Amp problem

sweave

Switched my amp (Trace Elliot Speedtwin C30 - 5×12ax7 + 4×el84) on today and seemed fine on standby but made a loud noise with bright purple light coming from power valves when switched on fully. Obviously switched this off very quickly! 
Questions - is this likely to be a dodgy power valve or something more sinister? If the valve, then is it safe to try swapping them out one at a time to identify problem, or will repeating the light show cause more damage elsewhere  or even be dangerous?
Cheers for any advice
Steve

Sassafras

One for @ICBM ;

ICBM

Valve failure - the purple light is ionisation from an air leak which causes the current to arc over... pretty much like how a neon bulb works, only not controlled. The purple/pink glow is characteristic of the Argon gas in air, which is not absorbed by the 'getter' (the silver flashing at the top of the valve).

Repeating the light show is a bad idea if you can avoid it. It's possible that some amp damage has already occurred, but if it hasn't then it's better not to risk it. If you closely inspect the valves it may be obvious which is the culprit - the printing on the outside or the grey plate on the inside may be discoloured, or the getter may be thin-looking or even turning white, depending on how bad the leak is. (This can sometimes develop over a few days.)

If you can't tell, I would replace all four power valves - which is best done as a set anyway - and see if the amp works OK. when the new valves are in, check that all of them glow correctly when switched fully on - you should be able to see a faint blue glow inside the plate structure as well as the typical orange heater glow. If any of them don't, the chances are that the bad valve has burned out the screen resistor as well.

Don't throw the old ones out yet - the faulty one may show itself after a while, or if not then you could get someone to test them for you. The other three will be OK, almost certainly.

This is the colour of an Argon lamp, you will probably recognise it .

https://upload.wikimedia.org/wikipedia/commons/5/53/Argon-glow.jpg

DJH83004

The Speed Twin C30 is 4 x EL84 cathode biased, and the output valves are biased quite hot, so I suspect one has flashed over (arced internally). It is doubtful this has done any damage to the rest of the amp, other than possibly blowing the mains fuse, which from memory is located in the IEC mains socket - I don't think there is a separate HT fuse. If you didn't clearly see which valve flashed over, I would change the whole set for a known good set on the basis that one has failed the others won't be far behind.   

ICBM

Actually, having thought about it - is the Speed Twin 30 one that's affected by the notorious shrinking plastic valve sockets problem? I can't remember - I haven't seen one for a while.

If it is, it's the shrinking valve socket which has cause a power valve to crack around the pins and let air in - and if so, do *not* fit new valves until the sockets have been changed, or they'll go the same way.

How to tell - remove one of the power valves and have a look at the pins. If they're bent inwards towards the middle of the valve, the socket has shrunk. You could look inside the amp as well, and if the sockets look like this (this is a Marshall, but they're the same type) then they must be changed - notice how they're discoloured, look melted and no longer exactly round.

sweave

Many thanks for the advice.
Good news is that the valve bases look a little 'tanned' but still round with no bent pins on the valves.
Bad news is that I replaced the dodgy valve but the amp wouldn't switch on at all. Checked the fuses and they seem ok and can't see anything obvious on the top of the PCB. Sounds like it might be time to get it to an engineer.
Having a good week - Samsung tablet died, central heating boiler died (and the heat exchanger) and now this - maybe time to write an epic ( obviously acoustic) blues song.

DJH83004

It definitely sounds like a fuse has blown which often happens when an output valves flashes over. Did you check the 20mm fuse located in the IEC mains socket as well as the fuse in the plug?

sweave

I must confess I didn't know that IEC sockets had fuses in them until today - apologies to DJH83004 that you had to tell me twice to look there  - doh! - I had checked the other 4 fuses within the amp.
This fuse has blown, which is excellent news. Just need to get a new fuse and see if the replacement valve does the job. 

DJH83004

No worries, just glad to help out 

sweave

Update - fuse replaced but now, when I switch the amp on to standby l can hear a slight hum, some lights come on (eg the ones indicating which channel is selected) but the main power light doesn't and neither do the valves start glowing. Any thoughts?

ICBM

That means the filament supply has failed - there are two fuses (FS2 and FS3), either one of which failing will stop it working. Check them with a meter, if they don't look blown - you need to take them out to do this because otherwise the transformer winding and valves will complete the circuit and give a false 'good'.

The blown valve most likely shorted from cathode to filament as well as plate to cathode (in fact it may have simply shorted everything, if it had got ionised!) which will take out one or both these fuses. They're T6.3A, if you need to get more before you start.

I'm not a big fan of filament fuses to be honest, they quite often 'nuisance blow' for no good reason, and they don't really protect the transformer much better than the primary fuse.

sweave

Both F2 and F3 seem fine.(checked with multimeter)

ICBM

sweave said:

Both F2 and F3 seem fine.(checked with multimeter)

Are you confident to meter inside the amp with it on? You don't need the HT on (ie keep the standby to 'mute'), so it will at least not be too dangerous if you keep well away from the mains side.

Power the amp up and meter from the chassis to both ends of each of those fuses, with them back in place - you should read just over 3V AC on all four points. Then meter across from one fuse to the other, or from TP1 to TP2 on the board - you should get 6.3V AC.

sweave

Put the fuses back in having tested them, reassembled the amp and it now works, when it didn't before I tested the fuses (for the third time). Maybe one of the fuses wasn't sitting in the holder tightly enough when I put it back in before? Anyway it is now back to normal so I don't have to borrow an amp for tomorrow. Will still buy some replacement valves as my 'spare' is one of the originals it came with in 1998!
Again, many thanks for the help - much appreciated. ICBM and DJH83004, if you are ever in the Wokingham area, I owe you a beer or two...
Steve

ICBM

sweave said:

Put the fuses back in having tested them, reassembled the amp and it now works, when it didn't before I tested the fuses (for the third time). Maybe one of the fuses wasn't sitting in the holder tightly enough when I put it back in before?

That's quite possible - sometimes you need to gently close up the clips a bit with a pair of pliers.

Let's hope it's not some other intermittent fault...

sweave said:

Again, many thanks for the help - much appreciated.

No problem .

DJH83004

Glad she is up and running, I would strongly recommend you swap the set of output valves sooner rather than later, my personal preference for EL84s in a cathode biased amp would be JJs as in my experience they are the most robust of the modern EL84s available.