High pass filter on reverb recovery

[jreese2001]

Hey guys I built an AB763 super reverb. It has some hum that increases as the reverb knob is turned up and disappears if the reverb pan is unplugged or if the reverb knob is on zero. I've checked filter caps, grounding, etc and I am convinced that it is simply being picked up by the reverb tank itself. I've read some posts on this topic but I did not find any about putting a high-pass filter on the recovery stage. I know that AB763 has the 500pf/1M high pass filter on the input to the reverb driver, but has anyone ever put a high pass filter either coming off of the reverb pan or after the recovery stage using the 0.003uF capacitor? I am just trying to get an idea of what has worked in the past. I am after a little bit of noise reduction without coloring the reverb sound too much.

Josh

[Enzo]

You don't need a high pass filter, you need to remove the source of the hum.

If it goes away when the pan is unplugged, then the pan seems at fault. Dismount the reverb pan and move it as far away from the amp's power transformer as possible. Like move it outside the cab and hold it at a distance. Does that reduce the hum?

When you installed the pan, did you mount it with the OUTPUT jack end away from the power transformer end? If not, reverse the pan. OUTPUT end should be as far as possible from the PT.

Take a piece of sheet steel and place over the open side of the reverb pan, making sure it makes contact electrically. Does that shielding help? No sheet steel? Got another reverb pan of same dimension sitting around? Invert it and screw them topgether. Thart will be a shield for test purposes. If it works we can come up with a less bulky sheet.

[Tom Phillips]

Also check the RCA connector grounding on your reverb pan.
Normal configuration for a Fender amp is pan input connector isolated / output connector shield connected to the pan chassis.
Ohm it out to be sure.
Tom

[jreese2001]

The pan is out, I have the chassis on my workbench and I have a brand new RCA cable connected to it. I've put a piece of sheet aluminum on the open side of the pan. The input jack sleeve does not connect to the pan itself. So if I remove the input jack at the chassis there is continuity between the pan and the chassis but if I plug the input back in and remove the output jack there is no continuity. I've also moved the pan around a bunch but no configuration seems to reduce the hum. By the way I am attaching the scope trace. It looks like classic 60 cycle noise to me but some of you much more experienced guys may see more in it. Thanks!

[Tom Phillips]

... I am convinced that it [hum] is simply being picked up by the reverb tank itself. ...

Josh,
There is an easy way to test that theory. Make an adapter cable and connect the output of the reveb pan into the input of an amp that does not hum. (The same input where you normally plug in your guitar) Leave the input of the reverb tank unconnected. Make sure you have sufficient lead length to place the tank at least as far away from the chassis as it would be in the normal position in the bottom of the Super Reverb cabinet. Now slowly turn up the amp volume. Is the hum still there? If not then the tank is not picking up the hum. touch the springs with your fingertip and you should clearly hear the rubbing sound. That verifies that the connection is working.
Tom

[Chuck H]

All AB763 reverbs hum at least a little. The ground scheme is less than ideal I suspect. Your ground to chassis reading is accurate for a stock tank so I it's probably the right tank. Have you tried different hook up cables or tested shield continuity on the existing cables? An audio file might help those who are familiar with the BF amps to comment on whether the amount of hum you have is "normal" (in quotes because IMHO it's excessive on the stock amps).

And +1 to tank orientation. Of course, if you have it out of the cabinet and the problem persists then you've eliminated that possibility.

I can't tell the frequency from your scope trace because that doesn't look like any AC supply voltage I know of. With your scope set at 10ms division you have 20ish swings across ten bars. So that's 200Hz?

[Enzo]

I suggested steel to block magnetic fields from the power transformer. But if moving the pan around makes no difference, it probably is not that.

[jreese2001]

Thanks to everyone so far. Tom that is a brilliant idea to plug the tank into the input of another amp. I am going to try that tonight and I'll report back.

[jrfrond]

Did you sub in another pan?

This is easy to troubleshoot. Short the reverb return input. If there's no hum, it's the pan, period. If it DOES hum with the input shorted, then perhaps the recovery tube is bad.

[jreese2001]

OK then it turns out I was wrong about it being the reverb pan. I shorted the grid of the reverb recovery to ground, with the reverb pan plugged in. No change in hum. Then I unplugged the reverb pan. The hum goes away, as I mentioned in the first post. THEN I shorted the grid to ground again, and the hum came back! What does this mean?

Does this mean there is a grounding problem? Just in anticipation of questions, this amp is star grounded. The preamp grounds are all daisy chained together and connected to the chassis at a single point. The power supply is grounded at a different point than the preamp ground. I followed Randall Aiken and R.G.'s articles out there on the web. No, it does not have a brass plate but all the pots are grounded to the chassis. The 220k on the grid of the recovery tube as well as the reverb transformer are grounded at the cathode of the recovery tube.

[jreese2001]

Sorry, by "power supply" I actually mean the PT center tap, the power tube grounds and the reservoir caps. The other caps are grounded at the same point as the preamp grounds.

Josh

[Chuck H]

Yup... Grounding problem. I suspect that by grounding the grid of the recovery tube you created a ground loop. That why it hummed even with the tank out. You need to consider that the reverb pans internal parts are grounded via the RCA jacks. Hunt down any part of the circuit that mat have two 0V reference points and make any changes to have those share the same ground. This may, or may not solve the problem. But it's a place to start.

[jreese2001]

I thought about this and I bet the ground loop is because I neglected to isolate the RCA jacks. I'll test it tonight by shorting the grid to ground again, but instead of shorting to the chassis near the RCA jack I'll run a temporary wire to the preamp star ground point. Correct me if I am wrong, but if the hum goes away this means I need to isolate the RCA jacks and run a wire from the sleeves to the star ground point.

[Chuck H]

That sounds right. I had a similar issue on a "reverb only" amp I built. My problem was a little different as I was sharing another amps ground too. But similar enough in that isolating the jacks was the first step in the solution.

Actual Fenders don't have isolated jacks IIRD. But they ALL hum too.

[jreese2001]

Well it looks like that's it. The hum went completely away when I grounded the grid like that. I am going to get some isolated RCA's and put them on and hopefully that should do it. Speaking of, does anyone have a good source for isolated RCA's? Or do you use plastic shoulder washers?