ShortScale.org

While I'm trying to sort out which pages of schematics fit together, here's a description of the problem.

Playing at low volume, everything's fine. Whack the volume up, and when you hit a chord hard, the sound disappears into a gated badly-biassed-sounding crackle, then slowly comes back. Same on both channels. Has anyone come across this?

Badly seated valve?

benecol wrote:Badly seated valve?

They're all sitting comfortably, though I don't rule out finding a dry joint when I rip it apart.

I'm no amp tech, but does the problem increase as you wind the amp up, or is there a point on the volume dial where shit goes down?

The fact it's related to output would immediately make look toward the power tubes, but that's a no brainer I guess..

Sounds like a similar problem I had on my Carlsbro 50 Top, hit a particular note and this kind of strange feedbacky noise would come up. I got it sorted and it was a power tube problem, a replace and service and now it's as awesome as ever.

i would also check the tubes. wait.....is this the jtm60?

Marshall JCM2000 DSL50 head problem

Doog wrote:Marshall JCM2000 DSL50 head problem

face broekn

I got a few minutes to look at it this weekend. In fault mode the signal grid to the phase inverter/splitter is biassed to 3V instead of around 36V - I'm suspecting breakdown of the input coupling cap.

No, it wasn't. Looks like the PI valve had two badly mismatched halves, but it looks instead as though the problem is the output transformer.
These two views show you how the end of one winding of the OPT is badly routed, pressed down against the other winding. The result appears to be breakdown of the insulation, which only shows up once you get a large voltage swing between the two anodes.

I'm tempted to see whether I can get a shot of this glowing in the dark.

I may be able to do a running repair by moving the end of the winding and putting an insulating sheet under it

[edit]Hmm, my poking about may have moved it sufficiently that I won't be able to get that picture of arcing. It's a bit late to crank it up to full volume at 11:25 pm

Well, I has video of the blue flames in .wmv but nowhere to host it. In fact, I did a little clip putting it in context too. Shout if you want it.

youtube yo.

fuck that is shit.

I love looking at gut shots of amps, but have no idea what any of it is or means. It's weird, I'm fascinated despite not having any of the knowledge that is usually needed to whet an appetite.

Mages wrote:youtube yo.

Yes, it took just an hour to create a YouTube accont.

So, folks, here's two seconds of arcing in the dark. Don't blink, you'll miss it. I had a longer video with commentary but brought the wrong one into work.
[youtube][/youtube]

I put it all back together and got like 5W output at max volume. Or full output if I plugged the guitar direct into the FX return and pressed the FX Loop Level button in.

Turns out that the original Marshall ECC83 phase inverter (a.k.a. phase splitter) valve is so low gain in one half that it's only any use in the PI position. I stuck an old Mullard ECC83 in and it's fine.

NickS wrote:I put it all back together and got like 5W output at max volume. Or full output if I plugged the guitar direct into the FX return and pressed the FX Loop Level button in.

Turns out that the original Marshall ECC83 phase inverter (a.k.a. phase splitter) valve is so low gain in one half that it's only any use in the PI position. I stuck an old Mullard ECC83 in and it's fine.

that's interesting that you had some crazy shit going on because of that.

for those that don't understand: a ECC83/12AX7 is a 'dual triode' which means that it is two triodes (plate, cathode, and grid) in one tube. kinda like two tubes in one package. so it is perfectly reasonable that one half will fail.