Presence Squeal On '69 Superbass Build

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jckid649
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Presence Squeal On '69 Superbass Build

Post by jckid649 » Mon May 21, 2018 3:30 am

I recently finished a Superbass build using the Metro build guide and parts from Valvestorm. While, I've read tons about others who have had this problem none of the things that worked for them is working for me. I have the high pitched squeal that comes when either the presence or treble is dimed out at a considerable volume setting. It's not as big of a problem when the guitar is not plugged in. It will manifest if the master volume is set above 7 or 8 though. The bigger problem is when the guitar is plugged in. If the master volume and channel volume is set to 10 and you begin to roll up the presence it quickly begins to squeal. Usually be 3 or 4 on the presence control.

I have checked every conceivable thing. I have metered every cap, preamp tube, power tube, and every other part that should or should not carry any voltage. Everything reads perfectly normal. I have verified it with a tech friend of considerable experience who also says he sees no problem with any of the readings. The 5.6k resistors are in place on all power tubes. The presence wire has been routed multiple different ways with no effect. I even tried a shielded wire on the presence pot. No effect. I removed the master volume and rewired everything without it in place. No effect. I have twisted every kind of combination of wires throughout the amp to prevent oscillation. No effect. I have checked all grounds, lead dress, solder joints, and possible mis-wiring. Found nothing. I checked for reversed primaries. Swapped the wires and got a horrible howl. So I swapped them back. Also, nothing is touching or shorting anywhere.

There is only one thing that completely stops it altogether. Removing the wire from the presence pot and off of the board. That 100% eliminates the problem. Unfortunately, the amp loses a lot from having no presence control. So that's not a viable option to leave it out. Since this is the only thing has any affect I have to think that it must be here. But I have no idea what the problem is. Currently I do not have a cabinet as I am waiting for Sourmash to finish it. So shielding could be a problem. But it may be several more weeks to determine that. I did try the aluminum foil on a wooden board trick. No effect. I have been told that doesn't always work because some of these high powered Plexi's need a good thick piece of metal for proper shielding. I can say that moving the chassis around does alter the sound of the noise. Lifting it off the 4x12 cabinet it is sitting on and flipping it upside down or at different angles alters the sound but does not eliminate it.

I and my tech friend, who has many many more years of experience and knowledge of electronics and amplifiers than I, are completely at a loss at this point.

Perhaps I should go to a hardware store and have a good thick piece of metal cut and fit it on the chassis to cover it to try that but other than that I don't know what else to try.

Tazin
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Re: Presence Squeal On '69 Superbass Build

Post by Tazin » Mon May 21, 2018 8:18 pm

It's not not uncommon for Super Leads to have that squeal when there out of the shell but, you don't hear about that too much with a Super Bass. Additionally, the squeal tends to start a little later when the Presence is turned up to around 7 or higher. Anyhow, I'd wait until you get the head cabinet to elimate lack of shielding as the possible problem.

jckid649
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Posts: 10
Joined: Thu May 28, 2015 12:10 am
Just the numbers in order: 13492

Re: Presence Squeal On '69 Superbass Build

Post by jckid649 » Mon May 21, 2018 8:27 pm

I actually went to a local shop today that builds sprayers for tractors and they were able to cut me a good piece of sheet metal that I applied for a shield. This appears to have solved my problem. Thanks.

Tazin
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Posts: 727
Joined: Fri Jan 19, 2007 8:54 pm

Re: Presence Squeal On '69 Superbass Build

Post by Tazin » Mon May 21, 2018 8:46 pm

jckid649 wrote:
Mon May 21, 2018 8:27 pm
I actually went to a local shop today that builds sprayers for tractors and they were able to cut me a good piece of sheet metal that I applied for a shield. This appears to have solved my problem. Thanks.
Amps can be finicky!

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