EL34 Failure

[PeteM]

Anyone care to help why two sets of valves should fail inside 18 hours of total use.

The amplifier is a Yarland EL34 single ended ( yeah its Chinese made but read on) The valves that have failed one was an Electro Harmonix heater filament gone open circuit the amp was on at the time and one channel just faded away silently no pops or bang. The other failure was a JJ El34L lost vacuum over 2 days of being used I noticed the getter had turned milky before switching on.

Before we blame the amplifier It has run fine day in day out with the Chinese supplied valves, it has also run for 4 days non stop in the middle of the garage with just load resistors away from flammables and with an RCD fitted. I also have some unknown re branded EL34's that red plate easily yet these show no signs of stress without bias adjustment.

From the vendors point of view this smells fishy 2 different brands fail from the same customer within a short period, I have to agree it does look suspicious but I can not get the amp to destroy the cheap valves, furthermore the valves that failed were brought up slowly by a variac.

I can not see beyond the obvious any fault with the amplifier it runs fine even the power transformer is only warm after a full days play, I have had 2 multimeter's connected one giving a stable 6.2 Volts heater supply, the other sitting across the 10 Ohm cathode resistor giving a voltage of 0.64 Volts B+ is 279 Volts

I should add the vendor gave me a full refund but would like some help before I buy further replacements.

Regards

Pete

EDIT.. I have uploaded the schematic http://www.petemoore.pwp.blueyonder....udio/FV34A.jpg and should add the failures were on opposite channels

[Alastair E]

Personally, I would say that you have just been VERY unlucky....

The two valves failed in different ways. I doubt that your equipment in any way 'killed' them. Maybe it was the shipping, stresses caused in transit, who knows....

[PeteM]

Personally, I would say that you have just been VERY unlucky....

Thanks for the reply Alastair.

I'm trying to see this from both points of view, the vendor thinks my bias was way off causing over heating.

If bias was way off would I have not seen plate glow in a dark room, I know for a fact the cheap Maplin Edicron re branded I purchased years back glowed in a World Audio design amp. At factory bias setting of the Yarland I have no glowing plates.

Correct me if I'm wrong bias on the Yarland is voltage across the cathode resistor. ie.. 0.64volts across 10 Ohm resistor is an idle current of 64mA * B+@ 279V = 18 Watts

Thought EL34's were good for 25W and the sample I purchased were suppose to be more bullet proof.

[Tweeker]

What the vendor really thinks and what the vendors says are not necessarily the same.

[PeteM]

What the vendor really thinks and what the vendors says are not necessarily the same.

True . I sent both sets back including the 2 good ones so he could see with his own eyes. Regarding the JJ's lost vacuum there was no sign of a cracked envelope it had to have leaked around the glass to metal seal, furthermore I have seen tubes lose vacuum while powered up this JJ was clean and no burnt filament.

The vendor had his reasons it would of been cheaper for him to replace 2 tubes but he insisted on a full refund.

Regards

Pete

[mississippi]

I read the story, and I gave a look at the circuit diagram.
I only have a small remark at the diagram, I don't think this could kill the tubes but some oscillations could appear.
The secondary winding (speaker side) is forgotten to give at a connection to the ground. The feetback is so not working!

[PeteM]

The secondary winding (speaker side) is forgotten to give at a connection to the ground. The feetback is so not working!

Not sure what you mean.

The feedback is screened cable and grounded one end.

[Frank Berry]

For the feedback to work, one end of the secondary must be grounded. According to the schematic, it's not.

[consort_ee_um]

The feedback connection is floating, the other end of the output transformer secondary winding must be grounded to have any effect.
Didn't the famed Marshall amps run without feedback to get a fantastic mid range presence?

[PeteM]

For the feedback to work, one end of the secondary must be grounded. According to the schematic, it's not

Thanks for the heads up

I think there is a discrepancy with the schematic, I will open up the amp and check. I know the power supply is much beefier than shown reason why I did not post that part.

I have tried again to blow up these bad EL34's I have, running them hot has not done anything.

Trouble the dealer planted a seed it must be my amp, I have taken his comments to heart.

Regards

Pete