First time builder looking for a project.

[JazzGuitarGimp]

I just went back and re-read your opening post to this thread. I see that you are new to electronics so I am going to offer this caution: please be very carefull when probing around inside the amp when power is applied. Voltages inside the amp are upwards of several hundred volts, which can be lethal.

It would be a good idea to make sure the output stage is not overbiased before you go any further. To do this, pull all of the tubes out (except V5) and measure the DC voltage at pin 5 of both V3 and V4. Adjust R27 so tha the measured voltage is the most negative with respect to GND. This voltage should be at most -40V and at least about -60V. In its present state, if the measured voltage is closer to GND than -40V, it's possible the output tubes could have been damaged. If by chance this fixes the hum issue, then you should set the bias to the proper level before you go any further. If this did not fix the hum issue, then leave R27 set for the most negative voltage for the time being. We can step you through the bias procedure when you get to that point.

At this point, some gut shots of your build would be very helpful. Also, how close is the PT to the OT? And do both channels respond the same way with regards to full volume at 0.5?

[JazzGuitarGimp]

If you pull V1, does all of the hum go away?

Do you have tube shields on V1 and V2, and do they help with the hum issue?

Can you measure the DC voltages at points A, B, C and D with respect to GND? This should be done with nothing plugged into the input and the volume and tone controls all the way down.

[DavidJNichols]

If you pull V1, does all of the hum go away?

Do you have tube shields on V1 and V2, and do they help with the hum issue?

Can you measure the DC voltages at points A, B, C and D with respect to GND? This should be done with nothing plugged into the input and the volume and tone controls all the way down.

Pulled V1 and still has hum. I do have shields on 1 & 2. I'm about to check the voltages you previously mentioned.

[DavidJNichols]

I just went back and re-read your opening post to this thread. I see that you are new to electronics so I am going to offer this caution: please be very carefull when probing around inside the amp when power is applied. Voltages inside the amp are upwards of several hundred volts, which can be lethal.

It would be a good idea to make sure the output stage is not overbiased before you go any further. To do this, pull all of the tubes out (except V5) and measure the DC voltage at pin 5 of both V3 and V4. Adjust R27 so tha the measured voltage is the most negative with respect to GND. This voltage should be at most -40V and at least about -60V. In its present state, if the measured voltage is closer to GND than -40V, it's possible the output tubes could have been damaged. If by chance this fixes the hum issue, then you should set the bias to the proper level before you go any further. If this did not fix the hum issue, then leave R27 set for the most negative voltage for the time being. We can step you through the bias procedure when you get to that point.

At this point, some gut shots of your build would be very helpful. Also, how close is the PT to the OT? And do both channels respond the same way with regards to full volume at 0.5?

Thanks for the words of caution. I really appreciate how everyone on this site stresses the fact that there is lethal voltage inside an amp. It's important information if you enjoy living.

So I have some strange things going on. I pulled all tubes except V5 and checked voltages of pin 5 on V3 and V4. They are sitting at -14.4. I took a few different readings after adjusting the bias trimpot and they still sit right at -14.4 (give or take .2).

[JazzGuitarGimp]

So I have some strange things going on. I pulled all tubes except V5 and checked voltages of pin 5 on V3 and V4. They are sitting at -14.4. I took a few different readings after adjusting the bias trimpot and they still sit right at -14.4 (give or take .2).

Okay, so the bias power supply needs to be checked. Since you have a negative voltage at the grids of the 6L6's, we know that diode D1 Is installed correctly. Check to make sure the filter cap, C13 is also installed correctly. Note its positive terminal is connected to GND, not its negative terminal. Next, with power off (and wait a few minutes for the caps to discharge), connect an ohmeter between point D and GND. At one end of the pot's (R27) rotation, you should measure 27K-ohms and at the other end of R27's rotation you should measure 77K-ohms. Note that the cap in the circuit (C13) will make the measurement a bit sluggish as you turn the pot. So wait a few seconds at each end of rotation for your meter's reading to settle.

[JazzGuitarGimp]

Double post. First time for me - I must be moving up in the world!

[DavidJNichols]

So I have some strange things going on. I pulled all tubes except V5 and checked voltages of pin 5 on V3 and V4. They are sitting at -14.4. I took a few different readings after adjusting the bias trimpot and they still sit right at -14.4 (give or take .2).

Okay, so the bias power supply needs to be checked. Since you have a negative voltage at the grids of the 6L6's, we know that diode D1 Is installed correctly. Check to make sure the filter cap, C13 is also installed correctly. Note its positive terminal is connected to GND, not its negative terminal. Next, with power off (and wait a few minutes for the caps to discharge), connect an ohmeter between point D and GND. At one end of the pot's (R27) rotation, you should measure 27K-ohms and at the other end of R27's rotation you should measure 77K-ohms. Note that the cap in the circuit (C13) will make the measurement a bit sluggish as you turn the pot. So wait a few seconds at each end of rotation for your meter's reading to settle.

Weird things are going on with my digital multimeter. On the 200k Ohm setting, I checked between GND and point D, and as I turned the trimpot, one end registered at 10.6 and the other at 12.3. I switched to the 20k Ohm setting and registered 3.77 to 3.95.

I'm confused as to why those readings would be different depending on the setting of my multimeter. It's a brand new multimeter, so I doubt it's fritzing out or anything, but I suppose it's possible. Or am I confused on how the Ohmmeter setting of my multimeter works?

I checked and rechecked my wiring for that section (and the rest of the amp) using the Weber wiring diagram and a highlighter to mark off each section as it's verified, and it seems to be wired according to the Weber layout, including appropriate orientation of C13.

[JazzGuitarGimp]

So I have some strange things going on. I pulled all tubes except V5 and checked voltages of pin 5 on V3 and V4. They are sitting at -14.4. I took a few different readings after adjusting the bias trimpot and they still sit right at -14.4 (give or take .2).

Okay, so the bias power supply needs to be checked. Since you have a negative voltage at the grids of the 6L6's, we know that diode D1 Is installed correctly. Check to make sure the filter cap, C13 is also installed correctly. Note its positive terminal is connected to GND, not its negative terminal. Next, with power off (and wait a few minutes for the caps to discharge), connect an ohmeter between point D and GND. At one end of the pot's (R27) rotation, you should measure 27K-ohms and at the other end of R27's rotation you should measure 77K-ohms. Note that the cap in the circuit (C13) will make the measurement a bit sluggish as you turn the pot. So wait a few seconds at each end of rotation for your meter's reading to settle.

Weird things are going on with my digital multimeter. On the 200k Ohm setting, I checked between GND and point D, and as I turned the trimpot, one end registered at 10.6 and the other at 12.3. I switched to the 20k Ohm setting and registered 3.77 to 3.95.

I'm confused as to why those readings would be different depending on the setting of my multimeter. It's a brand new multimeter, so I doubt it's fritzing out or anything, but I suppose it's possible. Or am I confused on how the Ohmmeter setting of my multimeter works?

I checked and rechecked my wiring for that section (and the rest of the amp) using the Weber wiring diagram and a highlighter to mark off each section as it's verified, and it seems to be wired according to the Weber layout, including appropriate orientation of C13.

If there were any remaining charge on C13, it would have caused your meter to do weird things. What I would suggest is if you have a few resistors of various values laying around (read, not soldered into a circuit), measure them with your meter as a way to rule out an issue with the meter.

[DavidJNichols]

If there were any remaining charge on C13, it would have caused your meter to do weird things. What I would suggest is if you have a few resistors of various values laying around (read, not soldered into a circuit), measure them with your meter as a way to rule out an issue with the meter.

Yeah, I actually just did that and the meter is working just fine. I rechecked my filter cap voltage and its down to .2 volts and doesn't seem to be draining much more than that. The Ohm readings are still the same.

[JazzGuitarGimp]

Is it possible to unsolder one side of C13 to remove it from the circuiut to do the test? Also, is R27 printed with its value, and have you verified it says 50K ?

[JazzGuitarGimp]

It would also be helpful to know the AC voltage at the cathode of D1, measured to GND.

[DavidJNichols]

It would also be helpful to know the AC voltage at the cathode of D1, measured to GND.

The voltage of D1 - cathode to GND is 47.7

[DavidJNichols]

Is it possible to unsolder one side of C13 to remove it from the circuiut to do the test? Also, is R27 printed with its value, and have you verified it says 50K ?

I removed C13 from the circuit and got similar results.

On the 200k Ohm setting: 9.6 - 11.1
On the 20k Ohm setting: 3.52 - 3.68

R27 does not have a value printed on it. The bottom has the numbers 85 03 on it. I did a Google search for that number in relation to a trimpot and turned up no useful information. I have big 50k Log pots and 20k Lin pots lying around. I suppose I could replace this Weber trimpot with one of those to test if needed.

[JazzGuitarGimp]

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.

[LeftyStrat]

Any just as a sanity check, in addition to what Lou said, I'd measure the two outer legs of the trimpot.