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

[sluckey]

I disconnected the primary leads and checked the impedance.

Reconnect the primary leads. Disconnect the two yellow leads ***AND*** the two brown leads. You can leave the two center tap wires connected to chassis. Now connect the power cord straight into the wall. Fuse blow? If so the PT is bad.

[prairie]

The primary DC resistance will be very low, a few ohms. Does the fuse blow with all secondaries disconnected?

Yep. Disconnected the 6.3v and 300v taps. Same result.

Could I have my power switch wired wrong or have the wrong type?

[sluckey]

Could I have my power switch wired wrong or have the wrong type?

The fact that you got voltage readings on the tubes eliminates power switch wiring. Looks like a bad PT.

[prairie]

Could I have my power switch wired wrong or have the wrong type?

The fact that you got voltage readings on the tubes eliminates power switch wiring. Looks like a bad PT.

Well that settles it. Ordering a new PT. BALLS.

[prairie]

UPDATE:

I ordered and installed a new PT (Heyboer HTS-5199) and everything fires up without issue (both through the limiter and directly into the wall). Though I don't have any guitar sounds coming through my speakers. That's not to say I don't have sound, however. When I turn the presence potentiometer, I get sort of a static sound through the speakers (think what it would sound like if you dragged a microphone across your pant leg), and a hum. So clearly there's output to the speakers (I've opted to wire a jack for each impedance, 4/8+8/16).

I'm using shorting jacks for my speaker jacks and have them grounded to the chassis (metal washers). My common lead (black) from my OT (Heyboer HTS-5200) is going to the ground lug of my 8 ohm speaker jack. I noticed that I'm getting continuity with ground across all jack contacts (I'm assuming that's from the centre tap being connected internally with the 4, 8, and 16 ohm taps and then the CT going to ground). Is that normal?

My center tap (Red) is going to my standby switch, as per the layout I'm using.

Judging that I have signal through my speakers from the presence control, I'm thinking my signal is getting dumped before the OT. Does that sound like what's happening?

[ViperDoc]

UPDATE:
My center tap/common from my OT (Heyboer HTS-5200) is going to the ground lug of my 8 ohm speaker jack.

Look at your schematic. If when you say "center tap/common from my OT", those are two different leads. Your OT PRIMARY CENTER TAP should be connected to the power supply filter cap node (80 uF). This will deliver supply voltage to your power tube plates. What voltage do you probe on your EL34 pin 3?

Your OT SECONDARY COMMON should be grounded, fine with the speaker jack ground.

Scratchiness on the Presence control is "normal".

[martin manning]

I ordered and installed a new PT (Heyboer HTS-5199) and everything fires up without issue (both through the limiter and directly into the wall).

good!

Though I don't have any guitar sounds coming through my speakers. That's not to say I don't have sound, however. When I turn the presence potentiometer, I get sort of a static sound through the speakers (think what it would sound like if you dragged a microphone across your pant leg), and a hum. So clearly there's output to the speakers...

A scratchy presence pot is normal for this circuit, but you still have something wrong somewhere.

...(I've opted to wire a jack for each impedance, 4/8+8/16)... I'm using shorting jacks for my speaker jacks and have them grounded to the chassis (metal washers).

Are unused taps still shorting the output to ground when you plug into any one of them? That won't do.

My center tap/common from my OT (Heyboer HTS-5200) is going to the ground lug of my 8 ohm speaker jack. I noticed that I'm getting continuity with ground across all jack contacts (I'm assuming that's from the centre tap being connected internally with the 4, 8, and 16 ohm taps and then the CT going to ground). Is that normal?

Are you talking about the secondary? There is no center tap. Sounds like you have 4, 8, and 16 ohm taps and a common.

Judging that I have signal through my speakers from the presence control, I'm thinking my signal is getting dumped before the OT. Does that sound like what's happening?

That could be DC on the presence pot getting to the speaker directly via the feedback loop.

[prairie]

UPDATE:
My center tap/common from my OT (Heyboer HTS-5200) is going to the ground lug of my 8 ohm speaker jack.

Look at your schematic. If when you say "center tap/common from my OT", those are two different leads. Your OT PRIMARY CENTER TAP should be connected to the power supply filter cap node (80 uF). This will deliver supply voltage to your power tube plates. What voltage do you probe on your EL34 pin 3?

Your OT SECONDARY COMMON should be grounded, fine with the speaker jack ground.

Scratchiness on the Presence control is "normal".

Apologies, my Common from the OT is going to the ground lug of my 8ohm jack. The center tap (red lead) is going to the standby switch.

[prairie]

Another update!

I disconnected the shorting jacks last night and replaced one with a non-shorting jack (and then wired the 8 ohm tap directly to it, common to the ground lug of the jack). I was able to get guitar signal through the speakers, though it was very quiet (with the volume control nearly maxed, it was at talking level) and pretty muffled. No more hum, though, so I'll call that progress!

I'm going to try and replace all jacks with non-shorting ones, try a new set of power tubes (I'm using a pair of used ones pulled from another amp for testing), and see what difference is realized.

I went through each connection as well to make sure nothing was wired in the wrong spot and that all connections were solid and with good contact. Everything looked good on that front.

I'll report back once I've changed out the power tubes and the speaker jacks.

[sluckey]

I'm going to try and replace all jacks with non-shorting ones

You don't need to replace the jacks. Just remove the jumper between the sleeve and switch lugs.

[prairie]

I'm going to try and replace all jacks with non-shorting ones

You don't need to replace the jacks. Just remove the jumper between the sleeve and switch lugs.

Just snip it off?

[martin manning]

Yes. The speaker should be the only load connected to the secondary, and the unused taps should be open (unconnected).

[prairie]

Yes. The speaker should be the only load connected to the secondary, and the unused taps should be open (unconnected).

Beautiful. I’ll make the revision and report back!

[prairie]

Corrected the output jacks, put a set of new EL34’s in and biased them, and while I’m getting more signal, it’s still very quiet. Like no louder than moderately loud talking. And it’s pretty muffled too.

Any ideas?

[romberg]

This is sorta a scientific wild a$$ guess based on it being quite and muffled. Maybe the tone stack is not wired correctly and or has a bad pot or something. Try lifting the tone stack out of the circuit by disconnecting the mid pot from the ground buss. Does that change anything?

Mike