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Post subject: Blackface vibrato problem
Posted: Sun Dec 09, 2012 11:12 am
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My 1964 Concert has developed a problem with the vibrato. When I switch it on there is a delay of anywhere from 5 to 30 seconds before the vibrato effect starts working. The amp is fully warmed up. I thought it might be a problem with the foot switch but there was no change when I tried another switch. Once the effect starts working it doesn't fade or cut out after that. I've also noticed that the vibrato level isn't really noticeable until the intensity level is turned up to about 5 or 6.. Any thoughts?


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Post subject: Re: Blackface vibrato problem
Posted: Sun Dec 09, 2012 12:30 pm
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Location: Natural Bridge, Virginia
When were the tubes last replaced?

http://ampwares.com/schematics/concert_aa763.pdf

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Post subject: Re: Blackface vibrato problem
Posted: Sun Dec 09, 2012 12:48 pm
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The power tubes were replaced a few months ago. The rest are older. I tried replacing the 12AX7A that controls the vibrato with another used tube and the problem persisted.


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Post subject: Re: Blackface vibrato problem
Posted: Sun Dec 09, 2012 12:55 pm
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Possibly bad intensity pot or photo resistor.

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Post subject: Re: Blackface vibrato problem
Posted: Sun Dec 09, 2012 7:29 pm
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May want to check the voltages on that section. Maybe you have a tired filter cap(s) supplying the power. When were the filter caps last replaced?

Also, the idle bias on the 12AX7 maybe a tad low. Indicating the bypass caps or bias resistors maybe out-of-spec.

Just a WAG... Here's the AA763 circuit schematic:

http://www.webphix.com/schematic%20heav ... _aa763.pdf


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