Fender Super 60

[eschertron]

I can start there. While I don't want to be accused of 'shotgunning', I think I can replace a couple of plate resistors with good conscience. I also saw a wirewound resistor in the power supply 10W, 30k Ohms, that has the epoxy cracked. I pushed on it with my pliers to see if it would change resistance (power off, Ohmmeter on!) but it didn't do anything that I thought was suspicious. If I'm going to be lifting leads, I'll check that one out again more thoroughly.

I'm pretty sure the existing resistors are film types, the amp is from early 90s +/-

I'm going to review Merlin's preamp book; the chapter on noise, because IIRC plate resistors and grid resistors contribute most to the gain stage's noise. And I need to get straight in my head the different causes of 'hiss' and the 'sputter' that is causing me grief right now.

[DRH1958]

In regards to that power resistor, see this thread, esp. post #9. Might not be a bad idea to replace it anyway. http://www.tdpri.com/forum/amp-centr...-red-knob.html

[g1]

That's a ballast resistor that puts a load on the supplies to drop the voltages. I liked the idea of replacing it with a chassis mount like seen here: http://music-electronics-forum.com/t11762/
There was a discussion about removing it and increasing the series voltage dropper, but the load from this resistor affects all the voltages upstream, right back to the main B+, so I think it is best to keep it in circuit.

[eschertron]

Good suggestions! As I'm sure I don't have any 30k power resistors in my bins, I'm free to choose the best option for that

[eschertron]

Today's story.

First, I found this thread http://music-electronics-forum.com/t36367/ about PSU caps since I was wondering if failing caps could make that sputtering, frying noise. And especially since each power-up event seems to make the sputtering/popping less and less. I have a few more diagnostic tests to run based on that hypothesis.

Also, following advice from above, I replaced the four plate resistors on the 12AX7 tubes. Did it from the top side. Neat trick. I still hear the sputtering, but as I said, it gets less annoying each time I run the amp. Ordered the 30k/10W bolt-on resistor from mouser. Was under $4.

[g1]

I take it that moving the tubes around in their sockets doesn't affect the crackling?

[The Dude]

Have you tried substituting known good preamp tubes? What you describe sounds like an arcing tube to me.

[eschertron]

Swapped V1 first thing, suspecting just that! I've had the preamp tubes in and out. Can't say I've rolled all the preamp tubes, but I could, i suppose. I did notice that if I tap V1 hard enough, I can get a dull 'pop' out of it. Not sure if that's mechanical, or if it's like you said - dirty socket contacts. Spraying a bit of contact cleaner on the tube pins and some gentle insertion motion can clean the socket? Definitely would have the amp unplugged for that, and left that way for some minutes after.

[The Dude]

Have a good look at the socket pins. I ,on occasion, see bent pins from someone misaligning a tube when they insert it. Or, the tube pins were bent when they tried to insert the tube into the socket.

[Jazz P Bass]

And try not to overlook a socket contact that is broken from the PCB pin part.

[eschertron]

And try not to overlook a socket contact that is broken from the PCB pin part.

Even though the PCB is fastened to standoffs right by the sockets, the board flexes more than I'd like when inserting/removing the tubes. So that's a possibility. Will look for sure!

[Jazz P Bass]

It wouldn't hurt any to look at the coupling caps.

As they should not pass Vdc from the previous plate, any crackling should show up on the non plate side with a meter or a scope.

[eschertron]

I will check the coupling caps; and especially after stage 3 (where most of the noise is coming from) which has the TMB tone stack in place of just one cap.

My oldest son is getting married in just over a week, I may not be on the forum much between now and then but I'll report back when I can.

[gearhead63]

And, as has already been said, look VERY closely at solder joints! I use my webcam at times and zoom in real close, you'd be surprised how easily you can spot a hairline crack in a solder joint on a monitor screen. If it looks at all like there is a line where the solder meets the component I would reflow it. A very good percentage of the amps I've had on my bench that made "crackling" noises had cracked solder joints on input jacks, filter caps, pots, larger power resistors etc. Definitely sounds like some kind of arching from a bad connection. You may even want to use an audio probe, I'd start at the input jack connections.

[eschertron]

Regarding suggestions for inspecting/reflowing solder points: At first blush everything looks OK. I have only chopsticked a portion of the board, will continue inspecting and probing. I noticed the screen resistors have been replaced at some point (they are on the topside of the board now) and one is showing evidence of crispness.

I replaced the screen resistors - as above, one was visibly stressed. Turns out the stressed one reads 730R out of circuit! Glad I replaced them

I also replaced the 30k 10W PSU resistor, a cap in the reverb return that looked iffy, and tightened up some solder joints. Much better now. Full disclosure soon.