AB763 Twin - Buzz when installed in cabinet only.

[AndrewM]

Help guys,

This amp is driving me nuts.

I have a late 60's fender twin reverb AB763 that has a buzz with all controls at zero that only occurs when I install the chassis into the cabinet.

The weird thing is that it only appears when you tighten the final bolt... the first three bolts do not effect the buzz and it doesn't matter which one you tighten last. Outside of the cab the amp sounds great....

When I pull the reverb recovery tube (V4) the buzz goes away. I've swapped the tube out but the buzz is still there. t.

I've checked all the grounds and everything appears as per the schematic. Any ideas?

[Jazz P Bass]

Fender used the rear of the pot shells for a ground reference on these amps.
So you have to make doubly sure that the pots and the chassis are clean & tight.
It's possible that something is flexing when you tighten down the chassis.
If that is not it then a wire may be getting moved.
That can be verified with a wooden chopstick with the chassis on the bench.

[Alex R]

also, check the four bolts that hold the mains transformer down.

[g1]

So when it is out of the cab you have the reverb tank connected?
Under the top of the cab there is a metal shield or screen that contacts the chassis when it is installed, could this be not connecting till the 4th screw is tightened?

[MWJB]

A buzz or a hum?

You checked the ground connections? Did you tug at the ground wires terminating at the brass buss? Sometimes these break free and still look connected.

I can't remember what year they started doing it but they hooked the main, screen & preamp filter caps up to the same ground point, this would cause a hum with all controls turned down?

You say that pulling V4 kills the buzz, not pulling V2 or any other tubes? Does any particular control kill the buzz, treble, middle on vibrato channel for example?

You could just turn something up? ;-) The amp won't pass any signal with all the controls down, so it can't be used in that mode.

Line the top lip of the chassis with electrical tape, tighten it up, what happens?

[AndrewM]

Thanks for the advice guys, I'll keep at it.

Definitely more of a buzz than a hum.

I cant get it to happen on the bench via chopsticking.

Pulling V1, V2 or V3 doesn't kill the noise.

The buzz seems to be affected by the reverb pot more than the volume pot. Tone pots don't seem to control it at all.

If I back any one of the bolts off a couple of turns, the level of buzz goes down.

The buzz also happens if I install it in another fender cab.

[Jazz P Bass]

Does the chassis act up if you disconnect the reverb tank?

[AndrewM]

Does the chassis act up if you disconnect the reverb tank?

Buzz is there with or without tank plugged in.

I've just redone the lead dress and gone over the grounds again. The buzz is still present when I install in the cab and happens when I tighten the second screw now... this is bananas.

[Danelectron]

AndrewM, Can you mute the buzz by placing a hand on the cabinet at some location? I have a Fender Deluxe that has a buzz that resonates at certain notes. I can just touch the back panel and it stops. If I loosen the chassis bolts a bit, the chassis leans away from the back panel and it goes quiet. I've been meaning to put thin nylon washers between the back panels, but usually just lean it against the couch. This also tilts it up at me for listening. Just curious because at first I thought I had a buzzing fret.

Edit: Let me add, I built this amp and may have mismounted the chassis creating this buzz.

[Danelectron]

AndrewM, Sorry, I just reread and picked up that all controls were zero'd. Not a vibration issue.

[Dave Curtis, dB AudioTech]

(I) usually just lean it against the couch. Just curious because at first I thought I had a buzzing fret.

I hope you're not blocking too much airflow when you lean it against something. Oddly enough, fret buzz is not nearly as noticeable when you're plugged in & cranked up, compared to unplugged.

[MWJB]

Thanks for the advice guys, I'll keep at it.

Definitely more of a buzz than a hum.

I cant get it to happen on the bench via chopsticking.

Pulling V1, V2 or V3 doesn't kill the noise.

The buzz seems to be affected by the reverb pot more than the volume pot. Tone pots don't seem to control it at all.

If I back any one of the bolts off a couple of turns, the level of buzz goes down.

The buzz also happens if I install it in another fender cab.

How much do you have to turn up the Reverb pot to kill the buzz (measure it in ohms, if it's a nominal amount fit a nwe 100KL pot and insert this resistor between LH tab & ground)? What effect did lining the chassis lip with tape have? Ground wire from V4 cathodes has been pulled about and is definitely soldered fast to the buss? The buzz is there with just V2 V4 & V6 installed?

Get a sheet of aluminium plate, drill it to match the amp strap holes, bolt it down on top of the chassis (you may need some spacers to use the amps chassis bolts?), now does it buzz on the bench? You are driving yourself nuts by hoping you have cured it on the bench, then putting the chassis in the cab to find you haven't...rig up a way to sort/test it on the bench.

It may be bananas....but tell me, what is more bananas? An amp that buzzes in a state that it cannot be used in, or a someone spending days trying to fix a buzz that (hopefully) does not affect the normal performance of the amp? I understand the desire to make it "just so", but assuming this is a 40+ year old amp, what evidence do you have that this wasn't always the case?

[Danelectron]

I hope you're not blocking too much airflow when you lean it against something. Oddly enough, fret buzz is not nearly as noticeable when you're plugged in & cranked up, compared to unplugged.

Dave, No, it is leaned against the corner of one leg so the upper back panel rests against it. Air flow from all around pretty much. It actually only resonates when it is cranked full blast. Which is most of the time.Thanks, for your concern, Dan