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Old 7th June 2009, 05:33 PM   #1
Minion is offline Minion  Canada
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Join Date: May 2006
Location: Cowican Bay , vancouver island
Default Feedback problem with Guitar amp ....

Hi folks , I recently built a Guitar amp and it sounds freaking amazeing but yesterday I started getting a Bad intermitant feedback problem .....

What happens is that it sounds Just fine and is dead quiet when not playing any notes but then suddenly i get a loud hum and unyielding feedback and after a few minutes it goes away and sounds awesome again but it comes back eventually....

The amp is a "PAIA Stack In a Box " Tube preamp kit , with a 15 band eq after it going into a TDA7293 Chip ....

here"s the schematic for the preamp ....

http://www.paia.com/prodimages/siabsch.pdf

Does it sound like a Bad Tube or something ?? The tube only has about 25 hours on it .....

any help would be great .....

Thanx
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Old 8th June 2009, 12:02 AM   #2
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Join Date: Jun 2005
Location: Fort Collins Colorado
Thermal problem? how hot is it in there?
Does it do it with the guitar volume off (on the guitar?)
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Old 8th June 2009, 01:38 AM   #3
Minion is offline Minion  Canada
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Join Date: May 2006
Location: Cowican Bay , vancouver island
I don"t think it is 2 hot , the chassis is quite large with lots of ventilation and the Tube only runs off of a 45v Plate voltage and the TDA7293 Chip is on a 8in x 8in x 2in Heatsink and barely even gets above ambient temp ......

There is no feedback with the Volume on the guitar off but there is a lot more hum than usual .....

What I am suspecting is because the Plate voltage is created useing a Cmos hex inverter as a charge pump that maybe the Plate Voltage is suddenly droping which causeing the 12ax7 to overload a lot more than usual which is causeing the feedback .....

Maybe there is a Bad cap in the charge pump which is causeing the Voltage to drop ..... ??

I wonder if I can use the +/-42v DC that goes to the TDA7293 to power the Plate ?? that way I can get 84v for the Plate and get more headroom and gain ....

Thanx
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Old 9th June 2009, 05:58 AM   #4
Enzo is offline Enzo  United States
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Join Date: Sep 2004
Location: Lansing, Michigan
SOunds to me like your input jack has an intermittant ground connection.
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Old 9th June 2009, 11:13 PM   #5
Minion is offline Minion  Canada
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Join Date: May 2006
Location: Cowican Bay , vancouver island
I wish it were that simple , but nope , all solder joints and connections are fine , I can even Bash the amp and the Guitar with a Hammer and it dosn"t Cause the Problem to happen or to stop happening .....


Cheers
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