Help with Fender Custom Vibrolux reverb squeal

Dennis Waltman · From: Wisconsin, USA

This amp was built in 1999. The squeal is only present with the reverb turned up past 5, is present in all channels goes away at zero on knob. I've tried two new MOD brand reverb tanks both two and three spring, tried grounding between tube chassis and either outer RCA jack on tank to no avail. I've also swapped the position 4 tube (from left viewing from back of amp) with a new tube, removed and plugged back in all tubes in an attempt to clean connections.
Any ideas on what to check?

Thanks, DW

Stephen Cowell · From: Round Rock, Texas, USA

Make sure your reverb tank is in a bag... I have wrapped them in bubble wrap before, that works great if you can stand the look. The cardboard piece across the bottom is important too... generally it needs to be acoustically isolated from the speakers.
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Dennis Waltman · From: Wisconsin, USA

Thanks,
I am able to reduce the tendency for it to produce this squeal when I have the reverb tank outside of the cabinet. I do need to add that this amp has a 15 inch Black Widow installed instead of the stock 2 10 inch speakers. Makes me wonder if the speaker magnet is effecting the tank.

Jim Sliff · From: Lawndale California, USA

i know this is going to seem ridiculous based on what you have posted so far, but.....

Have you reversed the reverb cables at the tank? That's the first thing we do t when reverb squeal is encountered in any amp with spring reverb - and fixes it 75% of the time.

OK, assuming you did already - please humor me and do it again.

Thanks! Now it's fixed. Or you feel dumb for listening to me. I don't. Very Happy

OK, NOW

... replace V3 with a new 12AT7. The recovery tube - the 12AX7 you already swapped out - is rarely the cause of squeal, but the driver tube is the second most common source, usually because of unwanted microphonics. There's not another in the amp - you should have a spare of every tube, so buy a new one if needed. I don't suggest a swap in vintage Fenders to "test" this tube anyway, as the other in vintage Fenders is the driver tube and may be also be microphonic without you noticing.

If those "fixes" don't solve it there's a possibility it could be interference from the speaker magnet - but I seriously doubt it, especially if removing the tank from the cab only "reduces" (but doesn't eliminate) the squeal.

You would be advised at this point taking it to a tech. The reverb transformer, other components in the circuit and all voltages should be checked/tested - all beyond DIY level unless you know what you're doing - and especially understand amp safety..
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Dennis Waltman · From: Wisconsin, USA

Thanks Jim ,

I had not heard of reversing the RCA leads at the tank but just tried it. No reverb at all with the leads reversed. I'll have to buy a 12AT7 for the 3rd position and see if that does the trick.

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