether the OT is the problem.