Peavey KB3 pre-amp blown?

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stevensec
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Joined: Sun Mar 07, 2021 9:25 am

Peavey KB3 pre-amp blown?

Post by stevensec » Sat Apr 17, 2021 12:07 pm

Hoping Enzo or someone else can give me an opinion...

I was playing electric bass into channel 2 on my Peavey KB3 keyboard amp (60 watt), and was using an ART Tube MP pre-amp to boost the input signal. I had the "Level", "Low" and "High" controls all set to about the 12-o'clock position. At some point, I suddenly lost all bass output from the coaxial speaker - - it sounded like I was only getting tweeter sounds. Although I had not been playing at a high volume, I assumed I had just blown the woofer portion of the coaxial speaker:
peavey KB3.jpg
peavey KB3.jpg (148.12 KiB) Viewed 4878 times
And so I disconnected the ART Tube MP, plugged my bass guitar directly into channel 2, and played around with the channel 2 controls. I noticed that I DO still have normal-sounding bass sound, but only if the "Low" tone control is set to the 4:00 position or higher; below 4:00 I get no bass response. The channel 2 "Level" and "High" controls work normally. Channels 1 and 3 work normally.

Do you think I "fried" something in the channel2 low tone control circuit? If so, what is the most likely part? Here is a schematic of the channel 2 pre-amp:
Peavey KB3 Channel 2 Pre-Amp Schematic 2of2.jpg
Peavey KB3 Channel 2 Pre-Amp Schematic 2of2.jpg (90.52 KiB) Viewed 4878 times
Peavey KB3 Channel 2 Pre-Amp Schematic 1of2.jpg
Peavey KB3 Channel 2 Pre-Amp Schematic 1of2.jpg (107.18 KiB) Viewed 4878 times
and here is the spec for channels 1 & 2:
Peavey KB3 Specs.png
Peavey KB3 Specs.png (179.75 KiB) Viewed 4878 times

This amp has been used very lightly, and has never left my basement.

Appreciate your input!!

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dak
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Re: Peavey KB3 pre-amp blown?

Post by dak » Fri Dec 24, 2021 7:52 pm

Well, this is half a year old, but the same opAmp serves high and low tone control, and if the bass comes into being when cranking up the low tone control almost fully, then that's where the VR6 tap comes into balance between the output from U2 and U3. Not a whole lot of possible culprits here: there is rather likely a break in the R6-VR6-R5 path. I consider it unlikely that anything manages to burn off at those voltages, so there is some cold solder joint among those 3 parts, or the VR6 track broke somewhere. If the latter is the case, the return of the bass would occur suddenly rather than gradually at one pot position.

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