A friend repairmen was demo some cables for the owner of a BAT pre + power set, a VK-51SE pre and VK-75SE stereo power amp.
To install the new cables he fullly turn down the remote volume control at the preamp to turn-off both units, but after it the power amp's fuse blow to his surprise.
The authorized dealer inform the volume pot on these units can not be fully down, otherwise the fuse would blow.
So I wonder why the power amp fuse blow??
And how I would identify a tube amp which may have this characteristic??
Thanks
To install the new cables he fullly turn down the remote volume control at the preamp to turn-off both units, but after it the power amp's fuse blow to his surprise.
The authorized dealer inform the volume pot on these units can not be fully down, otherwise the fuse would blow.
So I wonder why the power amp fuse blow??
And how I would identify a tube amp which may have this characteristic??
Thanks
Tubes are different than solid state.
Tubes are different than solid state. You cannot just pull wires out of the back and hook them back up. You need to turn off the unit then swap wires then power back up. The fuse probably saved the amp. Do not ever remove the load from a tube amplifier while running.
Tubes are different than solid state. You cannot just pull wires out of the back and hook them back up. You need to turn off the unit then swap wires then power back up. The fuse probably saved the amp. Do not ever remove the load from a tube amplifier while running.
Always smart to turn off the power amplifier before unplugging the audio input cable to it. The transient that can be generated during such an event can be speaker damaging if the amplifier has sufficient gain, and with RCA jacks and plugs where the ground usually (but not always) disconnects first a very large transient can be generated.
Tube amplifiers in general should always have a load connected to their outputs when powered - this is to protect the output transformer from flyback and other similar transient events.
Tube amplifiers in general should always have a load connected to their outputs when powered - this is to protect the output transformer from flyback and other similar transient events.
Could be missing coupling caps, or missing ground leak resistors, or poor output stage stability. The 'repairman' ought to have known that you never have a valve power amp running with no load connected, signal or no. Of course, one might assume that someone who 'demos' cables might not know enough to repair anything anyway.
I think we have to try interpretting this from Brazilian to English.
It seems he's trying to say that the "repairman" simply turned down the volume on the preamp to kill the signal power through the power amp so he could change the speaker cables without turning off the amps.
My take..so he disconnected the speakers and then the fuse blew.
The dealer told him that if you just turn down the volume to swap speaker cables you would probably blow the fuse.
Much like Midnightmayhem has said.
It seems he's trying to say that the "repairman" simply turned down the volume on the preamp to kill the signal power through the power amp so he could change the speaker cables without turning off the amps.
My take..so he disconnected the speakers and then the fuse blew.
The dealer told him that if you just turn down the volume to swap speaker cables you would probably blow the fuse.
Much like Midnightmayhem has said.
Probably the right scenario, if the amp is marginally stable it may start oscillating without load.
Poor design anyway
Yves.
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?
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.
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.
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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