Tube failure

[ChurritoTierno]

Hi guys.
A friend of mine brought a Marshall guitar amp for second hand. DSL40C model.
So, he brings the amp to me house to a revision... he says that he went playing and suddenly stop sounding.
I opened the amp and I found the HT fuse blowed.
I replaced that fuse with a new one of the same specs.
Playing for a 5 minutes the amp stop working again. The same fuse blowed again.
So. First suspect the power tubes, two EL34 in push pull.
I taked out those tubes of off te amp. Placed a new fuse and turned on the amp. It worked fine. Measured volt of all pins. Bias was -36V. Plate and screen about 470V.
So I haved to test those EL34.... I put a bias metter.
I messured 29mA and 34mA respectively for each EL34.
I left the amp on for about 2 hours and nothing wrong.
So suddenly I have the idea of testing for microphonic..
I took a screwdriver whith plastic handle. And did some gently knocks on the EL34s and suddenly when I hit one of them it made orage sparks and blowed again the HT fuse.
Never I have seen this.
The spark that I saw was between the plate and glass envelope.... but this makes no sense... looking the plate it has some squared holes. . So I guess that the spark was originated between plate and another inside electhrode and what I saw was part o it that escape through the squared hole of te plate.
So my question is if this type of tube failure is the classic "shorted tube".

 

[kevinkr]

Relocated to Instruments & Amps per forum policy..

The sort of failure you describe is not uncommon, I have run into the same problem. In an instrument amp given the handling it receives both in transport and use it seems to be a more common failure than in home use scenarios.

Replace the EL34 with a matched set and you should be good to go.

 

[GeorgK]

Unfortunately the quality of new tubes is not that good any more...
Especially Chinese EL34s, whatever the reason is.

When I see an amp with a blown HT fuse and there is no other obvious problem I usually replace the power tubes immediately.

 

[TonyTecson]

some users deliberately do that, some even as crazy as using both the el34 and the 6l6gc in the same guitar amp, they say they like it that way....

for hifi, that is a big no no.....

 

[Richidoo]

Same happened to me even with the very high quality New Sensor Gold Lion KT88, and brand new from very reputable tube seller, who custom tested and matched octet. When the tube shorted it took out a resistor which is there for that reason. I noticed on removing the shorted tube from the amp that it rattled when shaken. I asked the tube seller to make sure the replacement tube does not rattle. I think this has something to do with internal tube shorts. In future tube purchases I will request no rattling tubes!