Strange problem...

[surfsup]

I built a SE amp and it was working fine. I added an FX loop and it was working fine but there was still a bit of hum.

So the FX is a 12ax7 and I had both triodes grounded at the PT bolt.

So I took these leads and started moving them around. As I'm playing for a while, with hum minimal (one grounded at input jack, other at PT bolt), the amp suddenly starts generating static and noise, and fades in and out louder and louder like a slow tremolo. After about 5 seconds it was LOUD (like the amp was set to max).

I shut it down and wait a few mins. Start it up, no guitar plugged in. Starts doing it again. Move the grounds back to where they were, same issue. Move them anywhere and same issue. So I think its not the grounds but a component must have fried while I was playing? Any thoughts?

[T Wilcox]

Are you using switchcraft input jacks? Shorting type.
If so check there first. I just spent alot of time with same issue and found it at the jack..
Just a start but could be many other causes.

Todd

[selloutrr]

Voltage to tube?
Have you tried swapping or at least testing the tube?

Did anything slip off or touch other parts during your test run?

[surfsup]

The jack is just a regular metal three "prong" el-cheapo.

I swapped the rectifier, both 12ax7s, and the el34. I also put each one in a different SE amp, the tubes are all fine.

Nothing slipped off. Everything was soldered and worked fine for a few weeks. I just had the two grounds for each FX tube triode disconnected and clipped in with gators. I moved them many times.

Everytime I start the amp, after the 15sec warmup, the amp starts to hiss and crackle something fierce (with vol completely down) and get louder and louder, after about 3-5 seconds, it is screaming loud like amp is at max vol. When I power down, after hitting the killswitch, the amp still produces sound, making loud crackles and hisses for about 5 seconds then it finally dies out.

I feel it is related to a cap or something due to the prolonged noise...sounds like a demon is possessing it but that demon definitely can't play guitar!

I did not test voltages because it is so loud and I don't have a 220R 5w across the output jack to provide a little bit of load for the ot.

[selloutrr]

the trailing volume after you shut down is normal it's the caps bleeding off the power.

I'm not sure it's capacitor related. it sounds more like a missplaced ground wire. assuming your output transformer leads are in proper phase.

The first thing to do is to put everything back to stock pre the issue. make notes before the move and if that fixes the issue you can then reference the notes to understand why.

also check your other grounds. did you create a cold solder for another contact when you lifted the other grounds to experiment with aligator clips?

[surfsup]

fx tube cathode grounds, both ran to the PT bolt. Both on terminal ring. I took them off and moved them (separately). No soldering required. Weirdness happens. I move them back to same bolt. Weirdness persists. I move them all over the place, weirdness persists.

Swapped tubes.

The problem happened while sitting in front of the amp noodling around on the guitar. Nothing moved. Its as if something just "broke" while the amp was being used.

[selloutrr]

Get a chop stick one hand in your pocket get a buddy to play and poke the leads and parts to try and recreate or fix the amp.

If the noise is to loud you can use a dummy load or airbreak to tame the volume. Or ear plugs.

[surfsup]

Yea, I was just hoping the description would have triggered a memory from someone here because with this I have no idea where to start looking. Thanks.

[Zippy]

The first thing to do is to put everything back to stock pre the issue. make notes before the move and if that fixes the issue you can then reference the notes to understand why.

also check your other grounds. did you create a cold solder for another contact when you lifted the other grounds to experiment with aligator clips?

+1 for removing the FX loop to determine whether the problem is in the loop or the rest of the amp.

[selloutrr]

after the obvious things

Start at the output and work back. You know you have output. Work with what you know.

[surfsup]

Zippy, if you recall the FX loop worked before and I wasn't even using it while I was playing. the purpose of moving the grounds is I just wanted to see if I could reduce the small bit of hum I was getting. I think the hum is due to the FX loop tube being added - hence moving the two cathode ground leads.

I'll pull the FX loop out again and check. Thx for the suggestion.

[M Fowler]

I don't think it is a grounding issue but check the volume pot grounding.

To me this sounds like a phase issue and I have had that start suddenly after the amp has been played not on initial fire-up.

OT phase whether you try the speaker jack leads for primary leads. SE amp you said.

Mark

[ampdoc1]

It kinda' sounds like your loop is feeding back. That low hum may be just be low frequency feedback. Try shorting the input of the loop to ground. If this is the problem just replace the jack.

a'doc