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Old 10th September 2012, 12:01 AM   #1
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Default Lowering the Volume Control on Tube Amps

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
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Old 10th September 2012, 02:54 AM   #2
TheGimp is offline TheGimp  United States
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Bad design.
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Old 10th September 2012, 03:48 AM   #3
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Default 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.
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Old 10th September 2012, 03:52 AM   #4
kevinkr is offline kevinkr  United States
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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.
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Old 10th September 2012, 10:46 AM   #5
DF96 is offline DF96  England
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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.
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Old 10th September 2012, 12:33 PM   #6
oshifis is offline oshifis  Hungary
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The dealer might believe in occultism, but the volume control on the preamplifier has nothing to do with the blown fuse on the power amplifier.
BTW, was that the speaker protecting fuse, or the mains fuse?
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Old 10th September 2012, 12:38 PM   #7
TheGimp is offline TheGimp  United States
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If he turned the volume down, AND turned both units off, there is no reason for the fuse to blow.

The dealer stated that the unit should NOT have the volume turned all the way down or the fuse will blow.

This is thus a known condition, and therefore a bad design.
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Old 10th September 2012, 01:59 PM   #8
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Quote:
Originally Posted by DF96 View Post
Of course, one might assume that someone who 'demos' cables might not know enough to repair anything anyway.
+1
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Old 10th September 2012, 02:02 PM   #9
20to20 is offline 20to20  United States
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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.
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Old 10th September 2012, 02:30 PM   #10
Yvesm is offline Yvesm  France
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Quote:
Originally Posted by 20to20 View Post
. . .
My take..so he disconnected the speakers and then the fuse blew.
. . .
Probably the right scenario, if the amp is marginally stable it may start oscillating without load.

Poor design anyway

Yves.
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