(Solved)50w JMP clone sounding woofy and broken up all of a sudden

[sdn25]

The amp in question is my ceriatone plexi 50. While playing, all of a sudden the sound just went dark and woofy and started to break up (not in a good way). I have no clue as to what it could be, but the audio clip I attached may hopefully be of help. The first weird sound you will hear in the clip is what happens now when I flip the amp off of standby. Toward the end when the sound fades away, that's when I turned the standby back on. Any help will be appreciated! I have also attached a layout of the amp from ceriatone.

Sound clip: https://soundcloud.com/user-691083744/amp-issue-sound-demo?si=468b21fb5e6648fcabc6dd55611bc4b2&utm_source=clipboard&utm_medium=text&utm_campaign=social_sharing

 

[Pete Farrington]

Try reseating all valves a time or 3.

If that doesn’t help, do any control settings affect it, especially presence, treble?
Can you get a decent clean sound out of it?
What’s the output valve idle cathode or anode current?
Undertake a voltage survey, V DC to chassis at every pin of every valve, and at the chassis and board mounted can cap terminals.

 

[Matthews Guitars]

If it's not a tube, and the voltages are OK, my money would be on a leaky/failed coupling capacitor. This is the time to break out your oscilloscope and signal generator and trace the signal until you find the stage at which it all goes to hell.

 

[sdn25]

Try reseating all valves a time or 3.

If that doesn’t help, do any control settings affect it, especially presence, treble?
Can you get a decent clean sound out of it?
What’s the output valve idle cathode or anode current?
Undertake a voltage survey, V DC to chassis at every pin of every valve, and at the chassis and board mounted can cap terminals.

Reseating the tubes did not change the sound. BTW did you mean to swap the positions of the tubes or just reseat them? Because I just reseated the tubes in the same position.

The presence knob did change the brightness of the signal but the rest of the controls didn't seem to do anything.

Yes, I managed to get a decent clean sound by turning the volume down all the way.

I will report back with the voltages later when I have some bench time but you should also know that the sound progressively gets worse the longer the amp is kept on. On a cold start, the sound is not as bad as the audio clip but you can still tell something is wrong. When the amp is kept on for more than about 7 minutes the sound becomes worse and that whooshing sound is heard when I flip it off of standby.

 

[Pete Farrington]

Just reseating valves in the same socket is fine. Swapping sockets might introduce another variable.
Sorry, I should’ve suggested to try swapping in different, known good valves if the reseating thing didn’t help.

 

[sdn25]

Just reseating valves in the same socket is fine. Swapping sockets might introduce another variable.
Sorry, I should’ve suggested to try swapping in different, known good valves if the reseating thing didn’t help.

Right, I tried different output tubes but no change. Since the problem gets worse as the amp heats up, is it likely a pre amp tube or the leaky capacitor as Matthew suggested?

 

[paul-e-mann]

Right, I tried different output tubes but no change. Since the problem gets worse as the amp heats up, is it likely a pre amp tube or the leaky capacitor as Matthew suggested?

If you have a spare preamp tube try it in each socket to find the bad tube.

 

[sdn25]

If you have a spare preamp tube try it in each socket to find the bad tube.

okay so i changed all the pre amp tubes but the issue still persists.

 

[paul-e-mann]

okay so i changed all the pre amp tubes but the issue still persists.

Clean all the tube sockets, pots and inputs with Deoxit.

 

[sdn25]

the sockets are clean now. Problem still persists. Is that whooshing sound when the standby gets turned off a clue to determine what the issue could be?

 

[Matthews Guitars]

It's either a bad capacitor or possibly a noisy resistor. If it is a noisy resistor, it will get noisier as it heats up. This would be something that can be tested with a can of freeze spray. Chill down the noisy resistor and the noise will get quieter immediately.

Has the amp ever had its filter capacitors replaced? Being a newer Ceriatone, it shouldn't necessarily need that yet, but it's possible you got a cap with a short lifespan. It sometimes happens.

 

[sdn25]

It's either a bad capacitor or possibly a noisy resistor. If it is a noisy resistor, it will get noisier as it heats up. This would be something that can be tested with a can of freeze spray. Chill down the noisy resistor and the noise will get quieter immediately.

Has the amp ever had its filter capacitors replaced? Being a newer Ceriatone, it shouldn't necessarily need that yet, but it's possible you got a cap with a short lifespan. It sometimes happens.

The amp is about a year old. About 2-3 months after building it I replaced the stock mains and screens filter caps to F&T 50+50 caps but the can cap on the board is still stock. I wouldn't guess the F&Ts would fail but would a failing filter cap cause that noise you heard in the clip?

 

[paul-e-mann]

okay so i changed all the pre amp tubes but the issue still persists.

How about the power tubes?

Read this: https://robrobinette.com/Tube_Guitar_Amp_Troubleshooting.htm#Weird_Noises

 

[sdn25]

How about the power tubes?

Read this: https://robrobinette.com/Tube_Guitar_Amp_Troubleshooting.htm#Weird_Noises

Yep power tubes were the first thing swapped out. I will check out the article and try their troubleshooting methods and report back. Might just be a bad solder joint as I had recently swapped out my v2b cap from 330uF to 0.68uF

 

[sdn25]

Update, while probing the tubes for voltages, the 5k6 grid stopper just broke off and the tube started to redplate; I promptly turned the amp off. The break felt easy which would mean the joint was weak in the first place. Would a bad grid stopper connection cause the issues I had?

 

[sdn25]

Another update: I just decided to probe some of the coupling caps and are they supposed to have 180-190VDC on them? The 0.022 and 0.0022 coupler all have 180 VDC on them

 

[StingRay85]

They block dc along the signal path

 

[sdn25]

Just finished the voltage survey. PFA the image with voltages. Let me know if anything looks out of place.

 

[sdn25]

Update: Amp is back to normal now. Seems like the broken grid stopper was causing the problem. No more weird standby noise and no more woofy tone. Thanks for all the suggestions guys

 

[paul-e-mann]

Update: Amp is back to normal now. Seems like the broken grid stopper was causing the problem. No more weird standby noise and no more woofy tone. Thanks for all the suggestions guys

Awesome! I knew you'd figure it out! Enjoy that amp