Burnt 5W resistors - what next?

[raindownthunda]

I've had my Crack + SB kit running for a good 2 years with 100's of tube rolls with no issues. Today I turned the amp on with a newly arrived GE 6AS7G (appeared to be NOS or very lightly used) and both input & power tube heaters lit up as normal. As I let the tubes warm up for a few minutes, I started to smell a burning smell that got VERY strong. I quickly shut off the amp and unplugged the power. After about a minute I noticed both 5W 270ohm resistors were VERY hot to the touch and appear to have a brownish/burnt look. No headphones were plugged in thankfully.

Can anyone advise on what might have went wrong? And what I should do next?

Is it safe to assume the 5W resistors are toast? Anything else that would need to be replaced? Aside from building the kit I have no experience with circuits so am not sure where to begin troubleshooting and repair.

Thanks so much

 

[raindownthunda]

I just measured the resistance of the 5W 270 ohm resistors with a multimeter and am getting .240 kohm (240ohm?) for both. I am too afraid of causing a fire to try turning the amp on again with power since I'm not sure what's going on.

 

[Paul Birkeland]

If you fire the amp up with no tubes plugged in, do the resistors start to stink?  What's the voltage at terminal 13 with the amp on and no tubes plugged in?

 

[raindownthunda]

If you fire the amp up with no tubes plugged in, do the resistors start to stink?  What's the voltage at terminal 13 with the amp on and no tubes plugged in?

No smell after 5 minutes with no tubes. T13 = 236V with no tubes.

 

[Paul Birkeland]

Now try just the 12AU7 and check the voltages at terminals 1-5.

 

[raindownthunda]

1: 79.5
2: 228
3: 0
4: 228
5: 79.2

 

[raindownthunda]

I just checked the Speedball PCB with the stock 6080 added and the voltages are within the expected ranges per the manual.

Per a friend's suggestion, I checked the new 6AS7G and found continuity between pins 6 & 7 and 6 & 8. From what I understand this means the 6AS7G I got has a short and would be the culprit?

Since the SB voltages are checking out per the manual, is the amp safe to run with known good tubes? I'm worried given the burning smell and slightly burnt looking resistors that there may be some repair needed before the amp is safe to use again with known good tubes. Thank you

 

[Paul Birkeland]

Yeah the next step would be a voltage check with the 6080.  It sounds like you have a tube that needs to get binned.  The 270R/5W resistors should still work OK if the ceramic body hasn't cracked on either of them.

 

[raindownthunda]

Thanks Paul, appreciate all your help. I’ll try to return the tube. So you’re saying I don’t need to be concerned about them failing catastrophically with good tubes despite the previous burning from the short? Out of curiosity, if the 270ohm 5W resistors were to fail/crack during usage, what would happen?

 

[Paul Birkeland]

The white casting goes around the actual resistor body that's cast inside.  If the body cracks, it can fall off and then the resistor doesn't dissipate its heat so well and it can fail.

 

[Mucker]

The resistors are affordable parts:

https://www.amazon.com/Ceramic-Resistor-Completely-Insulated-Printed/dp/B07Y4J4W83/ref=sr_1_9_sspa?crid=27I20ULFHJZRE&keywords=270r+5w+cement&qid=1648062366&sprefix=270r+5w+cement%2Caps%2C94&sr=8-9-spons&psc=1&smid=A28ZWXW3ZSVNZU&spLa=ZW5jcnlwdGVkUXVhbGlmaWVyPUEyMjlKU1lHVlJKOEtXJmVuY3J5cHRlZElkPUEwNDY1MjEwMU9QWjREVkVWSDFFMCZlbmNyeXB0ZWRBZElkPUEwMTU1NjA3MVQ1Q0VEVVpGNzU3VCZ3aWRnZXROYW1lPXNwX210ZiZhY3Rpb249Y2xpY2tSZWRpcmVjdCZkb05vdExvZ0NsaWNrPXRydWU=

Would these be adequate replacements PB?

 

[raindownthunda]

I was actually looking at these Dale Vishay wire wound resistors and wondering if they would work and be slightly higher quality than the stock components: https://www.mouser.com/ProductDetail/Vishay-Dale/CW005270R0JE12?qs=sGAEpiMZZMtTURnxoZnJANcbPKGiGzhoDuVuTlBEOU8%3D

 

[Paul Birkeland]

These resistors are outside of the signal path, so they are not super critical.  I would tend to favor a resistor with some inductance over a non inductive part though.