fender pro junior pcb

[yunger]

well, i noticed that one of the el84's in my pro junior appeared to be scorched. the red lettering that says "jj el84" was faded to a grayish-white. the other el84 looks fine. i pulled the circuit board out that holds the sockets and the circuit board looks scorched around that particular tube (v4).
could something inside the tube have caused this? it was a few years old. i bought some new el84s a few months back but had not yet put them in. i'm just wondering if i should go ahead and change tubes now or if i can check/test anything in the socket to be sure i don't burn up my new tubes?
what do you guys think?
david

btw, this is my first post on this forum. i'm obsessing about building my own amp and really enjoy reading all the info here. i guess i'm like a lot of you...not happy unless i'm building or tinkering with stuff.

[drewl]

Wow, looks like some major heat there!
They run the output tubes very hot in these, and I usually install a bias pot and dial it down.
It looks like you just have some discoloration from the heat and the board is probably okay.
If you have a meter try measuring the sockets pins to ground and each other to make sure the board isn't compromised.

[yunger]

Wow, looks like some major heat there!
They run the output tubes very hot in these, and I usually install a bias pot and dial it down.
It looks like you just have some discoloration from the heat and the board is probably okay.
If you have a meter try measuring the sockets pins to ground and each other to make sure the board isn't compromised.

yeah, i checked for shorts in the pins. pins 4 & 5 are continuous but i think they are heaters. pin 2 on both power tube sockets read about 8.4 vdc....if i remember correctly. perhaps a bad el84 caused the overheating?

[drewl]

Yes 4 and 5 are the filaments (heaters) and should read a very low resistance.
that's a negative 8.4 on the grid (pin 2) right?
which should be higher as they run these things hot as hell.
You need to increse the negative bias voltage to cool them down!
Look at the schematic and you'll see a 22k (I think) resistor to ground in the bias supply you need to raise this, or put a pot in series with it to give more negative voltage.

[yunger]

yeah drewl it is -8.4. what should the negative voltage be? or better yet, how do i use this number to calculate ma or wattage?