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stueyp 13th March 2006 08:32 PM

Sovtek 6CA7-EH Producing smoke...
Hi everybody, this is my first post. I originally joined diyAudio for DIY speaker tips but now I really need some help from you Tube guys. I have an integrated using four EL34/6CA7-EH SOVTEK tubes. Today, when I wasnt using it but was powered on, one of the tubes went mad and a REALLY loud high frequency sound came from the right speaker. Then came the smoke, and the stench....I shall never forget it... The sound now wont go away. Something a bit strange though was that looking into the tube (like one does) on occasions before it blew, it looked different to the other three, it glowed brighter and somehow looked like it was wired back to front inside?! Does anyone have any idea what went on? And whether I've damaged the amplifier and/or speakers? Or would just replacing a duff tube do the trick?

The image shows the only visible break (though I assume the glass went if smoke could get out...)

Thanks, Stu.

kevinkr 13th March 2006 10:54 PM

Your description sounds like an output tube in full thermal runaway.
You can't simply replace the tube, there is obviously a problem in the amplifier that needs to be fixed. Also you should not run it this way, your output transformer if not already fried could be the next item on the list to fail.

The smoke you saw came from burning resistors inside your amplifier, most likely the screen resistor and possibly a cathode resistor as well depending on the topology.

The envelope of the tube itself may have a scorched logo, but unless the getter (silvery stuff) on the inside of the tube has turned white you did not let the smoke out of the tube itself although it is definitely no good. Sometimes when an internal connection is vaporized by a short the sudden presence of a large internal pressure (shock wave?) may fracture the envelope and let in the air.. (And out the smoke as well :D )

I would guess the tube developed an internal short possibly due to excessive dissipation or a manufacturing defect.

You should expect to replace the pair on the affected channel.

You will need to find someone to fix your amplifier, alternatively if you are willing to learn to do it yourself there are people here who can help you. Note that before proceeding further you should read the thread on high voltage safety as well as the newbie thread. I assume you can solder competently, if not find a technician to do the repairs...

The first step is to tell us what kind of amplifier you are using and to post the schematic if you have it.

lndm 13th March 2006 11:54 PM

Just theorising about the cause, it sounds like you may have had oscillation.

This can be caused/exacerbated by such things as circuit layout, lack of grid stopper resistance, a faulty or badly made valve or by an unfortunate mix of components.

gingertube 14th March 2006 02:56 AM

Fuses for tube cathodes
I have 5 EL34/6CA7 Amps and have auditioned heaps of modern production plus NOS tubes. I have had both NOS and modern production tubes fail for no apparent reason with symptoms of either screen to cathode short or (less likely) anode to cathode short.

All my amps now have 10 Ohm 0.25W resistors in the cathode circuit. Yeh - its a great place to measure tube currents for balancing BUT mostly they are great individual tube fuses. Cooked a few 10R resistors since but never an output transformer.

Next time you see a push pull EL34 schematic with 10 Ohm 5 Watt (or 2W) resistors specified in the cathode circuit - just ignore it and use 0.5W or less. Of course this does NOT apply to 270 Ohm cathode BIAS resistors - they need the full power rating BUT put the 1/2 or 1/4 watt 10 R in series with them.


stueyp 14th March 2006 09:09 AM

First off, thankyou for replying to this post, it is very much appreciated. The amplifier is a Classic (Yushang Audio) No. 16 Integrated amplifier. I have emailed both Classic, and the distributor with no response.

Here are some pics:

Kevin you certainly know your stuff. It looks like that resistor has gone. Now, would replacing the right channel tubes and the resistor fix do you think, or does the problem lie deeper? Although it is annoying that I can't play music at the moment, I am keen to learn a bit...

baggystevo82 14th March 2006 09:27 AM

Hi Stu,

Found this...which I assume youalready know, but might give a few more clues as to type of circuit...

I would suggest replacing the tubes, replace all burnt components, check the output transformer primaries (probably best to compare DC resistance with the undamaged channels transformer, though the laminations may need degaussing if they were subjected to a sustained current imbalance. Not the end of the world if they do need replacing) and just ease up on the bias a little until the amp proves itself reliable. I think I would suspect a dodgy tube more than anything, but just keep a good eye on the bias currents and make sure none of the tubes are doing anything untoward.

Unfortunately I live all the way over in Hereford so it's a bit far to road trip over there.

Hope this helps,

stueyp 14th March 2006 09:53 AM

Thanks Steve, I have ordered a pair of sovteks and several new resistors. Your post has reassured me a lot! I was not looking forward to a trip over China...

Another thing, that particular tube that died always made more mechanical noise than the others, and does the fact that it looked "back to front" mean anything? Or am I being a complete newbie....

To lndm, for now I choose to believe it was a faulty tube!

Thanks again everyone.


baggystevo82 14th March 2006 10:02 AM

If it was brighter too, maybe the heater was shorter and passing more current? My KT88's jangle around a lot at startup with the inrush current into cold heaters. They're quiet as a mouse once they've warmed up and heater current has calmed down though. I had a little 6n1p that was like that, DC resistance of the heater was half that of normal ones, the heater was brighter, and with AC heaters used to hum like a *****, so I never used it again. Given time I'm sure it would have given up the ghost.

It could potentially be oscillations, but it looks like a very commercial product and I would hope they would iron all those problems out before release onto the market, but who knows...


Nordic 14th March 2006 10:14 AM

This document starts off on discussing catastophic, degenerative and subjective failures in tubes... maybe you find something of value...

baggystevo82 14th March 2006 10:19 AM

link doesn't work for me, sound interesting though!

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