Deluxe Reverb (ab763) Reverb RCA's wiring

[Lifto]

Hi all,

I just finished my 4th tube amp, this time a BF Deluxe Reverb based on the ab763 circuit. I'm really happy with the way the amp turned out. It sounds very good, all controls are working and I had low noise, so far so good.
I'm only experiencing some problems with connecting the reverb. Initially I had no reverb. Connecting the RCA cables to the pan only resulted in a strong hum which dissapeared as soon as I disconnected the reverb output cable from the amp. I knew there was something wrong with the way I connected the RCA jacks to the chassis. At first I had the Reverb pedal + reverb output + reverb input isolated from ground (only hum no reverb). By playing around with some alligator clips I found out that the reverb does work when I ground the RCA sleeves to the chassis. So at this moment I have all RCA sockets grounded with their sleeve to the chassis. (and tip connected as shown in the ab763 layout). Is this correct or should the reverb input be isolated?
So at this moment I have a working reverb but a constant strong hum (even with the RCA cables disconnected).

Is there someone who can help me understand the way the RCA's should be wired?

Layout:
http://www.thevintagesound.com/ffg/s...763_layout.gif

Schematic:
http://www.thevintagesound.com/ffg/s...b763_schem.gif

[Tom Phillips]

...Is there someone who can help me understand the way the RCA's should be wired?...

The first bit of information I'll offer is that all four of the chassis mounted RCA Phono sockets are grounded to the chassis mechanically via their physical mounting method. Therefore, if you are building a DR clone then first make sure that your work matches that and we will go from there.

[Chuck H]

What Tom says is true. After that there are specifics for the reverb pan itself. Some have an isolated input, or output, or both and others have neither. It's all specified in the part numbers. The point is that for stock Fender units all the isolation occurs at the reverb tank. And as Tom mentioned everything at the chassis is grounded. Reverb signal from the pan is VERY small and requires a great deal of reamplification. Any mistake in grounding that results in a loop will be hugely amplified and hum terribly. IMHE it's very easy to suffer ground loops when implementing an on board reverb. All vintage stock Fender reverbs hum. But it's an "acceptable" amount. It's possible to do better IMHE, but you will never eliminate all the noise that comes with an on board reverb.

All grounds associate with the reverb are critical. Where, if and what is grounded applies to separation of the functional circuit that actually shakes the springs, it's input from the preamp that drives it and the recovery circuit that feeds the mixing circuit. Even then it's possible to discover fault in pan orientation, microphony or, in one case I dealt with, a power transformer with EMF that just couldn't be avoided.

Making quiet reverbs is a royal PITA. I'm sure that's why Tom is questioning why you veered from the grounding scheme of a known working circuit. Like I said above, you CAN do better than the stock Fender grounding scheme, but it required either an acute knowledge of all the elements in play or the patience to work it out with research and experimentation.

[Lifto]

Thanks for your replies.
I will try to clear up how everything is wired now. I have followed the schematic/layout of the Deluxe reverb AB763 circuit as close as possible. I got confused with the connections of the RCA's because of the specifics of the reverb pan (input being isolated and the output tip going to ground via a 220k resistor). Normally I will refer to the schematic to see what is exactly going on but in this case it wasn't quite clear to me. I tried looking it up online but English not being my first language didn't help either.

At this moment I have all RCA sleeves grounded to the chassis. The only problem I'm experiencing now is (probaly) a ground loop which is too loud (I understand the reverb circuit always producing some hum) and which is also there when the reverb is turned off (by using the footswitch). When I had the rca's isolated from the chassis the amp there was almost no hum.

What would be the best way to see if I can reduce the hum? Would this be by adjusting some ground points and try to follow original as close as possible? It is a build from scratch so I would have to do some research on how it should be grounded originally.

[Chuck H]

Most tanks actually say "input" and "output" on them. So that should be alright. The tank should have the code on it somewhere? That code will indicate how the jacks are grounded though you may need to go to the manufacturers site to interpret that and compare it to a stock tank specs. Which may also want to look up (It's ground iso input and tank grounded output). If you grounded the circuit according to the layout there shouldn't be a ground loop. The other thing that can cause hum is the tank location. It would seem at face value to simply be "the bottom of the cabinet", but it's not always. I've had particular trouble with the MOD tanks being most sensitive to picking up EMF from the power transformer and even had to mount an "open side down" tank on it's side once to eliminate EMF inducting into the transducers. So scootch and move the tank around in the amp and see if some more hum can't be eliminated that way too.

[Jazz P Bass]

"Scootch"?

[Lifto]

I seem to keep forgetting vital information. I bought an Accutronics 4AB3C1B. The code translates to: Type 4, 8ohm input, 2250 ohm output, Long spring, Input insulated, output grounded, no lock, horizontal open down.

I will try to see if I get better results by 'scootching' the tank around. FYI the tank is in a reverb bag.

[Leo_Gnardo]

"Scootch"?

Yeh, scootch it just a skoshe. One of those "professional adjustments" ya know. Slide it over there just so, about a microfurlong.

- - - - - -

Anyway, if the pickup (output) end of the tank is under the power transformer then you're guaranteed it will hum.

[Chuck H]

"Scootch"?

Yeah! Scootch!..

Fine. My word. You can't use it

Turns out the actual spelling is scooch. So it IS a word (though autocorrect won't recognize it), I just misspelled it.

[Lifto]

Hi all,

Just thougth I let you know I managed to fix the issues. All it took was a critical view on my own work. In the end I found I used a form of semi-star ground mixed with the Fender layout. I changed it by using the scheme from EL34world and now it's all good. Reverb for days with limited/acceptable hum.

This is the layout I used: Grounds

Thanks for your suggestions and the willignness to help a rookie like me.