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'5f2 sqealing'
Author:Audiosalvage (registered user: 282 posts )
Date: Tue, Jan 10th, 2017 @ 18:38 ( . )

Hi folks, I have an issue with a new build. Ive done this amp before with success but am experiencing a high pitched oscillation similar to an old TV set. It comes on whether or not guitar is plugged in and not effected much by vol control. I've tried swapping OT wires and preamp tubes. I've poked around with the wires with no change. I used the hoffman layout and all components are new except the tubes. PT is used and it gives me a B+ of 256v ( a bit low but clean volume not my concern ). Does anyone have any tips?

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'5f2 sqealing'
Author:Audiosalvage (registered user: 282 posts )
Date: Tue, Jan 10th, 2017 @ 21:23 ( . )

Here's a pic [link]

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'5f2 sqealing'
Author:sajyho (registered user: 162 posts )
Date: Wed, Jan 11th, 2017 @ 10:14 ( . )

It's funny that I had the same issue with my new 5f2a a month ago, it's basically a 5f2a except the power tube that is a 6AQ5. I tried everything; changed all the parts including all the tubes, changing the layout, cleaninig all the jacks with electronic cleaner spray,etc... and the high pitch noise and cracking overdrive is still in there... I gave up the amp totally!
I searched the whole web and found out most of the people say it is because of a damaged/old tube, but in my case it seems it wasn't...

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'5f2 sqealing'
Author:Audiosalvage (registered user: 282 posts )
Date: Wed, Jan 11th, 2017 @ 16:23 ( . )

Another thing I've tried is putting a 6k6 tube in there and the sqealing stops........until you plug a guitar in, then it continues. I think this means that this tube has less power and less tendency to create oscillation.

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'5f2 sqealing'
Author:Audiosalvage (registered user: 282 posts )
Date: Wed, Jan 11th, 2017 @ 17:57 ( . )

Wow, tough crowd. usually I'm bombarded with helpful advice. Anyhow, I removed NFB wire and the problem stopped. Don't need no stinkin' NFB.

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'5f2 sqealing'
Author:steve25 (registered user: 2829 posts )
Date: Thu, Jan 12th, 2017 @ 07:23 ( . )

Reverse the wires on the primary of the OT and hook the feed back up again , is the amp then good?

It takes 3 gain stages for the signal to pass thru to have the first stage and 3rd stage in phase either each other and it's between these two stages that the amps layout can do you in with Oscillation issues if it's not the hook up of the OT that I posted about before.
Please report back when you have swapped the OT wires.

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'5f2 sqealing'
Author:Audiosalvage (registered user: 282 posts )
Date: Thu, Jan 12th, 2017 @ 21:17 ( . )

Ok. good now. Thanks Steve, I thought this phenomenon was only true of push pull circuits.

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