What has happened to my 6V6?

knuckleberryfinn

One of the Tung Sol 6V6GTs in my Tweed Deluxe has started glowing bright yellow (like a lightbulb) but not on the plates.  From what I can work out from the diagram below, it is the two ceramic mount supports that are glowing (labelled '5').  There is a whole lot of crackling and popping going on, so obviously it's buggered.  I'm just wondering what happened.

http://faculty.frostburg.edu/phys/latta/ee/wing813amp/813diagram.jpg

ICBM

Those can't be glowing, but they will be reflecting a lot of light from the screen grid which is glowing white-hot inside the plate.

Screen failure is the most common cause of blown 6V6s in my experience. Watch it doesn't take out the choke resistor while it's at it! This is also fairly common in 5E3s because there's no HT fuse usually and no way of making that resistor big enough to withstand a full-on screen short for any length of time. ie don't run the amp again until the valves are changed. The other 6V6 will be OK to keep as a spare.

digitalscream

That's exactly what happened to a KT66 in my JCA50H a year or so back - it developed a short and took out the screen resistor (I think) on the board as well.

Might be an idea to get the amp looked at when you change the valves just to check there's no damage.

knuckleberryfinn

Yes, I noticed it was reflecting light from somewhere else after having another look.  Thanks for the clarification.  I'll check the screen resistors when I've next got the back off the amp.

knuckleberryfinn

Can I run the surviving 6L6GC with a JJ 6L6S if they are measuring similar mA draw?  

ICBM

knuckleberryfinn said:

Yes, I noticed it was reflecting light from somewhere else after having another look.  Thanks for the clarification.  I'll check the screen resistors when I've next got the back off the amp.

5E3s don't usually have screen resistors. There's a 4K7 resistor on the board which is the 'choke' resistor (5E3s don't have proper chokes either) and is the feed to the screens - it's this that tends to smoke.

knuckleberryfinn said:

Can I run the surviving 6L6GC with a JJ 6L6S if they are measuring similar mA draw?  

Yes, if it's 6L6s you have in it. You don't need to worry about the matching at all, this is a very conservative circuit for 6L6s.

I'm pretty sure you can even get away with running one 6L6 and one 6V6 together in them!

knuckleberryfinn

Shit, I meant 6V6S.  I just didn't know what the 'S' stood for.  I tried it anyway - though in an Epiphone Futura amp - and it seems fine.  The tremolo sounds different though.

knuckleberryfinn

ICBM said:

knuckleberryfinn said:

Yes, I noticed it was reflecting light from somewhere else after having another look.  Thanks for the clarification.  I'll check the screen resistors when I've next got the back off the amp.

5E3s don't usually have screen resistors. There's a 4K7 resistor on the board which is the 'choke' resistor (5E3s don't have proper chokes either) and is the feed to the screens - it's this that tends to smoke.

Just got round to looking at this again and sure enough the 4K7 resistor is discoloured.  It must have been dissipating a serious amount of heat.  However, it still measures the correct resistance, and is there is still a 22VDC drop across it.  @ICBM Should I change it or not bother?  I doubt I have the right value kicking around so it would be a minor hassle.

jpfamps

knuckleberryfinn said:

Shit, I meant 6V6S.  I just didn't know what the 'S' stood for.  I tried it anyway - though in an Epiphone Futura amp - and it seems fine.  The tremolo sounds different though.

S stands for Slovakia, country of origin.

ICBM

knuckleberryfinn said:

Just got round to looking at this again and sure enough the 4K7 resistor is discoloured.  It must have been dissipating a serious amount of heat.  However, it still measures the correct resistance, and is there is still a 22VDC drop across it.  @ICBM Should I change it or not bother?  I doubt I have the right value kicking around so it would be a minor hassle.

I wouldn't bother. If it's a wirewound - it hopefully will be - they can stand a fairly severe short-term overload without burning out or changing in value even if they discolour, and there's no real point in changing them - it's unlikely to fail by itself without another valve short.

If it does go it's a benign fail that just stops the amp working without doing any other damage, and if the valve has shorted the amp has effectively stopped working anyway! The only problem is that you then can't get it going again (say at a gig) with just a new valve.

knuckleberryfinn

Lovely job - I'll leave it alone.