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FullRangeMan 10th September 2012 12:01 AM

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??

TheGimp 10th September 2012 02:54 AM

Bad design.

Midnightmayhem 10th September 2012 03:48 AM

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.

kevinkr 10th September 2012 03:52 AM

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.

DF96 10th September 2012 10:46 AM

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.

lcsaszar 10th September 2012 12:33 PM

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?

TheGimp 10th September 2012 12:38 PM

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.

Evenharmonics 10th September 2012 01:59 PM


Originally Posted by DF96 (
Of course, one might assume that someone who 'demos' cables might not know enough to repair anything anyway.

+1 :up:

20to20 10th September 2012 02:02 PM

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 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.

Yvesm 10th September 2012 02:30 PM


Originally Posted by 20to20 (
. . .
My 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 :down:


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