Fried resistor in a SE-5a

Just got an email from a fellow I sold an ampmaker SE-5a, it seems the screen resistor has overheated and burnt out. This surprised me very much, as it's usually a very reliable amp. The first one I build nearly two years ago still has the same tubes and it's been played to death by a pro player. As the amp is over 5 hours away I can't check it at the mo, I can only assume it's been caused by something else failing, the tube(EL84) or a cap?

It's the 3rd one along in the filter chain..2.2K

This is the amp when it was new, still waiting on pics of the damage


http://i757.photobucket.com/albums/xx217/telenut62/SE-5/SE-5aChassis002.jpg

http://i757.photobucket.com/albums/xx217/telenut62/SE-5/SE-5aschematic.jpg

Tube failure can take out the Rg2. Maybe they ran the EL84 into the ground. I've seen other guys ruining their amps this way before.

Rg2?


Ah tech term for screen resistor?


Bit early here :lol:

Looking back at my notes the screen voltage was 250V, which is 10V higher than recommended by Barry

Tube failure can take out the Rg2. Maybe they ran the EL84 into the ground. I've seen other guys ruining their amps this way before.

Yeah, that'll do it. That's usually your screen resistor failure mode right there.

Rg2?


Ah tech term for screen resistor?


Bit early here :lol:

Looking back at my notes the screen voltage was 250V, which is 10V higher than recommended by Barry

Yes sorry. In a tetrode or pentode Rg is the grid resistor, Rg2 is the screen grid resistor (g2 being the screen'n'all, g3 is the suppressor grid in a pentode and so on)

Exact same thing happened to a friend of mine.

He gave me the amp not knowing what the heck happened but I'm pretty sure he ran it without a speaker.

It smelled ugly and I replaced the resistor but I want to rebuild it because he got the components bolted in the wrong place.

I am trying to learn here. Is the problem resistor the resistor that is number R2 on the schematic above? Isn't that a plate resistor?
OR...is it the 'third in the filter line. as the OP calls it...which is R10.
.2.2K/1w??? HTat is a droppping resistor, correct?
This 'screen resistor' thing has me confused. What am I missing?

@ Sir Wally
I'm a visual person :)

Okay, Andy thanks for making me take a better look at things....and I see that 2.2k resistor dropping off of the power source to the power tube now. Duh on me!!!!
I can only hope that the rest of my day is spent in a more 'wakeful' state, right?

I can only hope that the rest of my day is spent in a more 'wakeful' state, right?
No way! It's "Sa-tur-day, Sa-tur-day, Sa-tur-day, Sa-tur-day, Sa-tur-day, Sa-tur-day..."
-E. John...

S-A-T-U-R
D-A-Y
Night

Name that band...

S-A-T-U-R
D-A-Y
Night

Name that band...

Bay city rollers :)

I know dat song....dang :roll:

Anyway the offending item..

http://i757.photobucket.com/albums/xx217/telenut62/SE-5RR.jpg

Have sent a new tube and resistor to the owner but not heard back yet.
Interestingly someone has added a capacitor (near the blue wire) on to the second cathode-pin 8.....sure wasn't me.

S-A-T-U-R
D-A-Y
Night

Name that band...
Herman Brood and his Wild Romance?

S-A-T-U-R
D-A-Y
Night

Name that band...

One of the lesser Aussie bands :mrgreen:

H_Izu7tTwqk

Chisel it was...ta :wink: