Fender Blues Junior IV guitar amp not working

[daigs]

Hi,
I need to repair the guitar amp in the title. I'm new on tubes, it's the first time for me. Usually I repair keyboards.
After a time from 10 to 25 seconds about from power-on, the amp start randomly with one of the following issue:
-some intermittent little pop/cracking sounds on the speaker followed by a “uuuuuhmmmmm” continue sound
-a continue big big distorsion sound, like a big noise (with the volume and master to zero, see the video)
Yesterday I powered on the amp and all went well without crack sounds and with no audio distorsion for about 30 minutes. The sound was very good. I thought that the amp was repaired just moving the valves but I was wrong. Today the distorsion was back again.
Some notes:
-When the little crack sound appears, touching the first 3 gray ribbon cables (starting from left) with a plastic pencil, each touch is a crack sound on the speaker. The same on the EL84 valves.
-The first EL84 is immediately very very hot compared with the near one. The preamp valves are not hot.
I checked the diodes and are OK. To check the capacitors I have to remove the board.
Where I can start to do some measures?

One of the example above:
https://www.youtube.com/watch?v=kTSnM-uoD3k

Thanks a lot.

[geolangston]

Re: Fender Blues Junior IV guitar amp not working

A very very hot EL84 suggests the negative bias (pin2) is intermittent or not present. One of the two power tubes out of bias would cause tremendous hum sound. Either the ribbon cable supplying the negative bias voltage has a bad solder connection to either the main board or to the poweramp board, or the tube socket on pin 2 of the hot EL84 has a sloppy connection to the tube. First try re-tensioning pin 2 on the hot EL84 socket. Of course be sure all power is disconnected. You can do this with a jewelers screwdriver or similar tool.

If that doesn't fix it then you will need to look at the solder connections on the ribbon cables which means pulling the main board and the poweramp board and inspecting the solder joints closely.