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Author Topic: Gallien Krueger 800RB  (Read 529 times)

ilyaa

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Gallien Krueger 800RB
« on: July 22, 2019, 03:22:39 PM »
frustrating issues -

this amp has blown a few woofers - a pair of 10s in a 4x10 and a beefy EV 15 - it blew the 10s after about an hour and a half or so and blew the 15 in under a minute -

problem is, on the bench, it has no measurable issues. no DC on the output - shows me a perfect sine wave at full power on the scope - runs for as long as youd like with no visible problems.

im hesitant to plug it into an actual speaker because i dont wanna blow anything - any ideas what could be happening?

i asked the client about cabs and speaker wiring - the cabs and speaker cables were being used with other amps with no problems.

hmmmm....

phatt

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Re: Gallien Krueger 800RB
« Reply #1 on: July 22, 2019, 10:07:38 PM »
Just use a resistive load in place of speaker then you can safely measure the DC on the output.
Phil.

ilyaa

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Re: Gallien Krueger 800RB
« Reply #2 on: July 23, 2019, 03:03:24 AM »
i am - thats how ive been testing it -


phatt

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Re: Gallien Krueger 800RB
« Reply #3 on: July 23, 2019, 09:17:04 AM »
Sorry if i misunderstood that point.
Maybe use different loads like  4 Ohm then try 16 Ohm to see what makes it go crazy DC. I vaguely recall an amp years back that did crazy things ,, but found the Zobel network was burnt open causing strange DC problems.
Replaced and all was good.  Just a thought :-\
Phil.

Jazz P Bass

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Re: Gallien Krueger 800RB
« Reply #4 on: July 23, 2019, 11:36:01 AM »
The amp may be unstable & is oscillating at a high frequency (above hearing)
A scope should show that.

g1

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Re: Gallien Krueger 800RB
« Reply #5 on: July 23, 2019, 01:23:56 PM »
A beefy EV still might not handle 300W and this amp will do that easily into 4 ohms.
Is he pushing it to clippping?
The fact that it only blew 2 out of 4 tens makes me think over-powering rather than some DC fault.
Autopsies of the voice coils can be instructive.

ilyaa

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Re: Gallien Krueger 800RB
« Reply #6 on: July 23, 2019, 01:58:45 PM »
let me see if i can get my hands on the blown speaks -

how would overpowering vs. a DC fault look differently on a blown voice coil?

g1

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Re: Gallien Krueger 800RB
« Reply #7 on: July 24, 2019, 01:45:22 PM »
A voicecoil overheated from overpowering will be fairly uniformly roasted.

A DC condition will push the coil in or out of the gap, leaving part of the coil exposed.  The exposed part will cook first, leaving part of the coil cooked, and part of it intact.

Others here may have other comments, but that is probably the first thing I would look at as far as the coil.

phatt

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Re: Gallien Krueger 800RB
« Reply #8 on: August 05, 2019, 06:30:57 AM »
A voicecoil overheated from overpowering will be fairly uniformly roasted.

A DC condition will push the coil in or out of the gap, leaving part of the coil exposed.  The exposed part will cook first, leaving part of the coil cooked, and part of it intact.

Others here may have other comments, but that is probably the first thing I would look at as far as the coil.
Agre+1,,, Well explained. :tu:

ilyaa

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Re: Gallien Krueger 800RB
« Reply #9 on: August 19, 2019, 03:42:15 PM »
Finally got my hands on the speaker -

Autopsy results point to DC condition -

https://ibb.co/VjKdSzD
https://ibb.co/9WjmPJ5
https://ibb.co/TR55dgC

Agreed?

If the Amp is testing fine on the bench - any thoughts as to how I’d go about tracking this DC failure down??

phatt

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Re: Gallien Krueger 800RB
« Reply #10 on: Today at 02:22:45 AM »
Well have you checked the voltages on the schematic?

https://elektrotanya.com/gallien-krueger_800rb_sm.pdf/download.html

If that 5 Ohm resistor at R57 is running hot then I believe that is a sign that there is Hi Freq instability.

What is the DC reading at idle?
Then how much does it drift as you increase power?
I'd haz za guess the issue is early in the signal path.
If C11 is stuffed that may send it into oscillation.
A scope here would be the best way to tell you what is going wrong.

Also don't forget to keep a keen eye on simple things like dried /cracked solder pads.
Phil.

 

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