Not sure if anything on my amp was damaged.

[MadAsAHatter]

I had a bad moment with my KSR Gemini yesterday. About 5 minutes into playing I noticed the left pair of tubes start to red-plate and the volume dropped. I killed the power immediately. After giving everything a bit to cool down I turned the amp back on to see if the fault light on any of the tubes came on. The amp went through it's startup checks fine, no fuses blew and no fault lights on. No smoke or burning smell either. With no instrument cable plugged in, I took it out of standby and got a bad buzzing sound through the speakers. Turned everything off immediately.

I'm pretty sure the culprit was a bad speaker cable. With the cable plugged into the cab I measured the ohms and was getting all kinds of crazy numbers bouncing all over the place. I grabbed another cable and everything checked out fine, so nothing wrong with the cab/speakers. I checked the bad cable for continuity. There was good continuity on the sleeve of the cable, but spotty and eventually no continuity on the tip. I did get continuity when checking the wires directly. So it looks like one of the 1/4" plugs crapped out.

At this point I have not changed the tubes that red-plated yet. I did hooked it up with the good speaker cable and turned it on long enough to see if I got that bad buzzing again. Everything power up fine and the buzzing seems to have gone away with using the good cable.

I will replace the tubes and bias it, but don't want to go any further until I'm reasonable sure nothing was messed up. Aside from burnt components or wires what should I look for and/or test? Should I send the amp to Kyle for a once-over or do you think I was able to catch the mishap before anything was damaged?

 

[311splawndude]

Sounds like you did all the right things. Have you opened it up and looked yet? Assume the power tube pairs are next to each other and not inner/outer?

I'm not a EE or tech but that amp should have screen grid resistors whos sole job is to protect the power tubes from over current. So if they look good and any HT fuses look good - I'd say you are good to go. I'm thinking that amp can probably handle it.

What I'm not smart enough to know would be how a bad speaker cable causing a mismatch for the OT would cause red-plating

Good luck though MaaH

 

[Chubbs]

I think your right on, to me it sounds like a tube issue. Only thing I’d do is open it up and look for any burnt (like you said), and post the pics here. Then swap the tubes and fire it up while it’s open. Provided nothings burnt, odds are the fuses will save the important bits.

 

[Dave M]

Sounds like you did all the right things. Have you opened it up and looked yet? Assume the power tube pairs are next to each other and not inner/outer?

I'm not a EE or tech but that amp should have screen grid resistors whos sole job is to protect the power tubes from over current. So if they look good and any HT fuses look good - I'd say you are good to go. I'm thinking that amp can probably handle it.

What I'm not smart enough to know would be how a bad speaker cable causing a mismatch for the OT would cause red-plating

Good luck though MaaH

I think your right on, to me it sounds like a tube issue. Only thing I’d do is open it up and look for any burnt (like you said), and post the pics here. Then swap the tubes and fire it up while it’s open. Provided nothings burnt, odds are the fuses will save the important bits.

Paging: @raiken

 

[MadAsAHatter]

Sounds like you did all the right things. Have you opened it up and looked yet? Assume the power tube pairs are next to each other and not inner/outer?

I'm not a EE or tech but that amp should have screen grid resistors whos sole job is to protect the power tubes from over current. So if they look good and any HT fuses look good - I'd say you are good to go. I'm thinking that amp can probably handle it.

What I'm not smart enough to know would be how a bad speaker cable causing a mismatch for the OT would cause red-plating

Good luck though MaaH

Yes, the tubes that red-plated are next to each other not inner and outer. I haven't opened it up yet and probably won't have a chance to get to it until the weekend.

When I was measuring resistance on the bad cable it should have read an 8 ohm load. I was getting numbers anywhere from 50 to 300+ ohms. My best guess is the cable caused the transformer to see those wild swings in resistance and possibly pushed high voltages across the tubes. Only my guess.

I think your right on, to me it sounds like a tube issue. Only thing I’d do is open it up and look for any burnt (like you said), and post the pics here. Then swap the tubes and fire it up while it’s open. Provided nothings burnt, odds are the fuses will save the important bits.

I'm hoping my worst case scenario is the tubes were damaged from red-plating, but everything else is fine. I know Kyle built the Gemini with several protections in place and the amp wasn't on for very long. I'm also thinking the cable was on it's way out but hadn't completely crapped out while in use. So maybe the resistance I measured wasn't as bad as what the amp may have been seeing while in use.

 

[MadAsAHatter]

I'm happy to report that all appears to be well with the amp.
I gave the insides a good visual inspection and didn't see any burnt components. Then I put a new set of power tubes in and set the bias. Once it was all situated I played it for a good 30 minuets. No red-plating of tubes, no volume drop, no strange noises. Everything worked as normal. Looks like I caught things in time where the only thing that may have been damaged was the tubes.

 

[Dave M]

I'm happy to report that all appears to be well with the amp.
I gave the insides a good visual inspection and didn't see any burnt components. Then I put a new set of power tubes in and set the bias. Once it was all situated I played it for a good 30 minuets. No red-plating of tubes, no volume drop, no strange noises. Everything worked as normal. Looks like I caught things in time where the only thing that may have been damaged was the tubes.

Did you just get lucky, or actually diagnose and resolve a problem?
Something to think about.

 

[MadAsAHatter]

Did you just get lucky, or actually diagnose and resolve a problem?
Something to think about.

Probably a little bit of both. Lucky in the sense that I spotted the the symptom right as it happened and nothing was damaged. I'm fairly certain I got to the root cause of the problem with the bad cable. It is possible one of the tubes was already going bad and happened at the same time as the cable issue. No good way to test for that though. I am however positive there is no issue with the circuit itself, but I'll still keep an eye on everything while in use.