Repeated arcing on pins 2 and 3 - JTM45 content

[johnnyreece]

Got a problem that's been plaguing me since (I thought) I finished this project...It's a Weber 6M45 kit (their take on a JTM 45). I beefed up the bias filtering and added separate bias controls for each tube. Also went with cap cans vs. their standard cap board (which is quite a pain). The screen resistors have been upgraded to 1k 5W. Everything seemed to be going okay, until it wasn't. Here's a rundown of what's happened so far:

1st failure: Everything checked out fine. Was playing when all of the sudden it crapped out. Realized a speaker wire jiggled loose (was using spade terminal connectors rather than soldering. Learned my lesson, I thought). Socket shorted between pins 2 and 3 (inside chassis) on V4. Popped B+ fuse. Was running 6L6 tubes.

2nd failure: Cleaned the socked with alcohol well and replaced the tube at blown socket. Rebiased. Plugged in to a 2x12 cab wired in series. One of the speakers sounded like it may have been blown. Mistakenly kept cranking it until it zapped again. Once more shorted between pins 2&3, but this time was at the tube (still V4). Again popped B+ fuse. Used EL34 tubes.

3rd failure: Replaced suspect tube socket. Replaced tube at the same socket. Rebiased. Plugged in to the same 2x12 cab, but wired in parallel this time. Replaced suspect speaker, just in case. Played for 10 minutes at lower volumes. Immediately after turning it up (both channels jumpered and the volume was at about 5 on each) and playing, popped the B+ fuse. Speaker connection definitely not the problem, but the short happened in a different spot this time, but still V4. Again EL34 tubes.

4th failure: Switched taps on my secondaries, bringing plate voltage from around 480 to 430. Replaced impedance switch and tube socket. New set of tubes. Rebiased. Plugged into 2x12 cab, again parallel with 2 good speakers (8 ohms each, running 4 ohm tap on the amp). Played for 20 minutes or so, gradually increasing volume until it was screaming. Got almost through "Wang Dang Sweet Poontang" when it hit again. Pins 2&3 on V4, again...again EL34's.

Here's a couple pics. This one's a shot after just attempting to clean V4 with alcohol:

[IMG768]http://i218.photobucket.com/albums/cc16 ... 1_2594.jpg[/img]

Shot of my cap cans:
[IMG768]http://i218.photobucket.com/albums/cc16 ... 1_2591.jpg[/img]

General overview:
[IMG768]http://i218.photobucket.com/albums/cc16 ... 1_2588.jpg[/img]

Please let me know if I can clarify anything, with pictures or words. So, it seems I'm having an issue with flyback from my OT. I just can't figure out what's causing it. The amp plays fine, until it doesn't...I bought this as a project with gremlins from someone else. I'm starting to think that maybe it's the OT with an intermittent open/short. Does that sound reasonable? I'm thinking of scrapping this entire project... Thanks for any ideas y'all have, or even sympathy...I accept sympathy in lieu of advice.

edit: Thought I had a miswired resistor after looking at the pics...then realized why you don't judge your handiwork A: Upside down and B: After 4 glasses of wine.

[Reeltarded]

The out selector is definately wired correctly?

In my long-time experience with the Marshall Major... hmm.. Once you have an internal arc in the OT, it's on it's way to see Elvis, and James Dean. Flyback in the 2M/kW range easy! lol China Syndrome!

Let's hope it's just an overlooked thing in the output wiring?

I'll look too!

Umm.. I don't like naked push-back wiring in areas like that. I would shrink wrap every wire with voltage back there.

[johnnyreece]

I've checked the switch (and checked again when I wired a new one in) but you never know. I'll give it another look.

By the way, although it's cloth covered wire, there is some kind of rubber insulation underneath it. It's the Weber wire, if that makes a difference.

It is killing me that it only seems to happen when it's playing hard. If it popped a fuse as soon as I flipped the switch, I'd just order an OT, having replaced everything I have. It's working JUST enough to make me think it's a small issue I'm overlooking.

[Alexo]

Maybe try this? Seems like a band-aid fix, but may be telling if it helps...

[johnnyreece]

Yeah, I've heard of doing that. Don't the diodes fry on the first incident, though? I agree that it's cheaper than tube sockets/OTs, but I really want to get to the root of the problem. I may implement that once I get the final solution to help prevent future damage, though. Definitely can't hurt!

Of course, if it doesn't destroy the diode(s), I'll put it in lickety-split! I'm dying to get it fixed so I can run it with KT-66 tubes (will require changing the taps from the OT to the switch, but I'm cool with that).

[Structo]

Hi John, I'm not familiar with that impedance switch.

I think most of us use the three position one like this.

Hoffman sells it as well as others but it is a heavy duty switch with wide blade contacts.

[img:600:336]http://www.hoffmanamps.com/MyStore/cata ... G_2266.jpg[/img]

$9.50

http://www.hoffmanamps.com/MyStore/perl ... =506153869

I think at this point I would get another OT.

If that were my amp, I would probably get about three or four colors of 20 ga Teflon wire and rewire at the very least, the power section.

I like the 63/37 eutectic solder as it sets all at once and has a less chance of a cold joint.

Try to use only enough solder to wet the joint and leave it nice and shiny.

[johnnyreece]

Hey, Tom!

I must've read your mind...I have that same switch now (that's an old pic where the Weber switch was still installed).

If I rewire it, I may go ahead and build a different amp into this, anyhow. It really just has too much headroom for me (and I don't like pedals all that much). Thinking of a cathode-biased version of something...we'll see!

I've been using good ol' 60/40 solder. I've not had any trouble with cold solder joints, but I may take your advice if I go on a spending-spree for parts soon.