Amp trouble:

[noodleplugerine]

Hmmmm... Turn on, start playing, then as I play the volume drops untill the fuse blows.

I replaced the fuse, and turned the amp to see what was going on...

So basically - Of the 2 big 6l6s, they both start to glow, and then the one on the right glows loooooads, and then there's a blue light in it (spark I guess) and the fuse blows.

This is probably something really obvious, but its my first valve amp, I can't recognise the signs for dying valves etc.

[Ratrod]

Stop playing and don't turn it on again.

Your power tubes are probably done. You'll need a matched pair of new ones and the amp needs to be rebiased.

Take it to a tech.

It might also be the output transformer.

Anyway, take it to a tech.

[ailean]

Stop playing and don't turn it on again.

Your power tubes are probably done. You'll need a matched pair of new ones and the amp needs to be rebiased.

Take it to a tech.

It might also be the output transformer.

Anyway, take it to a tech.

+1

Power valves would be top of my list of culprits.

If you feel confident then change the power valves and try again, but you'll need to re-bias, if you don't know how then take it to a tech.

I'm going to do the government warning bit: Valve amps are $%&#ing dangerous to tinker with if you don't know what you are doing, if in any doubt take it to a tech.

[Adam.M]

Stop playing and don't turn it on again.

Just in case you didn't get the message first time around.

Yep, tubes are a goner!

[indysmith]

This should be in Tech

[viking]

Hmmmm... Turn on, start playing, then as I play the volume drops untill the fuse blows.

I replaced the fuse, and turned the amp to see what was going on...

So basically - Of the 2 big 6l6s, they both start to glow, and then the one on the right glows loooooads, and then there's a blue light in it (spark I guess) and the fuse blows.

This is probably something really obvious, but its my first valve amp, I can't recognise the signs for dying valves etc.

  What amp is it ?This looks like a simple short-cut induced by a bad power tube (the 6L6 with the blue light,i guess).If your amp is a Mesa&Boogie or another fixed-bias amp,you just have to replace the 2 6L6's with another similar matched pair  :).But,if it's not a fixed-bias amp...follow our friends advices (go to a tech,unless you know how to bias it yourself  :)) Anyways,no panic,it's quite ordinary for tube amps...Cheers!

[jpfamps]

There is a recent thread on this in the tech section.

Looks very much like a valve shorting out causing the fuse to blow (ie do its job!).

You will need a new pair of output valves.

I would advise taking it to a tech to get this sorted out (then I would say that wouldn't I!).

By the way, it is always very useful when trying to trouble shoot amp problems to know which make/ model of amp we are talking about as there are often issues specific to that type of amp.

[Twinfan]

New valves may not cure the problem - I had something similar where the valve socket was shorting out needing a replacement.

TAKE IT TO A TECH!

[viking]

Jpfamps is a tech,isn't he  ?

[jpfamps]

New valves may not cure the problem - I had something similar where the valve socket was shorting out needing a replacement.

TAKE IT TO A TECH!

From the description — blue/white sparks inside the valve — it does sound like a valve shorting out rather than an external short. However this can and does cause the screen grid resistors to expire, so a trip to the tech might be advisable.

By the way I saw the consequences of a quite spectacular external short the other day. The main problem if this happens is that a trail of (conductive) carbon is left behind that unless cleaned up will make another short a racing certainty.

It's very easy to see if there has been any external tracking by checking the valve sockets for carbon deposits/ burning.

[noodleplugerine]

Heya,

It's an Engl Screamer, and the plan is to take it to my normal amp tech, but was just curious as to what was going on, expanding knowledge etc.

So, any recommendations on new power tubes?

[Ratrod]

Tubes is a matter of taste but since you're asking.....

I really like the Tung-Sol 5881 reissues.

When the regular 6L6 tubes in my Engl Classic expire, they will definately be replaced by Tung-Sol 5881's.

[Copperhead]

The main problem if this happens is that a trail of (conductive) carbon is left behind that unless cleaned up will make another short a racing certainty.

THIS!!!

[HTH AMPS]

New valves may not cure the problem - I had something similar where the valve socket was shorting out needing a replacement.

TAKE IT TO A TECH!

From the description — blue/white sparks inside the valve — it does sound like a valve shorting out rather than an external short. However this can and does cause the screen grid resistors to expire, so a trip to the tech might be advisable.

By the way I saw the consequences of a quite spectacular external short the other day. The main problem if this happens is that a trail of (conductive) carbon is left behind that unless cleaned up will make another short a racing certainty.

It's very easy to see if there has been any external tracking by checking the valve sockets for carbon deposits/ burning.

I've seen quite a few old amps where the common heater-to-cathode short has arced the pins leaving a carbon burn and some 'tech' has cut out the burnt track with a stanley knife.  It's asking for trouble doing that imo - if the socket arcs, just bite the bullet and replace it.

[jpfamps]

New valves may not cure the problem - I had something similar where the valve socket was shorting out needing a replacement.

TAKE IT TO A TECH!

From the description — blue/white sparks inside the valve — it does sound like a valve shorting out rather than an external short. However this can and does cause the screen grid resistors to expire, so a trip to the tech might be advisable.

By the way I saw the consequences of a quite spectacular external short the other day. The main problem if this happens is that a trail of (conductive) carbon is left behind that unless cleaned up will make another short a racing certainty.

It's very easy to see if there has been any external tracking by checking the valve sockets for carbon deposits/ burning.

I've seen quite a few old amps where the common heater-to-cathode short has arced the pins leaving a carbon burn and some 'tech' has cut out the burnt track with a stanley knife.  It's asking for trouble doing that imo - if the socket arcs, just bite the bullet and replace it.

Absolutely, get the socket changed.

However carbon is often deposited elsewhere in the amp and this will need removing. For example I had a DSL401 in with a toasted screen resistor that left carbon all over the PCB.

The best short out I've seen is on an AC30 where one of the valve pins melted into the socket!