Metropoulos Forum

I built a 50w kit (not Metro as the don't have them anymore ) and when I turn up to about 4 or more the fuse would blow. I had a bad cap in the bias circuit which I replaced. Before I replaced it my tubes redplated and one of the screen grid resistor got a bit burned. So I replaced that. I played for about 30 minutes at pretty low volume and eveything seemed fine. As soon as I turned up I saw a flash and the fuse popped. I got some scorching between pins 2 and 3 on one of the power tubes. Now these are the same tubes I've had in since I built it, and the bad cap seems like it was like that when I got it. I put in a new pair of rubys and turned up even louder and everything SEEMED fine BUT after a few minutes I THINK I might have smelled something burning. I turned it off and haven't tried it since. I don't know if the smell was risidual from when the tube went or is something else burning. I've searched for something shorting but haven't had any luck.

If pin 2 and 3 have arced there can be carbon traces left on the tube socket that you may not be able to see. Sometimes testing for continuity between those pins will show which sockets may have carbon traces but not always. They may test okay (no continuity) but the high voltage and current while playing can cause them to arc again and blow the HT fuse.

You can try scraping between pin 2 and 3 of the socket with a razor to remove carbon. If that doesn't fix it, you'll have to replace the socket if the HT fuse continues to blow.

What would cause the arcing in the first place? Bad tube? I could have screwed the tube when I was having bias issues. What a faulty tube run at lower volumes?

What appears to have happened is the power tube screen failed, creating a ground fault that overloaded a screen resistor. Replacing the faulty power tube should fix the problem, but ...

You might want to play the amp (or have someone else do it) while having the tube slightly lifted off the base just enough to see any arcing at the socket. So long as there is none or any other unusual behavior, you should be ok. Should something else rear its ugly head, you'll likely get a blown fuse.

Well I tried again and after a few minutes of LOUD playing it blew again. No signs of arcing. I didn't smell anything. It actually blew while I wasn't playing. I stopped for about a second and it blew. All the other times it was when I turned up loud and hit the first chord. I did notice some white/blue "flashes" inside the power tubes. I've never really watched tubes before while I played so I don't know if this is normal or not. It was between the plates and not very noticable if you weren't watching them closely.

White/blue flashes, sparks or otherwise inside the power tube is indicative of internal arcing between electrodes, and that creates havoc. Take note of which tube(s) are affected and replace.

This was a brand new Ruby EL34. I removed the old chinese tubes that arced and put these in.

What's the bias look like?

Ted B wrote:What's the bias look like?

33mA on one and 36mA on the other.

Just to clarify as I didn't convey it to well. When I built the amp I had bias issues which red plated the Chinese tubes. THey were biased VERY high. I stupidly played thru it and it was fine at low levels and as soon as I turned up it blew the B+ fuse. One of the screen grid resistors smoked a bit and turned a bit brown. I found out that I had a bad cap in the bias circuit. I fixed the bias circuit and got my bias down but still the fuse would blow at higher volumes (same tubes, same resistor). I replaced the resistor (actually both as they were 2Watt and I put in 5Watt). I tried it again (same tubes) and this is when I got the ARCING but it was on the other tube, not the one with the burned Screen Grid resistor. SO I figured maybe i cooked the tubes so I put in the new Rubys and tried again. It was good for a few minutes and then kickied out. The white?blue flashes are in the tube that had the original burned screen grid resistor, NOT the side that Arced.