Tube amp makes noise on shutdown

Archived from groups: rec.audio.tech (More info?)

Neil,

Is this a new occurrence for this amp, or a new amp to your system? It
is quite possible that some amps will put out strange stuff as the power
supplies come down, and most tube amps don't have protection relays to
disconnect the speakers at power down. It would also be helpful if you
mentioned what amp and preamp you are using. Lastly, it could be the
preamp powering down in a less than graceful manner. (Try shutting off
the power amp a couple of minutes BEFORE shutting off the preamp.)

Regards,
Tim Schwartz
Bristol Electronics


Neil Peters wrote:
>
> There is no signal going to the amp and it's time to shutdown until
> another day of listening arrives. Flip the power switch to off like I
> always do EXCEPT this time after about five seconds or so there is this
> brief sound out of the speakers. Sound lasts about two seconds and
> sounds like a sine wave sweep down in Hz and amplitude.
>
> BBBBBEEEEEoooooooppppp p p
>
> How bad is this? Can it be fixed?

Archived from groups: rec.audio.tech (More info?)

Hi,
If this amp has always done this, I would not worry. Some amps employ
both current and voltage feedback to enhance the damping factor and
linear frequency response. This is normal in some amps as the different
stages loose power upon shutdown. But, if the amp has never done this
before, it could be foretelling a problem that is starting to rear it's
ugly head. A lot of modern amps use all kinds of solid state regulators
and such in their power supplies and some of these can cause some
strange oscillations as the amp initially powers on and when it is
powered off.
Bill B.

Neil Peters wrote:

> There is no signal going to the amp and it's time to shutdown until
> another day of listening arrives. Flip the power switch to off like I
> always do EXCEPT this time after about five seconds or so there is this
> brief sound out of the speakers. Sound lasts about two seconds and
> sounds like a sine wave sweep down in Hz and amplitude.
>
> BBBBBEEEEEoooooooppppp p p
>
> How bad is this? Can it be fixed?

Archived from groups: rec.audio.tech (More info?)

It sounds like an instability in a feedback circuit. But, I have
actually seen a bad tube cause this. The only way to really find out for
sure is to do some tests on the amplifier. It is probably nothing too
big. My only concern would be if the amp is producing a high frequency
oscillation you can not hear when it is on, and that oscillation only
becomes apparent when the amp is turned off and the voltage dropping is
allowing the oscillating problem to drop enough in frequency so it
becomes audible. It would not be a bad idea to have the amp checked out
with a signal generator and scope. If you can not find anyone locally to
work on it, I service tube audio here in north Florida. People send me
amps no one else can fix from all over the world. You can check me out at:
http://home.alltel.net/wbittle1

Ozzy 2005 wrote:

> Neil Peters wrote:
>
>> There is no signal going to the amp and it's time to shutdown until
>> another day of listening arrives. Flip the power switch to off like I
>> always do EXCEPT this time after about five seconds or so there is this
>> brief sound out of the speakers. Sound lasts about two seconds and
>> sounds like a sine wave sweep down in Hz and amplitude.
>>
>> BBBBBEEEEEoooooooppppp p p
>>
>> How bad is this? Can it be fixed?
>
>
> Thanks guys!