First time builder looking for a project.

[DavidJNichols]

Here is what I'd do next:

Lift one end of R28 so that both R27 and R28 can be measured out of circuit. Measure each one individually, making sure that when you measure the pot, you have the wiper parked at the end that is tied to the wiper. R27 should measure at about 50K, and R28 should measure 27K. Leave one side of R28 lifted and with the output tubes removed and C13 back in the circuit, power up the amp and measure the DC voltage at point D. You should see approximately -67V. If you see something substantially different, then there is something wrong further down. Point D doesn't have many connections, so it won't take long to check it out. It is also possible thad capacitor C7 or C8 is leaky.

R27 and R28 both check out; they're almost dead on 50k and 27k, respectively. With C13 back in the circuit and R28 removed, the voltage at point D was -18.

[JazzGuitarGimp]

So something connected to point D is pulling its voltage toward GND. The only other things connected to D are the folliwing:
R19
R20
C7
C8

Follow your wiring from C13/R27 to R19, R20, C7, and C8 to see if anything looks amiss. The only other thing that comes to mind is the possibility that C7 or C8 could be leaky. To check this, lift one end of each of these caps and measure the voltage at D again.

[DavidJNichols]

So something connected to point D is pulling its voltage toward GND. The only other things connected to D are the folliwing:
R19
R20
C7
C8

Follow your wiring from C13/R27 to R19, R20, C7, and C8 to see if anything looks amiss. The only other thing that comes to mind is the possibility that C7 or C8 could be leaky. To check this, lift one end of each of these caps and measure the voltage at D again.

The wiring all looks good. I checked the resistance o R19 and R20, as well as R21 and R22 and they all checked out. I also lifted C7 and C8 individually and tested voltage and got the same readings as before.

I'm fairly certain it must be a wiring error on my part rather than faulty components, since this is my first build, I just have no idea where it could be. Could a faulty grounding scheme do this? I've checked my grounds and experimented with moving a few of them around, but nothing helped.

[JazzGuitarGimp]

Divide and conquer. Here's what I'd do next:

At the pot, there is a wire that goes over to the two 220K resistors. Lift this wire at the pot, and recheck your voltage at the minus side of C13. Again, C13 is in the circuit, but one side of R28 is lifted, and we're looking for about -67V to GND.

As for your grounding scheme, though I wouldn't be willing to rule out anything at this point, I am pretty certain that this has nothing to do a ground being connected over there or over here. The bias supply is a very straight-forwartd circuit, so my approach would be to methodically isolate the problem there before moving on to the next issue. The 6L6's will likely end up being biased somewhere in the -40V to -45V range to be biased correctly. And -18V will bias the tubes so hot, that they will pull heavy current from the B+ power supply, which can cause significant AC ripple on that supply. That much ripple will likely prove to be the culprit of the hum you are experiencing.

[DavidJNichols]

Divide and conquer. Here's what I'd do next:

At the pot, there is a wire that goes over to the two 220K resistors. Lift this wire at the pot, and recheck your voltage at the minus side of C13. Again, C13 is in the circuit, but one side of R28 is lifted, and we're looking for about -67V to GND.

As for your grounding scheme, though I wouldn't be willing to rule out anything at this point, I am pretty certain that this has nothing to do a ground being connected over there or over here. The bias supply is a very straight-forwartd circuit, so my approach would be to methodically isolate the problem there before moving on to the next issue. The 6L6's will likely end up being biased somewhere in the -40V to -45V range to be biased correctly. And -18V will bias the tubes so hot, that they will pull heavy current from the B+ power supply, which can cause significant AC ripple on that supply. That much ripple will likely prove to be the culprit of the hum you are experiencing.

Lifted the wire that goes to the 220K resistors and lifted R28. Kept C13 in the circuit and measured voltage on negative side. Still -18.

[JazzGuitarGimp]

So we are down to four parts in the circuit:

Power transformer bias tap (BLUE/RED; measures 47.4 VAC to GND)
D1
R26
C13

Some questions:
- Can you verify R26 is indeed 1K-Ohms?
- Is the RED/YEL wire from the power transformer connected to chassis GND?
- Using your ohmmeter on the lowest scale, what is the resistance from the RED/YEL chassis connection to the plus side of C13?

[DavidJNichols]

So we are down to four parts in the circuit:

Power transformer bias tap (BLUE/RED; measures 47.4 VAC to GND)
D1
R26
C13

Some questions:
- Can you verify R26 is indeed 1K-Ohms?
- Is the RED/YEL wire from the power transformer connected to chassis GND?
- Using your ohmmeter on the lowest scale, what is the resistance from the RED/YEL chassis connection to the plus side of C13?

R26 is indeed 1K. Red/Yellow form PT is connected to chassis GND. When
I check resistance on the lowest Ohm setting (200) from red/yellow GND to positive side of C13, it bounces all around. I think I saw it settle around 4, then again around 6, then 44 - it's all over the place.

[JazzGuitarGimp]

Progress! It looks like you've got a weak connection to GND, either at the RED/YEL chassis GND, or at the point where C13+ ties to GND. Do you have a clip lead? If so, clip one end to the solder lug where you've got the RED/YEL wire attached to chassis, and the other end to C13+. Then check your voltage at C13- ...... fingers crossed......

[DavidJNichols]

Progress! It looks like you've got a weak connection to GND, either at the RED/YEL chassis GND, or at the point where C13+ ties to GND. Do you have a clip lead? If so, clip one end to the solder lug where you've got the RED/YEL wire attached to chassis, and the other end to C13+. Then check your voltage at C13- ...... fingers crossed......

I clipped them off and still read -18. When looking very closely at C13's negative side, it looks like there is some slight erosion or rust - similar to a dirty battery terminal. Possibly a bad cap?

[JazzGuitarGimp]

Possibly. If you have the ability to take pictures, I'd like to see the corrosion on the cap, as well as the RED/YEL termination to chassis and C13+ termination through to the brass plate. It seems like there may still be an issue with a GND connection - possibly a cold solder joint, or a loose screw, etc.

[JazzGuitarGimp]

I clipped them off and still read -18. When looking very closely at C13's negative side, it looks like there is some slight erosion or rust - similar to a dirty battery terminal. Possibly a bad cap?

In re-reading this, I just wanted to make sure you understood my use of the term 'clip lead' to mean a piece of wire with an alligator clip at each end. I got a little concerned when you said "I clipped them off...."

[DavidJNichols]

Possibly. If you have the ability to take pictures, I'd like to see the corrosion on the cap, as well as the RED/YEL termination to chassis and C13+ termination through to the brass plate. It seems like there may still be an issue with a GND connection - possibly a cold solder joint, or a loose screw, etc.

Yeah I can take some pictures, but it'll have to wait until tomorrow afternoon. Thanks so much for all the help, by the way. I really appreciate it.

[JazzGuitarGimp]

I enjoy helping - and I am hopeful we will get to the bottom of this shortly. Until tomorrow....

[DavidJNichols]

Possibly. If you have the ability to take pictures, I'd like to see the corrosion on the cap, as well as the RED/YEL termination to chassis and C13+ termination through to the brass plate. It seems like there may still be an issue with a GND connection - possibly a cold solder joint, or a loose screw, etc.

Well I'm having trouble uploading pics at the moment, but in order to ensure good ground connections, I will recheck ground washers on PT lug. At the moment I have the other grounds individually soldered to the brass plate. I can create a buss system for those, so there is only one place they attach to plate.

[JazzGuitarGimp]

I've never encountered the 'brass plate' grounding system. I assume the plate is connected to the chassis by virtue of bring sandwiched with the chassis at the pot shafts and input connector? I didn't see anything on the weber layout that suggests a ground wire to be soldered to the brass pate at one end, and then connected to a chassis screw at the other end. Are all of the pots and input connectors tightened securely to the chassis?