Mission 5E3 problem, burnt 270Ohm 5 Watt

[Ravian]

Hi there,

Last night I was playing with my band my amp suddenly stopped making sound! When I checked the amp I saw smoke coming off of it so I immediately turned it off. Today I turned it on again and saw that my 270Ohm 5 watt was glowing red hot! I turned it off again and checked all my wiring. I saw that the wire from the pin 2 from the V3 to the pin 2 from the V4 wasn't connected properly. Furthermore, I saw I used the thinner yellow wire instead of the thicker green wire for that connection. Could it be that the wire is too thin to be used on that position? Or is something else wrong? Some help would be greatly appreciated! I marked the two problems points on the Mission 5E3 diagram, see the picture:

[MWJB]

I doubt that the disconnection of the heaters at 6V6 pin 2 caused your 270ohm resistor to glow red hot. So long as the wire connecting the heaters is #20 awg, or thereabouts, I doubt that it is an issue...shorts/loose/intermittent connection is a worry though, heaters carry large currents so make sure connections are secure as you can quickly fry a power transformer if things go wrong.

Replace the power tubes, 270ohm resistor & its bypass cap, keep the new bypass cap (22uf/110v) physically away from the 270ohm resistor as even in normal operation this resistor gets hot.

It would be more helpful to see pics of the actual amp, rather than layouts/schems, if there is a deeper issue we need to see what you have actually done.

[Ravian]

The wire that was used for the 6V6 connection was yellow 22g. I will change the power tubes, 270ohm resistor and new bypass cap and make sure everything is properly connected. Here are some pictures:

[MWJB]

I'd shorten the leads on that 100ohm resistor from pins 2 to 8 on V3, may be optical effect but I don't like the look of it flapping about near the socket bolt like that.

[Bruce / Mission Amps]

The short 22g wire you used could handle hundreds of miliamps of current. That's not it.
To make a 5 watt 270 ohm resistor glow red.... and not blow up, I think it would take about 175ma-200ma of current flowing through it.
The bypass cap can't be shorted because then there would be nearly no voltage or current through the 270 ohm resistor.
Off the top of my head, the only thing that could do that would be a dead short from B+ to the common cathodes in the power tubes and the entire B+ is across the resistor.
The PT is rated with it's normal vac @120ma (a little over kill for a 5E3 but better safe the sorry). It will deliver about 150ma to 160ma for quite a while before it gets pretty hot. I know 'cause I've tested them in worse case scenarios at close to +150ma with fixed power resistors across the red secondary leads to the center tap for many many minutes to see what would happen.
But 150ma across the 270 resistor is around 6 watts. I doubt the 5 watt Mtl Ox 270 ohm resistor would glow red or burn up very fast at 6-7 watts.
Keep us posted on what you find....

[Enzo]

I can't tell from your photos, but any chance the center pin is broken off one of your power tubes and it was then installed facing the wrong way? That tube heater between pins 2 and 7 acts like a dead short if the tube is in the wrong pins. For example off by one pin leaves the heater between pin 8 and pin 3 - voila, dead short from B+ to cathode.

[Ravian]

Thanks for your responses, I ordered a new 270Ohm 5 watt resistor, a bypass cap and power tubes. Is it necessary to replace all three parts before turning it on again?

@Bruce: "be a dead short from B+ to the common cathodes in the power tubes and the entire B+ is across the resistor"

How could I check if there is a dead short? Could the not properly connected wire from the pin 2 from the V3 to the pin 2 from the V4 be the cause of this?

@Enzo: I don't fully understand what you mean by center pin. You mean the pin on the power tube itself to connect it properly? If so, those pins didn't break, there's really only one way to connect it to the socket! That's ok right?

The amp was turned on quite loud en long before the resistor started glowing and I saw smoke. I think it took about 2 hours before the amp completely died!

[Jazz P Bass]

Me thinks your output tube failed & ate the resistor.
Replace all three items & monitor the voltage across the cathode resistor.
You want to see that it remains stable at idle.

[Ravian]

Me thinks your output tube failed & ate the resistor.
Replace all three items & monitor the voltage across the cathode resistor.
You want to see that it remains stable at idle.

Thanks, will change all the items and check the voltages. What are normal voltages here and are there any voltages I need to check as well?

[Enzo]

Yes, I meant the center post on the base of the tube itself. It is indeed there so the tube can only go in the socket one way, and if yours are intact then my scenario is not the case here. In the service industry, I see broken off tube bases, and they cause trouble as described.

[Ravian]

I just checked my turret board and I think I wired some resistors closest to the filter caps wrong. Can someone confirm this?:

[MWJB]

They look right.

[Ravian]

They look right.

Ok, I thought maybe the 1K5 resistor needed to be the other way around!

[Ravian]

I just replaced the bypass cap, resistor and tubes. The amp seems to be working now! I'm going to measure some voltages tomorrow, will post them here! I ordered JJ 6V6 tubes, man those things are big compared to the EHX tubes that came with the mission kit!

[Bruce / Mission Amps]

I can't tell 100% in your picture:
http://music-electronics-forum.com/a...6-img_5042.jpg
but be sure the common point of those two 220K resistors is at chassis ground.