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Old 10th September 2012, 05:15 PM   #11
12B4A is offline 12B4A  United States
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In addition to what has been said about hot swapping cables (if that really is what is happening), it's also quite possible that unit has at least one bad 6c33 tube (or is about to go bad).
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Old 10th September 2012, 05:36 PM   #12
20to20 is offline 20to20  United States
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Quote:
Originally Posted by Yvesm View Post
Probably the right scenario, if the amp is marginally stable it may start oscillating without load.

Poor design anyway

Yves.
So, what design, good design, poor design allows this or would keep this from happening?

It seems there is a "blanket" rule that most tubers respect and that is to never run the amp without a speaker load.

So if you "know" (term used loosely) you have a "well" designed amp, that would suggest you could remove signal and then switch speaker cables all day long without a problem. If that's true then the question about any amp that blows the fuse is, why?

Could it be that an amp running tubes at high B+ near all the maximums, feel the sudden high reflected impedance back into the primary as the wire is "nervously" disconnected by a screwdriver and the connection is intermittently made for a few seconds. Does this high reflected impedance cause spikes in the B+ and the quiescent current that may blow a fast blow fuse or one too conservatively rated?

Does this indicate a marginally filtered B+ ?

Just what "poor design" triggers this?
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Old 10th September 2012, 05:59 PM   #13
DF96 is offline DF96  England
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This amp claims no feedback, so loop instability is not a problem. RF instability in the output stage is one possible cause. The cure is to revise the layout, use a better OPT, or add snubbers.

A DIYer might be forgiven for not checking for this, but a commercial product ought to be more careful - especially if people believe, wrongly, that a loadless valve amp is OK provided that there is no input signal.
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Old 11th September 2012, 03:38 AM   #14
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I just phoned him and confirmed, he just turn all down the volume control at the preamp and the power amp fuse blow instantly, of course the power amp goes down by the lack of the fuse.
He did not actually power off any equip or remove any cables.
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Old 11th September 2012, 04:08 AM   #15
20to20 is offline 20to20  United States
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Originally Posted by FullRangeMan View Post
I just phoned him and confirmed, he just turn all down the volume control at the preamp and the power amp fuse blow instantly, of course the power amp goes down by the lack of the fuse.
He did not actually power off any equip or remove any cables.
OK, that is a better description.

Unfortunately, it is also a very strange problem.

Now I'm in TheGimp's camp and think the preamp has a design flaw. It may cause the fuse to blow in any amp. But it surely is not normal.
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Old 11th September 2012, 04:35 AM   #16
20to20 is offline 20to20  United States
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I just looked at the VK-51SE owners manual and there were no warnings or notices about the volume control minimum limits.

Call the factory.

Last edited by 20to20; 11th September 2012 at 04:40 AM.
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Old 11th September 2012, 06:53 AM   #17
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I was just informed now, this amp also had a poor contact on a 6C33 socket(it use the cheap claw pin stardard septar socket) sometime after this incident, so maybe this prob is explained now.
I appreciated all yours expert advices.
Thanks
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Last edited by FullRangeMan; 11th September 2012 at 07:04 AM.
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