jaret wrote:Ok, I've isolated the rca shields and grounded one of them with the RT ground ...
Where only one tank connection is grounded, it's typically the output you want to ground rather than the input.
This is a combo, isn't it? The output transducer can be very sensitive to picking up hum from the PT (which then will go straight into V1B's grid), so you want the output side as far away from the transformers as possible.
That's the one I grounded- I figured it was more sensitive since it was essentially a long grid line.
The chassis sits on top of a sealed 10" cab with the reverb tank inside it at the bottom so it shouldn't be bothered by the trannies. I tried turning the tank the other way around inside the cab just to be sure, but moving it around didn't change anything. I also checked the springs while I was at it- all are intact.
jaret wrote:
Could it be a conductive tube socket?
Jaret
Since the noise is there even with the reverb tube pulled I don't think it is a grounding issue. I think you are on the right track with the DC on the V1B grid. Maybe you have a conductive board and there is continuity between the plate resistor turret and one of the 220K's turrets or the one on the other side of the V2B blocking cap. This can be a problem with certain board materials like XX phenolic or even the Fender style fishpaper sandwich if a blob of solder drips between the boards.
Yeah, I've cleaned up a lot of grounding issues and the hum is still there. I even elevated the heater CT just to be safe.
I was wondering about that too- how the hum can be present without the reverb tank connected and without V2? But still be dependent on the reverb knob setting?
I'm going to take a divide-and-conquer approach- disconnect the RT from the B+ and see if the hum remains. If it does, then I'll try disconnecting the 4.7n. Still humming? Then I'll disconnect the plate resistors of V1B and V2B, which connect to turrets on either side of the V1B grid turret. I think the board is phenolic, so conduction is something to consider.
That should help narrow things down right?
Unfortunately, this'll have to wait until after NYE!
jaret wrote: I think the board is phenolic, so conduction is something to consider.
I don't think you need to go back as far as the reverb trans. If the DC at the V1B grid is coming from the pot ( and it seems like it is because when the wiper is at ground there is no noise, and no voltage at the grid I am assuming) then just disconnect the wire coming from the coupling cap to the wiper at the wiper and check to see if the DC is at the wire. If it is then remove the coupling cap and measure for DC at the turret, which at this point shouldn't be connected to anything. If no DC is present then maybe you just had the bad luck of of having 2 bad couplers. If it is there then you probably should closely inspect the top and bottom of the board to make sure there isn't some other explanation, because if the board is conductive I think you know what that means - you should replace the whole thing.
KT66 wrote:Since the noise is there even with the reverb tube pulled I don't think it is a grounding issue.
Jaret, disconnect the wire to the reverb pot's wiper (at the pot) and see what happens. Reading above I guess you have the 4.7n cap soldered directly to the pot's middle lug? Since you can pull V2B and still get the hum, it seems like it has to be getting in along the path from V2B's plate to the pot wiper. Is the 4.7n cap close to anything that could be a source of hum? Maybe post a picture so we can see what things look like. BTW, what frequency is the hum?
Ideally it would be best to ground the pre-amp filters at the same place as the pre-amp cathodes. The screen filter is probably the highest risk if it's grounded along with the pre-amps since it's further upstream. I guess your 4x20uF can has a common ground for all the sections, so you are out of luck? On the up-side, its looking like the hum may be from something else.
Just wondering... Why does your 4.7n cap from V2B's plate go to the wiper of the reverb pot?
The typical Fender circuit (Deluxe Reverb, e.g.) would feed the output from the recover stage to the top of the pot (thus making a constant AC load for the recover stage), take the output from the wiper, and ground the bottom. A 220k from the wiper to ground then serves as the grid leak for the following stage.
