Some AB763 Type Reverb Questions

[Rocketrob]

Hi All,

I'm in the planning stages of a modified AB763 type amp (single channel, no vibrato) and need some advice.
1. In the reverb section, are the input, output, and pedal jacks isolated from the chassis?
2. If they are where do they ground?
3. Where does the 220k resistor that is connected to the pedal tip ground?
4. Where does the reverb transformer ground?

This will be my first amp with reverb so I haven't worked out these details yet. Any help is much appreciated. Rob.

[Smitty]

In the originals they were grounded right at the chassis with lock washers and a grounding tab. There is a little tab that comes with the switchcraft phono jack from antique electronic supply that you use with a lock washer on each side (jack/lockwasher/tab/lockeasher) to ground the 220K resistor right to the chassis.

Optimally you wound want to ground that grid resistor to the same place you ground the cathode for that stage to reduce circulating ground currents. But the whole reverb can is grounded to the back of the chassis so there is only so much you can do...

I prefer USA Switchcraft jacks even though I'm suffering a little with the contacts on the switches right now...

I like to ground the reverb transformer to the ground side of the reverb driver cathode resistor. Same as a grid resistor.

There are only minor improvements that can be made to the brass ground plate scheme that Leo used, but it's worth exploring. I ground B-1 to the transformer mount with a grounding lug, the output tubes to another lug but the screens to the brass plate. Seems to work OK. Then work from B-2 down to the input stage working down the brass plate. Make sure currents don't cross over each other and you should be OK.

This is an excellent circuit to to learn and master. Dave Funk likes to use the direct coupled cathode follower as an input stage for this circuit to drive the tonestack more efficiently. See the blond Bassmans for more...

[Rocketrob]

Thanks for the reply Smitty.

I'm not making a Fender clone, but using the circuits to achieve a simular result. I have been using a "star" ground system on my builds and wondering how the Fender reverb circuit would apply to this.
This is how I have been doing the grounds:
1. AC line in ground near IEC socket.
2. Ground point near PT for PT center tap grounds, first filter cap(s), output tube cathode(fixed bias).
3. Ground point near input jacks for everything else except:
4. OT ground at speaker jack.
So, ground the reverb tranny and 220k R with the others near the input jack, and the reverb in/out/pedal kacks at the chassis where they mount.
Thanks, Rob.

[Smitty]

Maybe one more...

You may want to consider a separate ground point for your screen supply if that is going to the star at the input. If you are running your reverb driver off your screen supply you may wish to ground the supply (cap) in the same place as your driver cathode and transformer.

Look at the chassis and imagine the flows of current (still high even though the voltage is low) in the chassis. As long as they don't cross (meaning the lowest resistance to ground is to a different ground point than the supply ground point for that stage) you should be OK. B-1 near the power transformer and B-n (input stage supply) near the input is the right way to think about it. You can break it up as much as you want as long as you don't create a ground loop. I break it up eight times--one for each power supply node. Seven are on the brass plate. No hum in my amps.

[Chuck H]

You can buy reverb tanks with an isolated input and/or output jack. Or you can buy insulated RCA jacks (if thats your chassis mount jack of choice). I would leave the shield for the input cable floating. As in not connected to both the pan and the chassis. Only one or the other. Be sure the pan input load resistor is grounded with the associated preamp circuitry.

[Rocketrob]

Thanks for the replies.
I really shouldn't write things "Off the top of my head"; there is not much up there. Yes, I forgot to add that the screen supply ground is at the same place as the first filter cap, near the PT. I am planning on powering the reverb transformer/driver from the screen supply node. I know that the screen supply should not be grounded with the pre amp, so, you are saying to create another ground point between the PT star and the input star and ground the screen supply, reverb transformer/driver, and driver cathode there? I am using a ground bus that runs across the back of all the control pots and grounds at the input star. All of the pre amp and supply nodes use seperate wires to connect to their respective ground points.

[Rocketrob]

Chuck, I have a 4AB3C1B reverb tank, which has an isolated input, and a grounded output. Reverb pan input load resistor? Which one is that? Rob.

[Chuck H]

There isn't one. I was seeing a common stage in my head for a second and trying to cover all the grounding issues.

I did build an outboard self powered and speakered reverb for a customer and there was a ground loop issue. Isolating the two amps grounds (tank input from host and output to it's own amp) cured the problem. Isolating the input on your amp but keeping the shield grounded on one end or the other should give the same result. Not that you'll surely have a problem with the standard wiring, but I never met a BF reverb that didn't hum.

[Smitty]

Yes. Separate ground for screen supply and everything running off it. A good location can be where you ground the cathodes of the output tubes which can be another lug on the power transformer mount.

[Rocketrob]

Thanks Smitty.
I'll try your suggestion. It does seem counter-intuitive to me to run the cathode ground of the 12AT7 driver tube across the amp to the PT though. Most comments I've read about the AB763 say that the reverb driver / recovery stages are the most frustrating parts to get right. I'm about to find out : ) Rob.