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Old 3rd March 2005, 07:59 PM   #1
jjai is offline jjai  United States
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Join Date: Mar 2005
Location: San Francisco, CA
Default Amp pops on power down?

I am using a Sunn Beta Lead - solid state 100 W guitar amp into a single 4 ohm, 12" speaker. I've noticed that when I turn the amp off, there is a very loud "POP." It's very loud - equal to if I hit the muted strings on my guitar with volume at about 7.

I suspect that this pop has now cost me two blown voice coils (I've killed 2 speakers playing at only moderate volumes with this amp since it started popping like this).

Any idea what is happening, and how I might fix it? Does this sound like a bad transistor? Capacitor? I doubt it's the switch because the pop happens a split second after I've switched the amp off.


I don't know if it's related, but I've also noticed that the reverb channels "leak" into each other. There are two channels on the amp with separate EQ and reverb (only one reverb tank though). If I have reverb turned up on one channel, but down on the other, the amp still sounds reverby even if I'm playing on the channel with the reverb level down. Maybe a bad/leaky transistor in the reverb amp?

Everything else sounds great! I'm baffled!

Any thoughts you have to offer would be appreciated, before I start pulling all the caps and transistors on the board!

Thanks a lot!

-jjai
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Old 5th March 2005, 07:34 AM   #2
Enzo is offline Enzo  United States
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Join Date: Sep 2004
Location: Lansing, Michigan
Pops ar quite common, especially on older designs. It slams the output against a rail for a moment. It is no louder than a clipped peak, and probably is not why your speaker failed. It would be more likely in my view that there was some leakage of DC into the speakers.

If you turn down the reverb control for the "other" channel, does it go away from the in-use channel?

Each channel uses both a shunt and a series FET to switch channels off and on, and if you are in the A&B jack, they both are on. SInce you are getting odd channel crosstalk, I must assume you are in the A&B jack, not plugging into the A or B jacks. The footswitch should still turn the channels off and on, but if no footswitch then they both run and you will get reverb from the other channel.

Are you using the post controls to kill the unused side? Or are you channel switching?
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Old 8th March 2005, 08:26 PM   #3
jjai is offline jjai  United States
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Join Date: Mar 2005
Location: San Francisco, CA
Hey Enzo! You're obviously familiar with this amp!

I am, indeed, using the A+B input and using channel switching via the footswitch. When I turn reverb down on the "other" channel, it silences reverb in the active channel.

Thanks for your help!
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