Please help with identifying the problem

[dlee6036]

I have a Jet City amp (JCA2112RC). A few days ago, I cranked up the gain and all of sudden the gain stopped working. It still makes sound, but there’s no gain, thus the volume is low. I swapped the power tubes but it didn’t fix the problem. Then I swapped one preamp tube one at a time, but failed to identify a bad preamp tube. I have no idea what’s causing this and what to do. I would really appreciate your advice.

The problem:
https://youtu.be/cnBbZW0w440


Diagnosing preamp tubes:
https://youtu.be/ajHWQ7OezQ0

Schematic:
https://drive.google.com/file/d/1fvTQ8OlqveceaU8TqF2fWPbWqVpPCIYP/view?usp=drivesdk

[olddawg]

Read the sticky at the top about the effects loop jacks.. before anything put a patch cord between the effects in and out. Report back.

[dlee6036]

My amp doesn’t have an effect loop

[nickb]

I have one of these in the workshop right now if we need it to compare notes. Since these don't have an FX loop and it's not a tube, something has died so we will have to take some measurements.

It would help to know what effect each of the controls are having. If you turn the reverb up and rap the tank can you hear it?

We also need to know what relevant skills and test equipment you have.

[dlee6036]

I have one of these in the workshop right now if we need it to compare notes. Since these don't have an FX loop and it's not a tube, something has died so we will have to take some measurements.

It would help to know what effect each of the controls are having. If you turn the reverb up and rap the tank can you hear it?

We also need to know what relevant skills and test equipment you have.

I don’t have any professional equipment or experience. Do you think I should buy 4 more preamp tubes and swap them all or just take to a technician? I tried with 2 new preamps and tried all the combinations but still no luck. Yeah I can hear the tap on the reverb tank.

[nickb]

I don’t have any professional equipment or experience. Do you think I should buy 4 more preamp tubes and swap them all or just take to a technician? I tried with 2 new preamps and tried all the combinations but still no luck. Yeah I can hear the tap on the reverb tank.

Waste of money. Take it to a tech. It should be an easy fix.

PS: Where are you (update profile location) ?

[alexradium]

pots in these are very inexpensive,i had to replace the gain once.

[dlee6036]

Waste of money. Take it to a tech. It should be an easy fix.

PS: Where are you (update profile location) ?

I’m in Ottawa, Canada

[dlee6036]

pots in these are very inexpensive,i had to replace the gain once.

I have a soldering iron. Is replacing a pot easy? How do I test the pots?

[Bloomfield]

Generally replacement is easy, especially if it is panel-mounted rather than PCB-mounted. Not sure what pot value or mounting type you need, but nextgenguitars.ca in Nepean has a decent selection if panel-mount is what you need. They don't have a storefront but apparently you can place an order for pickup at the warehouse.

[nickb]

I have a soldering iron. Is replacing a pot easy? How do I test the pots?

I wouldn't replace anything unless you have a very good reason to suspect it else you risk introducing new problems.

To test it you would need a DVM with a resistance range. Earlier you said you didn't have any test equipment or knowledge which is why I recommended you take it to a tech. If you really want to have a go get yourself a decent DVM ( really cheap ones tend to have all sort of cunning ways to give incorrect readings), read up on basic amp skills to avoid hurting yourself.

[Randall]

Impossible to diagnose from this video.