Engl Fireball 60 crackling on warm up

[johnathansrowe]

Hi, I recently blew a preamp tube in the V5 preamp spot. I found out it also took out the screen grid resistor after replacing the tube and getting no output from the preamp. After replacing the SG resistor, I was able to get sound back from the preamp, but now it crackles and pops for about 15 minutes and then subsides without an instrument connected. I checked the plate resistors on all the preamp tubes and they all checked out with good values. I reflowed solder on all the connections of the resistors and sockets of the preamp and power amp just in case. Jacks are solid and i cleaned all pots and sockets with de-oxit. I changed all the preamp tubes and power tubes one by one with new ones and all together incase there were more than one bad tube. I tested by pulling each preamp tube one by one and bringing the amp out of stand by, I get no change on V3 or V4. The sound is absent when i pulled the V5 or the V6 preamp tubes. The V5 drives the effects loop and V6 is PI. I don't know what else I can check. Any suggestions would be appreciated, Thanks

[Jazz P Bass]

Welcome to the Forum.

Thanks for the description of the tubes, as I do not have a schematic.

I would question how a preamp tube could 'take out' an output tube screen resistor.

That would seem to be an issue with the output tube itself.

[johnathansrowe]

Not exactly sure the blown tube caused the screen grid resistor to fail. Maybe the underlying problem could have caused both the preamp tube and resistor to fail. I also forgot to mention, the crackling and popping is not effected by any controls other than the volume control. The volume control doesn't change the crackle, it just raises or lowers the volume. I think I'm going to desolder the caps next and check those, although i don't think thats the problem.

[g1]

Can you post a schematic?
Where was the screen resistor you replaced?

[johnathansrowe]

http://milas.spb.ru/%7Ekmg/files/sch...ematic_new.gif

Here is the schematic ^^. I'm sorry I thought it was a screen resistor, but it was a plate resistor On the V3 A preamp tube socket there is a 100k resistors going from the plate to voltage in right next to a 4.7k resistor.

[johnathansrowe]

I'm new to working on amps, this being the first one. Connected to the same pin is a 47uf capacitor, coupling I think. If this were damaged also, could it be leaking dc voltage and causing this issue? Sorry for the confusion in the description. On the manual for the amp it called the preamp tubes v3-v6, but on this schematic they are labeled v1-v4

[johnathansrowe]

So I've re soldered every connection in the v3 area. The v3 plate resistor R29 was replaced and still no change. I think I can safely rule out a bad OT if I put signal straight into the return of the fx loop, bypassing the preamp and the crackle stops. The crackle is still apparent with v1 or v2 grounded at pin 2 and 7, so it has to be further along in the circuit. I removed C17 because it had a weird micro phonic sound when I tapped on it , unlike any others, but I tested it on my meter and it showed 106.5nf. Being a 10% capacitor is it still good or can this be the problem. I ordered some capacitors and resistors, but I'd like to know if I'm waiting my time. Thanks