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'noise and squeal'
Author:mategenomics (registered user: 50 posts )
Date: Mon, Mar 20th, 2017 @ 04:27 ( . )

Hi guys,

Out of blue my amp started doing strange things. I have left in a room in my apartment and I havent played it for like a month or have not even been moving it or touching so we can rule out any mechanical stress or environmental issues. The amp is not amplifying any signal, when I turn it on it is doing hiss and slight hum that reacts to volume and gain controls so we can rule out output section too I think.
When gain is maxed past 3 o clock it even starts to squeal with a high pitched sound.

I already experienced this and identified it as broken or microphonic tube but I already swapped both the tubes for new and it is still the same.

I have to admit I have not tried to open it up and measure the components or voltages, maybe you would give me some clue before I start checking everything.

So tu sum it up - I doubt it is the tubes and I doubt it is a lead dress - before this the amp has been dead quiet when playing and I was not moving it anywhere.

many thanks in advance.

metagenomics

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'noise and squeal'
Author:steve25 (registered user: 2847 posts )
Date: Mon, Mar 20th, 2017 @ 07:00 ( . )

Place a clip lead short across each preamp tube plate load resistor ( no plate load resistor = no gain ) but for the PI section, when the noise stops you've found the gain stage with the issue, or it's the next one back up stream towards the input jack.

At that point I would start confirming that any Cathode bypass caps have not shorted and in the same vein you do not have any failing power supply filters ( starting to short out) and yes new parts like electrolytic cans can go south from not being used!

Another way to go is to to set the amp volume wise so it just acts up, and then use a screw driver to to do a grid tap test as you progress back up to the input jack, the taps should make louder and louder crackles untill you get the failed gain stage.

Just two days ago I turned on one of my Bass amps that I had not used in over a year and it blew a fuse within 10 seconds of being on!
No that this relates to your issue, but always test your gear before heading out to a gig!!

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