Help! Flash-over on my amp :-( - diyAudio
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Old 16th February 2009, 06:09 PM   #1
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Default Help! Flash-over on my amp :-(

Yikes!
I've just had a flash-over on my amp, an Integrated World Audio Designs KIT6550 amp. It has 2 channels, each comprising class-A push pull 6550s, and 2 5U4G rectifiers in the PSU.
The amp has been working fine, without any crackles or anything, for years. I turned it on this evening, and as it warmed-up there was a blue flash from one of the 6550 valves. I quickly switched it off, then after a few moments, tried it again - this time, after ~10 secs, there was a flash from the top of one of the rectifiers. As I turned it off, the sound started to come through (I think it was on both channels), so maybe the o/p TXs have survived.
What do you guys think may have caused this (what checks can I make)? and what do you think is the best way forward (I have no problem with replacing all the valves - they must be 6 years or more old by now)?
Cheers!
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Old 16th February 2009, 07:21 PM   #2
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This seems to me like a classic case of a tube gone bad. The first place to start is to replace the bad 6550, or both. Simply testing it may not show anything because the test voltage won't be high enough. The output transformer is most likely OK since you didn't leave it run long. The arcing in the 5U4 is probably a result of it seeing a load on the circuit from the bad tube. It may still be OK because tubes are somewhat forgiving of short term abuse.

Be sure to keep an eye on things and your hand on the switch when starting up with the new tube(s). Unfortunately todays tubes lack the durability of the old time stuff. But after six years of service I don't think they owe you anything. Any smaller tubes will probably still be good. It's the output tubes that are under the most stress and fail first. Make certain that any fuses are of the correct size and type.
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Old 16th February 2009, 07:25 PM   #3
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Default Re: Help! Flash-over on my amp :-(

Quote:
Originally posted by Beelzebub
What do you guys think may have caused this (what checks can I make)? and what do you think is the best way forward (I have no problem with replacing all the valves - they must be 6 years or more old by now)?
Cheers!
Had that happen once with an 807. Internal arcing at power up. Once it settled down, you could see the thyratron-like blue glow filling the entire space between the cathode and plate. No doubt about what caused that. Gassiness usually manifests itself with bias that won't settle down, with a rising plate current, with or without the characteristic blue glow.

As for the 5U4, perhaps the flashover took that out too (my 807 project uses a SS power supply that can stand up to current surges better than hollow state devices). Looks like it's time to replace some VTs.
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Old 16th February 2009, 07:52 PM   #4
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Thanks guys,

Looks like I'll need to replace the 6550s and the 5U4Gs - they're all the same age, so now all are suspect.

What do you reckon is the best brand these days? and would putting KT88s in instead make any difference?

Cheers!
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Old 16th February 2009, 09:46 PM   #5
m6tt is offline m6tt  United States
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I had a solder glob between B+ CT on the OPT and ground. It was a filmosound "ship in a bottle" build, so I couldn't see it or dislodge it easily. Turned it on and saw the entire 5y3 redplate amongst the blue mist. The entire plate structure, pins, etc were cherry red and getting brighter by the time I hit the off switch. 1sec-2sec of overload after the filament warmed up, max. That 5y3 still works pretty well. Nice for dropping the B+, it has a helluva voltage drop now . Your 5u4 is probably not so bad, but I would check the B+ is in spec and maybe get a new one for reliability. If you do, you could keep the old one and get good service from it as a "bench" tube.
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Old 16th February 2009, 11:11 PM   #6
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Quote:
Originally posted by m6tt
I had a solder glob between B+ CT on the OPT and ground. It was a filmosound "ship in a bottle" build, so I couldn't see it or dislodge it easily. Turned it on and saw the entire 5y3 redplate amongst the blue mist. The entire plate structure, pins, etc were cherry red and getting brighter by the time I hit the off switch. 1sec-2sec of overload after the filament warmed up, max. That 5y3 still works pretty well. Nice for dropping the B+, it has a helluva voltage drop now . Your 5u4 is probably not so bad, but I would check the B+ is in spec and maybe get a new one for reliability. If you do, you could keep the old one and get good service from it as a "bench" tube.
It's amazing how tough these things are! Transistors and diodes would go in a flash
I think it would be best to 'retire' the rectifiers and output valves, they have done sterling service over the years
I might put in some KT88s, just to see if there is any difference in the sound - There doesn't seem to be much difference in price, although it will have to wait till pay-day, as it is going to cost a bit

Hmm... Just had a thought - one of the remaining 6550s could be pressed into service for a single-ended class A project - oh dear, more time and expense ha ha ha!
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