Reverb tank at a 45° angle inside of an amp head?

[joulupukki]

I recently built two 1-channel AB763 amps. I also built one combo cab and one head cab to house them in. Both of them do fine in the combo cab. But, one of them (the first one I built with a stand-up PT) squeals a bit when you crank the volume and reverb up too much. It’s a reverb tank proximity issue. If the tank is out of the cab it’s fine to be dimed.

V2 of this amp doesn’t have any squealing issues but definitely induced hum. However, if I rotate the reverb tank 45° at the bottom front of the head cab, the hum almost disappears entirely. Would it be weird to mount it permanently this way?

 

[chas.wahl]

I'm not seeing 45 degrees; looks like it's 90 degrees from "open side facing downward" to me. Tanks are nominally constructed to mount in some particular optimum orientation that's either open side down, open side up, open side to front or back, or even (like in some early Ampeg Reveberockets) tank end down with open side facing one side of the cabinet -- the alphanumeric code of the reverb tank tells which. The reason is that the mounting of springs to transducers is somehow "tuned" to the natural "hang" of the spring relative to the orientation of the transducers, to keep the springs as centered as possible in their mechanical openings. But that said, what works works.

 

[joulupukki]

Yeah, it’s not the best picture. The open side is facing the bottom front of the cab and laying at a 45° angle so that the plane of the top is pointing towards the back top of the cab. In this position, nearly zero hum and seemingly good performance. I will probably take it out of the bag and figure out some sort of mount so that the bag doesn’t get too hot right near the PT.

 

[trev333]

I screwed mine on the back when I didn't have room inside the cab after some transformer mods... works fine...

 

[chas.wahl]

Also, are you sure that your problem wouldn't be erased by flipping the reverb pan end-to-end, to get either input or output further from the PT? This article by Amplified Parts says that the output transducer, in particular, should be kept away from transformers. The article largely recapitulates one that used to be available online, authored by Accutronics, though I can't find it anymore. I did save both the text and illustrations as a Word document, and will gladly forward it (or could put it up on a DropBox location for download) upon request.

 

[Phrygian77]

You could also try a short tank.

 

[joulupukki]

Also, are you sure that your problem wouldn't be erased by flipping the reverb pan end-to-end, to get either input or output further from the PT? This article by Amplified Parts says that the output transducer, in particular, should be kept away from transformers. The article largely recapitulates one that used to be available online, authored by Accutronics, though I can't find it anymore. I did save both the text and illustrations as a Word document, and will gladly forward it (or could put it up on a DropBox location for download) upon request.

Yes. This is always the very first thing I (or anyone) should try. I’ve seen that doc. Lots of good info in there. Strangely, with it 1/4” away from the PT on the ~45° angle, it’s pretty darn quiet.

You could also try a short tank.

Agreed, though I don’t really wanna have to buy another tank. Definitely worth a shot though.

 

[joulupukki]

Right now I’ve got the medium-decay tank from Mojotone in there. I wonder how the Medium MOD tank would do? https://www.cedist.com/products/reverb-tank-mod-8ab2c1b-medium-decay-3-spring

Also, I’ve read a couple reports of people completely enclosing the reverb tank with MU Metal to drastically reduce or eliminate the magnetic interference. Would that be worth even pursuing? Anyone here done that?

 

[mountainhick]

Right now I’ve got the medium-decay tank from Mojotone in there. I wonder how the Medium MOD tank would do? https://www.cedist.com/products/reverb-tank-mod-8ab2c1b-medium-decay-3-spring

Also, I’ve read a couple reports of people completely enclosing the reverb tank with MU Metal to drastically reduce or eliminate the magnetic interference. Would that be worth even pursuing? Anyone here done that?

How would you do this? What kind of MU metal would you use?

 

[joulupukki]

How would you do this? What kind of MU metal would you use?

Great question. Maybe something like this? https://www.magnetic-shield.com/mumetal-stress-annealed-sheet/

…and then just build a box like I would building ductwork (With a sheet brake)?

 

[tubedude]

Right now I’ve got the medium-decay tank from Mojotone in there. I wonder how the Medium MOD tank would do? https://www.cedist.com/products/reverb-tank-mod-8ab2c1b-medium-decay-3-spring

Also, I’ve read a couple reports of people completely enclosing the reverb tank with MU Metal to drastically reduce or eliminate the magnetic interference. Would that be worth even pursuing? Anyone here done that?

Ignore the Mu metal posts. Mu metal does resist magnetic fields, but is very expensive, and bending it reduces it's magnetic shielding properties.
If you screw the tank down use Sorbothane O rings to avoid vibration. Also with Accutronics tanks, there are several holes in the chassis to allow repositioning of the support springs to allow for mounting the tank in different orientations. Maybe other brands also. Not sure who makes tanks for Mojo.

 

[Bill Moore]

Get a piece of sheet metal maybe 4 or 5 inches wide. and long enough to bend in a shield over the tank. With the tank on the bottom move the sheet over the tank, and see if it quiets some. If it does, bend some corners, and screw it down to hold it over the tank in that area.

 

[schmee]

Not sure how 45 degrees will come out, but I have use flat style tanks at 90 degrees in heads probably 3-4 times with no issues whatsoever.

 

[joulupukki]

I wish 90° was quiet because it’d be easier to mount. It’s not exactly 45°, but all I know is that the hum goes away.

 

[BigDaddy23]

If you are set on positioning at 45deg, why not make a couple cleats with 45deg chamfers for the inside of the head cab and fix the tank to them?

 

[joulupukki]

If you are set on positioning at 45deg, why not make a couple cleats with 45deg chamfers for the inside of the head cab and fix the tank to them?

Yep, I probably will. Right now it’s tightly wedged between the front baffle cleat and the opposite side inside the tolex bag. It’s not really moving, but if I don’t find a different solution, it should be easy enough to cut a couple of wooded wedges. The other way I could do it is just tack a couple of screws in place while it’s in the bag and call it good.

 

[Deebs3]

I think the biggest noise i ever heard in my life was when i was gently repositioning my reverb tank to reduce hum in my blues junior and it suddenly flew into the speaker magnet, hot dang that made a big noise....

 

[joulupukki]

I think the biggest noise i ever heard in my life was when i was gently repositioning my reverb tank to reduce hum in my blues junior and it suddenly flew into the speaker magnet, hot dang that made a big noise....

Oh man, way too familiar! I sometimes hate those big powerful magnets! I usually try to turn the volume knobs down before drastically moving them, but still. Yowsah! Surprisingly the creamback neo in my combo amp ... barely has any pull at all, kinda nice.

 

[joulupukki]

Get a piece of sheet metal maybe 4 or 5 inches wide. and long enough to bend in a shield over the tank. With the tank on the bottom move the sheet over the tank, and see if it quiets some. If it does, bend some corners, and screw it down to hold it over the tank in that area.

I mean, why not? I pillaged some sheet metal from one of those broken tankless water heaters I’ve got. Crudely cut and bent it in a few minutes. Just as bad as before with this orientation.

 

[joulupukki]

As it turns out, if I rotate it a full 90° so that it’s sitting at the very front of the amp head cab, 95% of the hum disappears (it’s the quietest orientation). As it turns out, both the AB763 amps (this one and the one in the combo cab) have this same behavior. I wish I understood why since it seems like it ought to have been just fine laying down flat as designed.