HELP! What's wrong with my amp (sound clip)

[Noe_1942]

It's a JetCity 50W head. When the amp is turned on and I strum my guitar.... It makes this crackling, terrible noise for 45 minutes, then all of the sudden it starts working fine. Once it starts working, it sounds great and I can play it for hours without any issue.

I've experimented with all of the following variables:
- I've tried replacing all the tubes
- Tried multiple guitars, multiple cabinets, multiple instrument/speaker cables
- Tried taking effects out of the effects loop, problem still persists
- Problem is consistent no matter what kind of power source I'm plugged in to (Direct in to wall, extension cord, surge protector, ETC)

Here's the clip....
https://soundcloud.com/northeast1942/ampproblem

I've read that a lot of JetCity amps have bad Bias pots.... is this noise consistent with a bad bias pot?

[M Fowler]

Sounds like bad tube but could be bad solder joint.

The Jet City amps are built very well with good parts except the bias pots.

I fixed a few 18w, 22w and last week an older 100w head. The bias pots were filmsy in each. The amp would start making noise and when you watch the tubes some would red plate.

Mark

[Noe_1942]

Forgive my ignorance.... but what does red plating look like?

[M Fowler]

Like a red Christmas light, instead of the tube idling nicely, the plates will turn red hot and will damage the tubes.

[Noe_1942]

Just played it for an hour and kept an eye on the tubes.... no hot plating from what I could see.... any other theories?

[Colossal]

Sounds like bad tube but could be bad solder joint.

The Jet City amps are built very well with good parts except the bias pots.

I fixed a few 18w, 22w and last week an older 100w head. The bias pots were filmsy in each. The amp would start making noise and when you watch the tubes some would red plate.

Mark

Mark, do they redplate on the same side every time, even if you swap known, good working tubes in those position(s)?

[Structo]

<<<<<<<<<<

Look at my avatar.

Red plating.

[HeeBGB]

The Plates get so hot they turn red. If you have ever seen a piece of metal heated to cherry red with a torch that is what you will see inside the the tube when it red plates. As Mark said look at his avatar!

[martin manning]

Googled...

[tubeswell]

If you've replaced the tubes and still have the problem, its time to take some voltages. For the output tubes this means the plate, screen, and grid voltages (starting with the grid idle voltage). As a ballpark indication with 6L6s, you should have somewhere around -35 to -50V, depending on how high your plates are.

[M Fowler]

Sounds like bad tube but could be bad solder joint.

The Jet City amps are built very well with good parts except the bias pots.

I fixed a few 18w, 22w and last week an older 100w head. The bias pots were filmsy in each. The amp would start making noise and when you watch the tubes some would red plate.

Mark

Mark, do they redplate on the same side every time, even if you swap known, good working tubes in those position(s)?

Dave, red plates in the same position each time. All tubes did not redplate. The trimmers on the 18w and 22w were falling apart, the 100w seemed solid but since only redplating on same side I replaced the trimmer, tested and then installed a matched quad as one tube pulled way down.

Mark