5F6A Clone Blew Main Fuse

[tonecat]

Hello Forum Members,
I was playing out with my band on Saturday night. The club we were playing in lost power twice while we were playing. My amp was under full load both times the power went out. Shortly after getting things working again for the second time, the amp started to lose output volume and it sounded a little weak. I turned it up a little bit and a few minutes later, it blew the mains fuse. Rather than trying to figure out what went wrong, I switched amps so I could finish the gig. I haven't had the time to try and diagnose the problem yet, so I thought I would post here to see if anyone had any ideas on where to start from the symptoms the amp had I described. Please post any thoughts or troubleshooting strategies any of you might have. Any help would be greatly appreciated!!!! I plan to start working on it later tonight. Thanks so much!!

[Enzo]

Remove the output tubes, replace the fuse, and see if the fuse holds up. If the fuse still blows, you have a circuit problem. If the fuse stays OK, then probably one of your power tubes failed.

[Diablo]

If it still blows a fuse with the tubes out, next step is the light bulb limiter for testing. Do a search on this forum if you don't already have one.

[tonecat]

Thanks for the tips everyone!! Thats what I would have done.
I pulled all the tubes and tested them on my Amplitrex AT-1000 tube tester which tests tubes under a load. All checked ok. In fact, they tested the same that they did last time I checked them all about 4 months ago. I put them all back in and replaced the fuse. I plugged the amp into my Sencore Powerite and powered the amp on. I observed on the Powerite that the amp was drawing about 400mA @ 117vAC in standby mode. I flicked the standby switch and with all contols at zero and no load, the amp was drawing about 750mA @ 117vAC. This was consistent with the specs I wrote down when I built the amp over a year ago. I hooked up my sig gen and load box and applied a 150mV, 1Khz signal to the amp. I set all tone controls at 12 o'clock and started cranking her up. At maximum output, the amp was only drawing 1.5 amps on the Powerite which is far under the 3A fuse. I then checked the bias at idle and it was about 43mA per tube with 455vDC B+ at pin 3 of the power tubes. A little bit hot, but still under 70% dissapation. I am using re-issue Tung-Sol 6L6GC STR tubes in the amp which sound better to me biased a little on the hot side. All other voltages in the amp read the same as they did when I built the amp. This is my main amp and I have lots of hours on it with no problems until now.
The question is, what happened that night? Did the amp overheat? I was pushing it hard, had pedals in front of it, had it tilted back in a chair, and the room was very warm. I use the amp like this all the time, but the room was never as warm as it was on Saturday night. Could it just have overheated?? Do you think because I am running the tubes a little hot it is drawing to much current? Am I missing something here? Should I put a small fan in the amp to keep it cooler?
This is a real puzzler. I was hoping to find a bad tube or some other failed component, but the amp works fine now. I can't have this happen at my gigs just because the room is warm. Does anyone have any ideas? Do I need to do some more tests? Please lend me some advice!!
By the way, I put an HT fuse in the PT centertap when I built the amp for extra protection and it didn't blow. When I checked the amp under full load, there was 287mA going through the fuse. Also, this is a pretty stock 5F6A amp clone. I am using vintage Shumacher transformers from a 64 Super Reverb, 2 ohm load, 50uf mains, 47uf screens, 22uf PI, and 16uf preamp for filtering, 1k screen grid resistors, 1k5 grid stoppers, and I have the heaters elevated with 75-80vDC and a virtual center tap. All other specs are stock. I also am using a vintage Mullard GZ34 rectifier.
Thanks for your help and let me know if there is any other info you need!!

[tubeswell]

Random short maybe? Maybe something like a chassis nut came loose and undone and shorted out one of the sockets. That has happened to me before (I'm embarrassed to admit)

[tonecat]

Thanks for the reply tubeswell. I don't think that is the problem because the amp started sounding weird and was lacking power right before the fuse blew. I checked for any loose or stray parts before I powered the amp on again.

[Enzo]

If I get a fainting spell, feel my heart racing, and pass out, then recover and go to the doctor the next day, he will find my pulse and blood pressure normal and my head clear. Just because your tube tester tests tubes well, doesn't mean they don;t have a problem. As you discovered, your amp is now working properly again.

You tube tester won;t find that little flake of loose material living inside the tube unless it shows itself at the time of test. That is just one possible failure mode. Or a broken grid wire swinging around inside might not be touching anything, but under certain vibrations it touches and POW.

Rap on the sides of the power tubes while running. ANy pops or crackles? Wiggle them side to side in their sockets. Any intermittancies? Is there ANY microphonic response? Pull the power tubes and vigorously shake them in front of your face. Hear anything loose inside them?

And don't forget the rectifier.

Thanks for including all the details, but none of that stuff is likely to be the problem. Not many things can blow the fuse. For the most part, the 12AX7s and such have no way to do so. The power supply and the power tube stage is most of the potential for failure. Power tubes are by far the most likely suspect. But a rectifier tube can cause trouble as well. Doesn;t sound like it here, but tube sockets can arc.

There is no standard failure of parts description. Bad parts do not necessarily go from perfect to totally blown like the flip of a switch. My main suspects remain your power tubes.

[tonecat]

Thanks for the reply Enzo!! The way you explained it from a "human" perspective really gave me insight into the problem. I noticed that the rectifier tube was making a strange buzzing sound and I think I saw a spark inside the tube out of the corner of my eye after running some more tests on it. I was listening to music the first night I tried to figure out what was wrong with the amp and I didn't hear the tube making any noise. From research I have done, I guess that rectifiers can make a buzzing sound especially when mounted upside down like in a bassman and still work fine. I figured I would swap out the rectifier as a starting point since the HT fuse didn't blow and give the power tubes one more chance. I had band practice tonight and I played the amp pretty hard for about 3 hours and had no trouble with it. I don't know if the rectifier tube had an issue or not but swapping it out seemed to work. I guess I need to carry extra fuses and spare tubes to gigs in case this happens again. Part of learning the troubleshooting experience is understanding that a part doesn't have to explode and be obvious to give you problems. I guess I thought since I was running a vintage Mullard GZ34 that it was/is indestructable. The other thing that is becoming clearer to me is that combo amps beat the hell out of electrical components. I think with the style of music that I play, a head/cabinet style rig is what I should run. Hopefully I won't have any more trouble with it. One last thing, is losing power to an amp under full load as bad for it as I think it is, or am I being paranoid? Could that possibly have sent the rectifier over the edge?
Thanks again to Enzo and this forum!! This makes it possible for a curious tinkerer to sucessfully come up with a solution that works!! Hopefully!! Thanks Again!!