5150 Screen Grid RESISTOR/Tube question...

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I have a 5150 head its my 2nd one. Ive always done my own biasing
and repair/electrical work. Im not a n00b at it. By no means the god of
electronics but my pops had some schooling in it, So I ended up learning alot.

Anyway way back, I was messing with this thing
when I had first biased this particular 5150,
I didnt have a bias probe at the time, And was trying to
get a lazy reading to the tube pin directly with the multimeter. Long story
short I accidently touched 2 tube pins at once and shorted it and meet
the REAL flame throwingmetal monster I almost poured water on it
but then I remembered I didnt wanna do that either it was out in a matter of seconds.
Like one of those mini sit on the ground fireworks.
It was just a slip of the hand and I should have known better anyway.


After that, Ordered a bias probe from eurotubes for 25$. Then took the amp
out of the box again to see what was dead on the board; Fixed 3 black resistors
and another resistor, I dont remember the values and Im not
on my computer at home at the moment anyway, Scraped the burnt
carbon stuff from the tube pins/socket. Just slightly started to melt it.

I actually plugged it all up and it worked to my surprise. No problems really
until like 2 years later, Yesterday I redid the bias mod. Just wanted to
clean it up a bit and make it look more factory while I was in there replacing
one of my LED for the power light. Used a 10k pot and made it adjustable from
outside when I biased it the first time btw.

Everything was good, I turned the knob all the way each way to see the blue glow like
I usually did, Sort of a bad habbit maybe but Before I knew it I remembered the 2nd
power tube from the left looking at the front of the amp
wasnt glowing slight blue like the other 3. Just the filiment
in the middle was orange. I turned it off, Checked it, Its the
burnt socket thats been performing fine for all this time...
I switched tubes around to make sure it wasnt the
tubes (new ruby 6l6mstr's)...And sure enough, Same spot
no slight blue glow when standby was flipped....

The amp sounded great, Seemed like just a tad better than before even
with this problem so I know my wiring and all is good....

QUESTION...
Could a Screen resistor have went bad or something like that?
What number is the screen grid resistors in the 5150 schematic
I reallllllly have a hard time reading a schematic, But I swear Im
pretty decent at wiring up electronics and doing clean work
but I can hardly read maps and or tech. diagrams
Dont know why but I just cant stand either of them.

And also I know its a good Idea to go ahead and upgrade
the screen G resistors to a different value, But its vague and nobody
really discusses it. I think Ive read they should be 470 ohm, 5 watt.

Drilling the rivets and all that stuff is cake walk. I need to figure this
one out along with the grid resistor location and value/replacement values,
ANy help, suggestions to try with multimeter/bias probe is appreciated,
I know its something that could easily be repaired more than likely.
.....Thanks MEF

[Gtr_tech]

Replace the socket that got charred. Check for cracked solder on *all* tube sockets while its apart.

[UNHALLOWED]

Replace the socket that got charred. Check for cracked solder on *all* tube sockets while its apart.

Alright, I will pick up a new socket. I checked the solder joints all around them when the incident happened a couple years ago and again the
other night before I noticed the tube not glowing I know theres no
cracked solder joints but I will of course be checking again soon being
I gotta fix it again. Ive saw a cracked joint before though in an ultra plus
and some cheap combo bass amp havent saw one on a 5150 yet.

Im picking up some 470ohm 5watt resistors (screen grid). I think there are 4.
And I think the 470ohm is what they should have been anyway it seems.

Might as well do that while I am replacing the socket, Although why would the socket work for 2 years then stop, Seems like it happened after turning the bias pot a couple times to the hot side.... ....."seems".

Thanks for the reply, Anybody else want to chime in

[Gtr_tech]

I wouldn't go to a 5w for screens on a 6L6. 1w is usually plenty since they don't draw as much screen current as an EL34 or other straight power pentode. Plus, the smaller wattage resistor acts as a fuse in the case of a tube failure, opening up the screen supply.

[Enzo]

The original screen resistors are 100 ohm, R207,209,203,205. They are the ones connected to the screen grids on the tubes. The originals are 5 watt types. I have never seen the need to vary from the factory design. The 5150 is not a tube eater.

The power tube board is facing away from you in the amp, the resistors are on the under side. If I have to change a screen resistor I usually unsolder it, poke its leads out through the holes till it falls free. Then fish out the loose resistor from the chassis. Now I solder a new resistor on the solder side - facing me - a whole lot easier than either trying to fish a new part into the holes from below or drilling out the 8 rivets to free the board. The circuit surely doesn't care which side of hte board the resistor sits on.

Yes, certainly a burnt open screen resistor will turn off a tube. Pull the power tubes. Power off. Remember ther is a 100 ohm resistor to the scrfeen supply from each tube pin 4. SO measure resistance between pin 4 of each power tube socket and one of the others. SHould be about 200 ohms from each socket. If one socket has no 200 ohm continuity to the others, then it has an open resistor.


You could also power up the amp and measure for B+ on pin 4 of each socket. Same deal, missing voltage means missing resistor.


And please don;t unsolder 32 pins to get the board off from around a bad socket.