SL+ cooked R37 and tube board connector

I was playing recently and the volume started getting wavy in my SL+ module and then that dreaded burning electronics smell. I pulled it out to find a black resister and a pin on the tube connector board was pretty cooked. Is this something a tube could cause or would you all think I have a short somewhere. It's like losing a friend, I can't rock without my SL+ ;-)

I've taken a couple pictures, one of the burnt connector and one of the resistor. I definitely appreciate any advice you all have. I've been a Randall fan for many years now. I really dig the mts stuff and hope Randall keeps it alive. Even though the Randall dealer closest to me (that actually carried the stuff) decided to drop Randall, i'm still keeping it alive.

I had a Blackface do that. I don't remember the exact name of the part that fried, but it was something like the power supply for the module.

Required a total rebuild so I just had Voodoo mod it for me.

Drop me a line if you would like to have it modded and I will repair it while it's here. It probably took a few other parts out along the way.

It can be rebuilt fairly easily. The question is, why did it happen? Bad tube? Faulty B+ supply?

What amp do you use? Have you removed the module recently?

usually an issue with B+ voltage.

Jeff Hilligan said:

usually an issue with B+ voltage.

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That's what I'm thinking and the SL+ was the unlucky victim.

I've only got the 3 mods right now (blackface, 1086, and SL+) so they aren't moved around often. The module came with the RM100 head which I bought used about 5 years ago (from this forum actually) and the only thing I can see that's been touched after factory is C3 and it wasn't soldered very well. Thanks for the help!

The resistor in question is part of a low pass filter on the B+ rail for the second tube. The pin that fried on the tube board is the pin that supplies that filtered B+ the second tube. The failures indicate excessive current flow, which would suggest a short (or low impedance path) on the B+ supply to the filament supply (I would say ground but there is no ground on the tube board). This would most likely indict the second tube, although I have never seen a 12ax7 fail this way. Essentially the plate would have to contact the filament.

Does the amp function fine with your two remaining modules?

If you would like the module repaired for free, PM me. It?s ~$8.00 to ship it each way.

so i got the unit from mtsweapon. The failure was at the header connector for the tube board, that pin that's fried in the pic. This was a classic case of the Randall "arc over" failure. The ground on the board is too close to the B+ on that pin. Under the right conditions the high voltage arcs to ground, once it does this the solder mask is penetrated and the arc will continue. This crates a low impedance path to ground drawing significant current, thus frying the R which is in series with that B+ line.

I ended up removing that pin, replacing the R, and adding a wire from the R to the tube board. It's all good and happy now, no other components or the tubes were damaged.

Interesting, thanks for the heads up!

eedude said:

so i got the unit from mtsweapon. The failure was at the header connector for the tube board, that pin that's fried in the pic. This was a classic case of the Randall "arc over" failure. The ground on the board is too close to the B+ on that pin. Under the right conditions the high voltage arcs to ground, once it does this the solder mask is penetrated and the arc will continue. This crates a low impedance path to ground drawing significant current, thus frying the R which is in series with that B+ line.

I ended up removing that pin, replacing the R, and adding a wire from the R to the tube board. It's all good and happy now, no other components or the tubes were damaged.

Click to expand...

Big thanks for the help EEDUDE, while i've got a decent knowledge of electronics I'm definitely deficient in tube knowledge. Sounds like a slight overlook in the pcb design that day. Can't wait to use that SL+ again, maybe i'll actually get mods done one of these days.

Eedude to the rescue again.
Man, Joe you are such a killer contributor on this forum and a cool guy as well. Glad you're around, bro.

Man, Joe you are such a killer contributor on this forum and a cool guy as well. Glad you're around, bro.

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+1
Joe's the man !

I agree, it's excellent having this forum going because otherwise I know there are quite a few of us that probably wouldn't even bother with the MTS stuff. In my area it's pretty much impossible to find. Thanks again to JF and Joe for all the support!!!

Ya, really... Thanks you guys

Happy to hear it all worked out and you are back in action!

thanks for the kind words guys.... sorry i have not been around much lately, seems life has gotten totally busy for me... my free time has gone negative. But I'm here to help out as I can.
I have also moved away from mts.... mesa triple recto is doing amazing things for me...