UPDATE: Finished my Express build last night > it's working!

[martin manning]

I wonder about the output transformer. Perhaps it was damaged by the same high current that took out the PT?

Measure the DC resistance of both halves of the primary and see if they are reasonably close and in the right ballpark, say 50 ohms each side. You can then put some low voltage AC through it and check the voltage ratio.

[prairie]

I wonder about the output transformer. Perhaps it was damaged by the same high current that took out the PT?

Measure the DC resistance of both halves of the primary and see if they are reasonably close and in the right ballpark, say 50 ohms each side. You can then put some low voltage AC through it and check the voltage ratio.

I’ve got roughly 43 ohms a side when checking impedance on the OT primaries

[martin manning]

That's good news. Not an absolute guarantee that the OT is fine, but I'd continue to look elsewhere for the problem, which is a loss of signal somewhere. Could be a bad solder joint, accidental grounding, or some other wiring error.

[pompeiisneaks]

One way you can see if you're losing signal would be to just put your multimeter on ac volts and read about how much each triode has on the grid and then it's associated anode. It should be going up a lot each time. Maybe at input you've got like 100mV of AC current or more, then at the anode maybe 1VAC, then the next stage maybe grid has close to that same 1VAC (sometimes losses like tone stack etc, but not all of it) and see if the anode of the output side is then quite a bit bigger yet again. If you suddenly see it going up and up and then boom down that will help you determine that's the stage where things are going south. (an oscilloscope is much better at doing this because you can SEE the waveform, but still a DMM works as well to at least see the voltage growing from amplification.

Some stages don't massively increase gain, due to say a tone stack with a cathode follower but they do not LOSE gain ever. That's a sign of something really wrong.

~Phil

[prairie]

I did a pop test on the plates, going from the power tubes back to V1 (to localize where my issue is). My plate on V1 is noticeably quieter than the other tubes, so I’m clearly having an issue somewhere near the front end of the signal.

[romberg]

It is a common thing to miswire the input jack so that the incoming guitar signal does not have a ground reference and is floating. This might cause what you are hearing. With the amp off and an instrument cable pugged into the input, measure the resistance from the tip of the cable to the chassis. You should see about 1M ohm.

Mike

[prairie]

It is a common thing to miswire the input jack so that the incoming guitar signal does not have a ground reference and is floating. This might cause what you are hearing. With the amp off and an instrument cable pugged into the input, measure the resistance from the tip of the cable to the chassis. You should see about 1M ohm.

Mike

Will do. Thank you!

[prairie]

It is a common thing to miswire the input jack so that the incoming guitar signal does not have a ground reference and is floating. This might cause what you are hearing. With the amp off and an instrument cable pugged into the input, measure the resistance from the tip of the cable to the chassis. You should see about 1M ohm.

Mike

Will do. Thank you!

[prairie]

It is a common thing to miswire the input jack so that the incoming guitar signal does not have a ground reference and is floating. This might cause what you are hearing. With the amp off and an instrument cable pugged into the input, measure the resistance from the tip of the cable to the chassis. You should see about 1M ohm.

Mike

Coming in at 1M ohm.

[romberg]

Do you have a smart phone with a headphone jack? If so, you can make a crude signal generator with it, a coupling capacitor and a cable.

There are many signal/function generators out there for smart phones. Most are free downloads. Then rig up a cable from the headphone out with one of the positive lines running into a 400-600V rated coupling cap. The value does not matter much say 0.1u. The point is you may not want to connect your phone to 400VDC by accident. Once you have a rig like this you can inject signal into points in the amp and see what is passing signal and what is not. And you don't have to keep strumming a guitar while you debug.

Mike

[prairie]

Do you have a smart phone with a headphone jack? If so, you can make a crude signal generator with it, a coupling capacitor and a cable.

There are many signal/function generators out there for smart phones. Most are free downloads. Then rig up a cable from the headphone out with one of the positive lines running into a 400-600V rated coupling cap. The value does not matter much say 0.1u. The point is you may not want to connect your phone to 400VDC by accident. Once you have a rig like this you can inject signal into points in the amp and see what is passing signal and what is not. And you don't have to keep strumming a guitar while you debug.

Mike

I will give this a shot this week.

For those just joining up here’s a recap:

Amp passes signal, but very quiet and muffled
voltages are good throughout
Ground contacts go to ground
Several tubes known to be good have been tried (both preamp and power tubes, biased to 60-70% plate dissipation using Bias King - will install 1 Ohm resistors to confirm bias levels through a manual process this week)
Components and chain have been checked and double checked
Shielded cable from input has been swapped for regular wire
Pop test on plates nets a much quieter result at V1
All connections have been cleaned up and reflowed
Removing bright switch removes all signal
Bypassing tone stack removes signal
All preamp sockets have been rewired and double checked
Different speaker and speaker cable combinations make no difference
Input jack has been rewired and confirmed to have 1M resistance from tip to ground
Output transformer has been checked (resistance between centre tap and each side is 43 ohms)


The two bolded items consfuse me, but maybe they help narrow down where things are getting dumped?

[romberg]

...
Removing bright switch removes all signal
Bypassing tone stack removes signal
...

The two bolded items consfuse me, but maybe they help narrow down where things are getting dumped?

Yea. These are two new bits of information. Your volume pot may be bad and/or the ground connection to the volume pot may be bad. I'd disconnect the pot and test it with a DMM. If it is not getting a good ground connection and or has an open contact then that would explain things.

Mike

P.S. Might as well test the treble pot while you are at it. That one can cause similar trouble.

[prairie]

...
Removing bright switch removes all signal
Bypassing tone stack removes signal
...

The two bolded items consfuse me, but maybe they help narrow down where things are getting dumped?

Yea. These are two new bits of information. Your volume pot may be bad and/or the ground connection to the volume pot may be bad. I'd disconnect the pot and test it with a DMM. If it is not getting a good ground connection and or has an open contact then that would explain things.

Mike

P.S. Might as well test the treble pot while you are at it. That one can cause similar trouble.

I took the volume pot out of the circuit and it tested 1M but I’ll definitely test the treble pot as well.

For what it’s worth, the tone controls affect the sound coming through the speakers.

[romberg]

I took the volume pot out of the circuit and it tested 1M but I’ll definitely test the treble pot as well.

For what it’s worth, the tone controls affect the sound coming through the speakers.

If you tested the pot just end to end for being near 1M *also* test out the center wiper. If this connection is intermittent, it would explain why you loose signal when the brite switch is taken out of the circuit (all the signal is going through the brite switch).

Mike

[prairie]

ITS FIXED.

I had wired the B+4 lead to the wrong side of a resistor.

Holy hell I feel dumb. I greatly appreciate everyone’s input and help.