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How can this Happen :(

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Posted on October 23, 2014 at 11:41:53
Nickel Core
Audiophile

Posts: 870
Joined: February 11, 2005
 Hi All,

Two days ago I switched off my phono stage while it was connected to my amp with the volume at 90%. Normally I turn the volume down, because switching off the pre-amp, always results in rather hefty cone movement and a low frequency 'plop' through my speakers.

This time the same, but now the 'plop' was followed by strange noises trough one channel and finally the destruction of the 83 rectifier of the PSU feeding one of my 300B power tubes. Also a fuse tripped.

It appeared both the 83 and the 300B died (filaments open). Of course a NOS RCA 83, and a EML 300BXLS :(. Luckily my WE 300B where not in the sockets...

Apparently a large current was induced to flow trough the 300B resulting in this disaster. Replacing both the 83 and 300B proved to be enough to get the amp running again with no damages to OPT or other components.

But what happened here? Why would a very low frequency (but large amplitude) signal on the input of the amp cause one channel to go KABOOM this way? And how to prevent this from happening again?

Thanks,
NC

 

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RE: How can this Happen :(, posted on October 23, 2014 at 13:28:16
Lee of Omaha
Dealer

Posts: 1800
Location: Omaha NE
Joined: September 8, 2006
 83s can take pretty ridiculous short-term overloads without damage. As both your 83 and 300B are open, it appears that some humongous voltage got on the filaments.

Or (guessing) your power supply discharged through the secondary, inducing a huge voltage on the 5V winding? I guess that could happen if the 83 shorted.

 

RE: How can this Happen :(, posted on October 23, 2014 at 13:31:23
Al Noakes
 OUCH ! No idea exactly what caused this but I would power the power amps off first in future .

Al

 

RE: How can this Happen :(, posted on October 23, 2014 at 14:02:55
Nickel Core
Audiophile

Posts: 870
Joined: February 11, 2005
 Hi,

I use separate transformers for the filament voltages, so your second scenario could not happen.

NC

 

Also...., posted on October 23, 2014 at 14:05:55
Nickel Core
Audiophile

Posts: 870
Joined: February 11, 2005
 The fuse that tripped was part of the B+ supply suggesting that a large current flowed though the B+ supply and not through the filment supply.

NC

 

RE: How can this Happen :(, posted on October 23, 2014 at 16:11:30
6bq5
Audiophile

Posts: 4385
Location: SF Bay
Joined: August 16, 2001
 I was taught to turn on the Pre-Amp first - then turn on the power amp,
and when powering down, to reverse the order - Power amp first and pre-amp second.
IIRC - it is because - unless the Pre-Amp is capacitor coupled - there can be a DC transient that will flow through the outputs - and artifact of the pre-amp dissipating - I am not sure if the REASON is correct - however I have maintained the practice (irrespective of the Pre-amp output coupling) and not had a problem yet - KNOCK ON WOOD....
Sorry to hear of your loss-
Happy Listening

 

RE: How can this Happen :(, posted on October 23, 2014 at 16:32:51
Paul Joppa
Industry Professional

Posts: 7295
Location: Seattle, WA
Joined: April 23, 2001
 A huge positive spike into the 300B might have resulted in a grid-cathode arc? I can see a cascading disaster if that happened, but not sure how hard it is to make it happen...

 

RE: Also...., posted on October 23, 2014 at 20:31:52
Triode_Kingdom
Audiophile

Posts: 10044
Location: Central Texas
Joined: September 24, 2006
 What is the mA capability of your B+ supply? Is it enough to burn out the filament if the tube saturates from positive grid voltage? I know that seems extremely unlikely, but I have to ask.

 

RE: Also...., posted on October 24, 2014 at 00:39:04
Nickel Core
Audiophile

Posts: 870
Joined: February 11, 2005
 HI,

That must be 10A easily, for surges maybe more...

I specified the B+ transformer to have very low DCR which can only be achieved by using thick wires so a large current capability.

I also feel the grid has become positive at some point. But how? I use fixed bias via again a separate supply.

NC

 

RE: How can this Happen :(, posted on October 24, 2014 at 03:09:20
sony6060
Audiophile

Posts: 1465
Location: USA
Joined: August 8, 2014
 I would guess an internal arc in the 300B tube caused an over-current condition thru the 83 tube. Further, the stored energy in the power supply capacitors arcing to ground was like a 'shot gun blast' in that 300B.

Some use a 50 ohm resistor in-between the last capacitor and the audio transformer to help quench internal arcs, but no guarantees of no damage in any tube with an internal arc.

 

EML 300BXLS , posted on October 24, 2014 at 03:15:33
Nickel Core
Audiophile

Posts: 870
Joined: February 11, 2005
 Hi,

The 300BXLS accepts a maximum plate voltage of 550V. I don't see an arc happen with in this tube, normally running at 425V. Always possible of course, but still....

NC

 

RE: How can this Happen :(, posted on October 24, 2014 at 05:03:51
Stephen R
Audiophile

Posts: 1428
Joined: January 11, 2002
 I have mute switches on the input of the power amps so I can mess about with cables/connectios and turn things on/off without big bangs.

Before my pre was tx out (no thumps now) I had a mute switch on the output. A number of manufacturers do this like ARC with a delay circuit and voltage sense so the output mutes before things can go bang ot you can tie the output mute into the power switch so off will always mute instantly.

cheers,

Stephen

 

The funny thing is that my pre is TX out... (nt), posted on October 24, 2014 at 08:01:02
Nickel Core
Audiophile

Posts: 870
Joined: February 11, 2005
 ...

 

RE: Also...., posted on October 24, 2014 at 08:42:54
Triode_Kingdom
Audiophile

Posts: 10044
Location: Central Texas
Joined: September 24, 2006
 10A B+?? Holy moley! Well, it sounds like whatever was output from the pre when you switched it off was high enough amplitude and duration to push the grid of the 300B positive. Wow, I never would have imagined a B+ supply could burn out a DHT filament. If this is heated with a CT filament transformer, you might consider installing a small fuse in the connection between the CT and ground (or between the hum pot/cathode resistor and ground). I would also scope the output of the pre when its connected to the amplifier to see if it's oscillating during power-down.

 

Your amp is not DC coupled, is it? (nt), posted on October 24, 2014 at 12:19:49
Chip647
Audiophile

Posts: 2649
Location: The South
Joined: December 24, 2012

 

No, (nt), posted on October 25, 2014 at 00:51:00
Nickel Core
Audiophile

Posts: 870
Joined: February 11, 2005
 .

 

RE: How can this Happen :(, posted on October 25, 2014 at 02:12:50
Joao
 Sorry to read this...

Think the problem is the EML300B (internal short)- I'd recommend to call Jac and ask him if his engineers can have a look into the tube in the factory. Unfortunately it happens when you did not turn off the volume level (it would happen when you turn down volume as well).

BTW, you know that the XLS-version of the EML300B is a different design and not comparable to the WE300B. Jac told me it's a powerful version and the filament current is quiet higher.

 

RE: EML 300BXLS , posted on October 25, 2014 at 06:25:19
amnesiac
Audiophile

Posts: 717
Joined: August 21, 2002
 Is one reason why I don't buy new tubes. I cant afford to lose them. I feel for you mate. I wonder if it was a old tube would it had failed the same way. I burnt a 30$ dac board the other day with similar circumstances. Havent checked if it the power supply or output amp.I just changed dacs. I suspect it was the output though. It was DC coupled.

I cant imagine any modern tube manufacturer having anywhere near the R&D budget the old ones had. Especially where reliability testing is concerned.

 

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