Cornford calamities

McToot

It's a Carrera, running either octal or nine pin... Ok. Maybe a bit OTT, but the amp went pop fizz bang yesterday on the 9 pin setting. It had done that before (on the octal) but only when I was experimenting with a very old kt66 that someone had lent me. I switched it off, removed the big bottle, replaced it with a very old Brimar 6v6 and fired it back up: it's been fine since. But yesterday playing on the the 9 pin setting (which was running an almost new JJ EL84). It went bang. Took the back off this eve and had a look at the 84; it was all frosty at the tip and when I tried to remove it, the glass sleeve just came away. It now looks like this: http://i671.photobucket.com/albums/vv73/juleslonghurst/b20ed25b037b5bb127c0bee896dfa1c2_zps7c0eed11.jpg Did I just get a dodgy el84 or should I be asking the local amp chap to be looking at this in more depth? Ta

ICBM

Probably a manufacturing fault in the EL84. From where it broke I would suspect a fine scratch on the inside of the glass from the mica spacer, which then caused a crack.

It could be that the valve was running massively too hot though, which could have been caused either by a fault in the amp, or - if it has a shared grid connection between the two valves - a fault in the octal valve. (The Mesa Blue Angel is like this and the one I had did that when one of the EL84s shorted - the 6V6 side then ran too hot, even when the switch was set to 6V6 only.)

Try a new valve, but watch it closely when it's running. If it starts to glow red-hot, you have a problem. If so, pull the octal valve and see if it stops.

McToot

Thanks John. I need to get another valve in, but might take it up to Henk Philipsen in Horsham for him to have a look. He was the one who lent me a few really old valves to have a play with, including a 50s coke bottle 6l6... Here are a couple of gut shots in case they are of any interest or use... They show where I've removed the 84. 

http://i671.photobucket.com/albums/vv73/juleslonghurst/47c439ce121ec54546a74d9cb402ca4f_zps7152a6e5.jpg

McToot

Not sure why they're not showing this time...

Sorted

fastboy

I was going to say Jules, take it to uncle Henk and he'll sort it