SluckeyAmps 1960 Vox AC15 Build

[sluckey]

Some DMMs can measure frequency. If yours can, then connect the meter to the speaker and get the amp to hum. Set the meter to measure ac volts. You should have a reading in the display that represents the amplitude of the hum. Now set the meter to read frequency (Hz).

You may want to build confidence in checking frequency by checking the filament voltage and switching to Hz mode. Should be a solid 60Hz.

[SkipJohnson]

Some DMMs can measure frequency. If yours can, then connect the meter to the speaker and get the amp to hum. Set the meter to measure ac volts. You should have a reading in the display that represents the amplitude of the hum. Now set the meter to read frequency (Hz).

You may want to build confidence in checking frequency by checking the filament voltage and switching to Hz mode. Should be a solid 60Hz.

My meter does measure Hz.  I recorded a frequency of ~120Hz when the amp produces the hum.

[shooter]

120hz is post rectifier hum, so PS, rather than filaments, unless filaments a rectified DC

[sluckey]

120Hz originates in the power supply. Doesn't necessarily mean that your problem is in the power supply though. Could still be a grounding issue.

Divide and conquer time... Pull V1, V2, V4, and V6. This leaves only the PI, power amp, and rectifier tubes plugged in. Do you still have the hum issue?

Next you will need a couple gator clip test leads and a filter cap to use as a test cap. 20µF at 450v is fine. Other values between 10 and 50µF are OK too, just be sure the voltage rating is sufficient for this amp. We will be bridging this test cap across each of the filter caps in the amp.Connect the negative lead of the test cap to chassis. Use the other clip lead to connect the positive cap lead to one of the cap terminals in the can and notice if the hum goes away. Now move to the other cap in the can and repeat. Next connect the test cap to each of the caps on the board. Did any of these tests kill or significantly decrease the hum? If so, replace that cap.

Report your findings.

[SkipJohnson]

120Hz originates in the power supply. Doesn't necessarily mean that your problem is in the power supply though. Could still be a grounding issue.

Divide and conquer time... Pull V1, V2, V4, and V6. This leaves only the PI, power amp, and rectifier tubes plugged in. Do you still have the hum issue?

There is still some hum, but different.  But other weird noises are now present as follows:
All knobs at 0:  Slight hum, less than original hum.
Tone knob at 3: Slight hum, a little different than the above.
Tone knob at 3, Nor Vol knob at 3: All quiet.
Tone knob at 0, Nor Vol knob at 3: Really loud squeal.
Tone knob at 3, Vib Vol knob at 3: Louder hum.

I will now work the other suggested tests.

[SkipJohnson]

Next you will need a couple gator clip test leads and a filter cap to use as a test cap. 20µF at 450v is fine. Other values between 10 and 50µF are OK too, just be sure the voltage rating is sufficient for this amp. We will be bridging this test cap across each of the filter caps in the amp.Connect the negative lead of the test cap to chassis. Use the other clip lead to connect the positive cap lead to one of the cap terminals in the can and notice if the hum goes away. Now move to the other cap in the can and repeat. Next connect the test cap to each of the caps on the board. Did any of these tests kill or significantly decrease the hum? If so, replace that cap.

Do I replace V1, V2, V4 and V6 for these tests?

[SkipJohnson]

Next you will need a couple gator clip test leads and a filter cap to use as a test cap. 20µF at 450v is fine. Other values between 10 and 50µF are OK too, just be sure the voltage rating is sufficient for this amp. We will be bridging this test cap across each of the filter caps in the amp.Connect the negative lead of the test cap to chassis. Use the other clip lead to connect the positive cap lead to one of the cap terminals in the can and notice if the hum goes away. Now move to the other cap in the can and repeat. Next connect the test cap to each of the caps on the board. Did any of these tests kill or significantly decrease the hum? If so, replace that cap.

Report your findings.

I just ran these tests with and without the V1, V2, V4 and V6 tubes.   None of the tests eliminated or changed the hum.

[sluckey]

1. Locate the two wires that go to the top cut pot. Disconnect these wires from the board. Does it still hum? Leave the top cut pot disconnected while doing the next couple tests.

2. Disconnect the wire from the normal volume pot center lug at the board. Does it still hum? Leave this wire disconnected.

3. Disconnect the wire from the vibrato volume pot center lug at the board. Does it still hum? Leave this wire disconnected.

At this point you have no top cut, no normal channel, and no vib channel, and hopefully no hum. What's the report?
 

[SkipJohnson]

1. Locate the two wires that go to the top cut pot. Disconnect these wires from the board. Does it still hum?

Note that the condition to create the hum is all knobs at zero.
When I disconnect either/both wires from the Top Cut Pot the hum goes away.  The amp plays perectly (minus tone control).
If I connect the two disconnected Top Cut wires with a gator clip, there is no hum.
I replaced the Top Cut pot with a new one, the hum is back.
It seems that Top Cut knob is at least related to the underlying issue.

I did not leave the Top Cut DISconnected for the following tests as the hum was gone in that state.
Disconnected the Nor Vol pot. The hum was still present.
Disconnected the Vib Vol pot. The hum was still present.

Reconnected everything.

[sluckey]

Could you post a hi-rez pic that shows the top cut pot with wiring to the board? I want to see that entire left end of the board.

Then remove the knob and nut from the top cut pot and fold the pot back over the top of the board. Use a piece of cardboard to prevent the pot from shorting to anything on the board. Is the hum still present?

[SkipJohnson]

Could you post a hi-rez pic that shows the top cut pot with wiring to the board? I want to see that entire left end of the board.

https://www.flickr.com/photos/23805934@N02/46792688424/in/album-72157626245738182/
https://www.flickr.com/photos/23805934@N02/46792688194/in/album-72157626245738182/
https://www.flickr.com/photos/23805934@N02/46792687814/in/album-72157626245738182/
https://www.flickr.com/photos/23805934@N02/46792687294/in/album-72157626245738182/

Then remove the knob and nut from the top cut pot and fold the pot back over the top of the board. Use a piece of cardboard to prevent the pot from shorting to anything on the board. Is the hum still present?

Still hum present.

[sluckey]

When I disconnect either/both wires from the Top Cut Pot the hum goes away.  The amp plays perectly (minus tone control).
If I connect the two disconnected Top Cut wires with a gator clip, there is no hum.

So, you actually disconnected the two red wires at the pot rather than at the board? And then you used a gator clip to connect the two red wires together?  And there's no hum?

This is crazy! Dead short and no hum. That's exactly what you have when you set the pot to zero (dead short), except you have hum.

Try this... With everything connected as normal, set the pot to zero. You should have hum. Now take your gator clip and short out the left and center pot terminals. Still hum?

[SkipJohnson]

This is crazy! Dead short and no hum. That's exactly what you have when you set the pot to zero (dead short), except you have hum.
Try this... With everything connected as normal, set the pot to zero. You should have hum. Now take your gator clip and short out the left and center pot terminals. Still hum?

Yes, hum still present 

I decided to re-run some tests and write down each step as follows:

All knobs to Zero
   • Hum Present

All knobs to Zero
Turn Tone up to 3
   • Hum gone
   • Amp operates properly

All knobs to Zero
Turn Nor Vol up to 3
   • Hum gets Louder

All knobs to Zero
Turn Vib Vol up to 3
   • Hum gone
   • Amp operates properly

All knobs to Zero
Disconnect left Nor Vol pot wire
   • Hum gone
   • Amp operates properly minus tone control

All knobs to Zero
Connect Disconnected left Nor Vol pot wire to left Nor Vol pot via gator clip
   • Hum Gone
   • Amp operates properly INCLUDING tone control
 

All knobs to Zero
Completely Disconnected left Nor Vol pot wire from pot and board
   • Hum Gone
   • Amp operates properly minus tone control

All knobs to Zero
Connect left Nor Vol pot connection to board with gator clip
   • Hum Gone
   • Amp operates properly INCLUDING tone control
 

All knobs to Zero
Install temporary new wire from left Nor Vol pot connection to board
   • Hum is Back!!!!!!!!!
 

I really do not understand this RELLY weird behavior.

[shooter]

left Nor Vol pot wire

which view? are you talking signal or ground?
As bizarre as this is, have you wired the pot backwards swap outside wires 

[sluckey]

Disconnect both green wires from the Brilliance rotary switch. Remove the rotary switch from the amp. Temporarily solder the free ends of the two green wires together. Got hum?

[SkipJohnson]

Disconnect both green wires from the Brilliance rotary switch. Remove the rotary switch from the amp. Temporarily solder the free ends of the two green wires together. Got hum?

Hum still present.

BTY:  In my previous post, the last 5 tests listed contained a cut/paste error.  The Nor Vol referenced should have been Top Cut

[SkipJohnson]

This is the test that boggles my mind more than any other:

All knobs to Zero
Disconnect left red Top Cut pot wire
   • Hum gone
   • Amp operates properly minus tone control

All knobs to Zero
Connect the Disconnected left red Top Cut pot wire to left Top Cut pot via gator clip
   • Hum Gone
   • Amp operates properly INCLUDING tone control

This makes no sence at all.
Has anyone seen anything like this before???  Where inserting an aligator clip into a connection changes the function of the amp.
Help appreciated as always.

In the mean time, I am starting some work on my next project.  Tearing down my AA764 Champ (https://www.flickr.com/photos/23805934@N02/47529605471/in/dateposted-public/) and rebuilding it into a 5F1 Champ.  I really never liked the AA764. It has ZERO character.  While both my 5F1 Champ (https://www.flickr.com/photos/23805934@N02/47529605951/in/dateposted-public/) and 2x5F1 Stereo Champ (https://www.flickr.com/photos/23805934@N02/47476826082/in/dateposted-public/) are awesome.  They scream character.

[shooter]

This makes no sense at all.

to me, it's an "antenna" issue, the gator clip provides better blockage to stray noise than your wire.

try this. leave the wire connected then parallel the gator-clip with the wire.

[sluckey]

BTY:  In my previous post, the last 5 tests listed contained a cut/paste error.  The Nor Vol referenced should have been Top Cut

No harm done. Just a short detour down a dead end street. 

I'm wondering if that yellow OT wire is involved. It's very close to those magical red wires. Disconnect that yellow wire from the speaker jack, pull it out from under the board (hope you can get to that tape!), re-route around the end of the board keeping it away from the top cut wires, and reconnect to the speaker jack. I would plan to make this a permanent change. You'll likely have to lengthen that yellow wire.

Any change?

[SkipJohnson]

This makes no sense at all.

try this. leave the wire connected then parallel the gator-clip with the wire.

Makes an even more annoying hum/squeal.
I agree with the "antenna" idea though. Here is more WEIRD behavior supporting this idea.
I connect the gator clip to the Tope Cut board side connection only, then set the other end on the work surface, and the hum goes away.
Put the unconnected side near my laptop, the hum returns.

[sluckey]

Another thought... Replace those two red wires. They look like low voltage control wiring (HVAC t-stat, or doorbell, or sprinkler system wires). That shouldn't be a problem for the top cut pot, but I think I also see some of that same wire used on the power tube screens. If it is in fact low voltage control wire I would replace it.

EDIT... Replace those red wires before you re-route the OT yellow wire. But rout the new top cut wires above the board, keeping them far away from any OT wires. Don't route them through those wire pass through holes on the board.

 

[SkipJohnson]

Another thought... Replace those two red wires. They look like low voltage control wiring (HVAC t-stat, or doorbell, or sprinkler system wires). That shouldn't be a problem for the top cut pot, but I think I also see some of that same wire used on the power tube screens. If it is in fact low voltage control wire I would replace it.

I am using 22 ga Solid 300v wire for all the turret board connections.  Is this wire not right for this application?

[sluckey]

Another thought... Replace those two red wires. They look like low voltage control wiring (HVAC t-stat, or doorbell, or sprinkler system wires). That shouldn't be a problem for the top cut pot, but I think I also see some of that same wire used on the power tube screens. If it is in fact low voltage control wire I would replace it.

I am using 22 ga Solid 300v wire for all the turret board connections.  Is this wire not right for this application?

That wire is fine. I'm talking about LOW VOLTAGE CONTROL WIRE. 24VAC stuff.

[Willabe]

I know Sluckey and others here have built this amp with Slucky's layout and not had this problem, but since you are;

EDIT... Replace those red wires before you re-route the OT yellow wire. But rout the new top cut wires above the board, keeping them far away from any OT wires. Don't route them through those wire pass through holes on the board.

Yes, agree, but make them a tightly/evenly twisted pair when you do it. Very important that they are tightly and evenly twisted for it to work.

Those 2 wires are out of phase and twisting them will give some cancellation/protection to them. It might be enough to knock out the problem. I would have done it from the start. It's just good lead dress practice. 

I'm wondering if that yellow OT wire is involved. It's very close to those magical red wires. Disconnect that yellow wire from the speaker jack, pull it out from under the board (hope you can get to that tape!), re-route around the end of the board keeping it away from the top cut wires, and reconnect to the speaker jack.

I'd try another tightly twisted pair here with the OT speaker jack wires. I'd have done this from the start, twist the OT secondary ground wire with the +speaker jack wire. Run them both together to the speaker jack and then, if your speaker jack is isolated from the chassis, run a single ground wire back to the power amp ground. Aiken is pretty strong on not having the OT ground current run through the chassis. So is Merlin and Kevin O'Connor.

Between moving those top cut wires away from the OT secondary wires AND twisting them tightly AND making the OT secondary wires a tightly twisted pair to the output speaker jack, that might knock out the problem. Probably have to do both. Unless, see below;

 
You did run a ground wire from the speaker jack back to the power amp ground, where you have the OT secondary common (ground) wire hooked up?

I don't see a ground wire going from the OT secondary to the speaker jack.


And since this amp has no -FB loop, I'd try moving the PI ground from the power amp ground to the preamp ground. 

Again, I know this worked fine with no problems for Slucky and others the way Sluckey did his grounding, but you've got a bug-a-boo. 

[Willabe]

Yes, I know he's using a much larger amp as an example but still,

From Aiken amps site;

What about the output jacks?

    The current in the secondary winding of the output transformer can be very large.  For example, in a 100W amp, the secondary current into a 16 ohm load is 2.5A.  It is even higher into a 4 ohm load, at 5A. This means that you need to pay special attention to the grounding of the output jacks and the output transformer.  It is important not to use the chassis for this return path. 

    The output transformer secondary has a common wire and one or more speaker taps, usually at 4, 8, and 16 ohms.  The speaker taps usually go to an impedance selector switch, and then a single wire goes to the output jack tip connection.  The common wire should never be connected to the chassis right at the output transformer.  It should be run all the way to the output jack and connected to the sleeve  connection of the jack.  This accomplishes two things.  First, it maintains the continuity of the connection in the event the output jack becomes loose.  Second, it keeps the heavy secondary ground return currents from flowing in the chassis. 

    Note that there still must be a ground return path to the rest of the circuit if the amp uses global negative feedback.  This should be in the form of a wire from the sleeve connection of the output jack to the preamp ground point where the phase inverter common connections are grounded, or to the ground of the portion of the circuit where the global feedback from the tip connection of the output jack is connected.  Note that there will be no heavy currents in this wire.  The speaker output jacks can be either isolated or non-isolated if you follow this plan, but it is usually best to isolate them to maintain control of the return current path for the global negative feedback, to insure it doesn't flow through a part of the chassis that may contain power supply ground currents.

    Sometimes it helps to ground the common (sleeve) side of the output jack to the chassis even when no global negative feedback is used.  Occasionally, an amplifier will have a high-pitched oscillation noise, or other type noise that will go away if you ground the output transformer common wire at the speaker jack sleeve terminal.   In addition, there may be a potential for a  small AC current to flow between the floating sleeve and chassis if the secondary is not grounded.  This current is due to  capacitive coupling  in the output transformer, and may cause a mild shock if the speaker plug sleeve and chassis (or guitar strings) are touched while running a signal through the amplifier.  Even though the potential for dangerous currents is low due to the galvanic isolation of the output transformer, the shock can still be annoying.  For this reason, it is best to always ground the common side of the secondary even when no global feedback is used.

From Merlin's site;

The secondary side of the output transformer (if one is used) should always be wired directly to the speaker jack using heavy-gauge wire. This is true no matter what ground scheme is used. A separate wire (which does not need to be heavy gauge) should then run from the negative connection of the speaker jack back to an appropriate star. If global feedback is not used then this speaker ground wire should be returned to the power-amp star. If global feedback is used then the speaker ground should be returned to the local star of whichever stage the feedback happens to be applied to, which is usually the phase inverter.

[ElusiveMoose22]

Hi,
New to your post but have been following silently a bit and maybe can help.

I agree with the "antenna" idea though. Here is more WEIRD behavior supporting this idea.
I connect the gator clip to the Tope Cut board side connection only, then set the other end on the work surface, and the hum goes away.
Put the unconnected side near my laptop, the hum returns.

But is it THE same hum.  Does the hum from your laptop register the same 120Hz you got earlier with the pot connected?  Even if it is the same frequency this could be a false positive. I think it would be unusual for shaft or any other part of your Top Cut pot to act like an antenna picking up outer noise, especially with the nob installed on the pot.  And it seems unlikely the red wires inside the chassis would be picking up noise from anywhere outside the amp as well.  It's a lot more likely they are picking up noise from inside the chassis somewhere.  That yellow OT wire sluckey mentioned is suspicious.  Also replacing the red wires and running them on top of the board as suggested can't hurt.  Neither can putting a twist in them.  Hopefully some combination of that advise from sluckey does the trick.  Good luck!

[shooter]

Didn't mean to infer outside noise, just any noise, and if I'm not mistaken  R, C, and wire, make a pretty good dipole antenna (top cut circuit) 

[Willabe]

Didn't mean to infer outside noise, just any noise, and if I'm not mistaken  R, C, and wire, make a pretty good dipole antenna (top cut circuit)

Yes, anything after a coupling cap to the next stages grid will act like an antenna, that's why we often use shielded cable for grid wires.

[SkipJohnson]

Another thought... Replace those two red wires. They look like low voltage control wiring (HVAC t-stat, or doorbell, or sprinkler system wires). That shouldn't be a problem for the top cut pot, but I think I also see some of that same wire used on the power tube screens. If it is in fact low voltage control wire I would replace it.

I am using 22 ga Solid 300v wire for all the turret board connections.  Is this wire not right for this application?

That wire is fine. I'm talking about LOW VOLTAGE CONTROL WIRE. 24VAC stuff.

I never use that in any of my builds.

[SkipJohnson]

EDIT... Replace those red wires before you re-route the OT yellow wire. But rout the new top cut wires above the board, keeping them far away from any OT wires. Don't route them through those wire pass through holes on the board.

Yes, agree, but make them a tightly/evenly twisted pair when you do it. Very important that they are tightly and evenly twisted for it to work.

You did run a ground wire from the speaker jack back to the power amp ground, where you have the OT secondary common (ground) wire hooked up?

I don't see a ground wire going from the OT secondary to the speaker jack.

And since this amp has no -FB loop, I'd try moving the PI ground from the power amp ground to the preamp ground. 

Working on the new red Top Cut wires, above board and evenly twisted.

I do have a ground wire from the speaker jack back to the power amp ground, where the OT secondary common (ground) wire is hooked up?

I'll move the PI ground from the power amp ground to the preamp ground. 

Tune back in tomorrow for an update..........................
Oh, and I really appreciate all the help.  I am learning a great deal on not only techniques but debugging.  And learning is one of the best things about this hobby.  thanks everyone!!!!!!

[Willabe]

Because it's probably a problem area, I'd take the utmost care with those top cut wires.

A lot of guys twist wire pairs in a battery hand drill driver. They come out very tight and even.

I'd run the 2 wire ends towards each other 1st from each turret staying on/over the turret board, but away from that middle turret, so they are twisted together as soon as possible, with just a little slack in the 2 wire ends, not taught like a guitar string. Then with the 1st twist sitting on the board, straight up from the turret board, to keep them away from the OT wires. Then start to make a nice smooth arched curve towards the top cut pot, with the top of the arch level with the pots lugs. And the arch will give the wires a little room to move, flex, if they need it from the amp getting bumped when moving it.

More twists are better than less twists. Even though it's a short distance, 2 or 3 or 4 twists are not enough. 

[sluckey]

I really don't believe the lead dress of those two wires is the issue. I rather believe this behavior is a crazy symptom of the real issue. Look at my wiring. I didn't do anything special with wiring the top cut...

     http://sluckeyamps.com/VAC15/big_guts.jpg

I wonder if there is a gremlin living near that workbench. Have you tried the amp in another room? Or, even better, take it to another location. Same behavior?

I also am not convinced that your grounding scheme is not a factor.

[Willabe]

I really don't believe the lead dress of those two wires is the issue. I rather believe this behavior is a crazy symptom of the real issue. Look at my wiring. I didn't do anything special with wiring the top cut...

I did and I noticed that your OT leads come through under the rotary tone control. Skip's come through right under the top cut pot. And, like you noticed, is very close to 1 of the top cut pot leads, part of the power tube gird circuit. That OT wire has a lot of current going through it and is very close to a grid wire. 

Plus, your OT wire then runs under the board on an angle that's different than Skip's.   

He might still have to move the OT secondary around the end of the board like you suggested.

His lead dress at the EL84 tubes should get cleaned up too, some wires shortened, moved.

There's a grid wire laying along side that OT secondary wire that I don't trust.

[Willabe]

I also am not convinced that your grounding scheme is not a factor.

????

Skip's grounding scheme could be the problem?

[SkipJohnson]

You guys are talking about my grounding scheme.  Let me tell you where I am at on this:

My original design and implementation used an Isolated Grounding scheme as I described in an earlier thread:
"I have decided to go with an Isolated Grounding Scheme.  This has proved very effective on my previous builds in reducing unwanted noise.  My goal is to isolate the Pre-amp grounding from the Power Amp grounding.
In order to do this I isolate the grounds for the input jacks, control panel components and the circuit board low power grounds from the chassis.  These grounds are run to terminal strips that are also isolated from the chassis.  Isolating the control panel pots and switches from the chassis is not necessary as these are already isolated from the component bodies.
For the input jacks I use Cliff Jacks, the jacks bushing is plastic so the ground is isolated from the chassis.
The terminal strips must be isolated by using plastic nuts, bolts and washers."
See 1st attachment ZZ-AC15 Ground Scheme V1.0

After my build was completed and the "HUM" was discovered the debugging process started.  At one point sluckey suggested I change my grounding scheme as follows:
"I don't like that ground scheme! I suggest you remove that orange wire that ties your preamp/control panel grounds to the power ground. The control panel grounds (I used a buss) and the NOR and VIB board grounds should all tie together to a chassis ground lug that is mounted very near the input jacks. The chassis will provide the return connection to the PT red/yel center tap.

The board PA and PI grounds, OT sec. ground(s), speaker jack ground, and cap can ground should all tie to the same chassis ground lug as the PT red/yel center tap. These power grounds should not rely on the chassis for current flow back to the PT red/yel center tap. This power ground lug should be physically located near the PT and cap can. You can eliminate one of those terminal lug strips."
See 2nd attachment ZZ-AC15 Ground Scheme V2.0

As part of my latest changes I am implimenting a change to the PI ground per Willabe:
"And since this amp has no -FB loop, I'd try moving the PI ground from the power amp ground to the preamp ground. "
See 3rd attachment ZZ-AC15 Ground Scheme V2.1

I am almost done with the 3 changes:
1) Re-route the yellow OT speaker wire around the chassis to the output jack
2) Move the PI ground from the power amp ground to preamp ground
3) Re-wire the red Top Cut wires as tightly twisted pair, precisely positioned wrt to the board and pot

Stay tuned...……………...

[Willabe]

Try lifting the EL84 grid wires up in the air and see if that makes a difference.

And try moving that OT secondary wire with a wooden chop stick, on both ends, 1 end at a time and see if that makes a difference. You have to do this with the amp on.

Look at Slucky's lead dress at the EL84 tubes and try to get yours closer to that.

[Willabe]

I am almost done with the 3 changes:
1) Re-route the yellow OT speaker wire around the chassis to the output jack
2) Move the PI ground from the power amp ground to preamp ground
3) Re-wire the red Top Cut wires as tightly twisted pair, precisely positioned wrt to the board and pot

Do them 1 at a time and test the amp to hear any change.   

[Willabe]

The control panel grounds (I used a buss) and the NOR and VIB board grounds should all tie together to a chassis ground lug that is mounted very near the input jacks. The chassis will provide the return connection to the PT red/yel center tap.

Did you install a chassis ground lug close to the input jack?

I don't see it on your drawing?

The PT high voltage plate wind CT should go  -directly-  to the 1st cap can ground, soldered to the can's ground lug, then run a wire to the power amp grounds, like you have.

And the green/yellow PT ground wire that goes to the AC power cord wire/ 3rd safety ground wire, is that the heater CT?

If so that should go to the power grounds. 

[ElusiveMoose22]

Stay tuned...……………...

I sure am!  This debug has been an interesting one, I'm pulling for you to figure it out!

I am almost done with the 3 changes:
1) Re-route the yellow OT speaker wire around the chassis to the output jack
2) Move the PI ground from the power amp ground to preamp ground
3) Re-wire the red Top Cut wires as tightly twisted pair, precisely positioned wrt to the board and pot

Do them 1 at a time and test the amp to hear any change.   

Right this could help isolate the issue so you can trace its origins and avoid similar issues in the future.  Also important for catching any new issues that could arise from these changes right away so you're not adding more mystery issues that have to be debugged.

I wonder if there is a gremlin living near that workbench. Have you tried the amp in another room? Or, even better, take it to another location. Same behavior?

This is a great thing to check as well.  Any luck trying the amp elsewhere, away from your work area and hopefully in a new location to make sure you're not experiencing noise unique to the area you've been testing in?

[SkipJohnson]

I am almost done with the 3 changes:
1) Re-route the yellow OT speaker wire around the chassis to the output jack
2) Move the PI ground from the power amp ground to preamp ground
3) Re-wire the red Top Cut wires as tightly twisted pair, precisely positioned wrt to the board and pot

Do them 1 at a time and test the amp to hear any change.

Too late.  I had already gone to distance on these changes so all were implemented at the same time.
Hum still present, no change.

[SkipJohnson]

Try lifting the EL84 grid wires up in the air and see if that makes a difference.

And try moving that OT secondary wire with a wooden chop stick, on both ends, 1 end at a time and see if that makes a difference. You have to do this with the amp on.

Look at Slucky's lead dress at the EL84 tubes and try to get yours closer to that.

Did this.  My EL84 wiring looks like Slucky's  as far as I can tell.  Moving wires with a chopstick yielded no changes in the hum.

[Willabe]

Try this now;

Try lifting the green(?) EL84 grid wires up in the air and see if that makes a difference.

And try moving that OT secondary wire with a wooden chop stick, on both ends, 1 end at a time and see if that makes a difference. You have to do this with the amp on. You already moved this wire.

Look at Slucky's lead dress at the EL84 tubes and try to get yours closer to that.

And the green grid wire should not be laying alongside that blue plate wire.

[Willabe]

And then this;

The control panel grounds (I used a buss) and the NOR and VIB board grounds should all tie together to a chassis ground lug that is mounted very near the input jacks. The chassis will provide the return connection to the PT red/yel center tap.

Did you install a chassis ground lug close to the input jack?

I don't see it on your drawing?

The PT high voltage plate wind CT should go  -directly-  to the 1st cap can ground, soldered to the can's ground lug, then run a wire to the power amp grounds, like you have.

And the green/yellow PT ground wire that goes to the AC power cord wire/ 3rd safety ground wire, is that the heater CT?

If so that should go to the power grounds.

The safety ground chassis connection should always be alone, nothing else connected with it.

[SkipJohnson]

The control panel grounds (I used a buss) and the NOR and VIB board grounds should all tie together to a chassis ground lug that is mounted very near the input jacks. The chassis will provide the return connection to the PT red/yel center tap.

Did you install a chassis ground lug close to the input jack?

I don't see it on your drawing?

Yes.  It is the 5 lug terminal strip. It is below the input jacks.

-------

The PT high voltage plate wind CT should go  -directly-  to the 1st cap can ground, soldered to the can's ground lug, then run a wire to the power amp grounds, like you have.

I'll need to make that change, it is on my list of changes (to be made one at a time  ).
-------

And the green/yellow PT ground wire that goes to the AC power cord wire/ 3rd safety ground wire, is that the heater CT?

If so that should go to the power grounds.

Yes it is.  That is the way it is on Slucky's (per the Big Guts picture).

[SkipJohnson]

Try this now;

Try lifting the green(?) EL84 grid wires up in the air and see if that makes a difference.

And try moving that OT secondary wire with a wooden chop stick, on both ends, 1 end at a time and see if that makes a difference. You have to do this with the amp on. You already moved this wire.

Look at Slucky's lead dress at the EL84 tubes and try to get yours closer to that.

And the green grid wire should not be laying alongside that blue plate wire.

I lifted the green grid wires up away from the other wires.  No change.

[SkipJohnson]

And then this;

The control panel grounds (I used a buss) and the NOR and VIB board grounds should all tie together to a chassis ground lug that is mounted very near the input jacks. The chassis will provide the return connection to the PT red/yel center tap.

Did you install a chassis ground lug close to the input jack?

I don't see it on your drawing?

The PT high voltage plate wind CT should go  -directly-  to the 1st cap can ground, soldered to the can's ground lug, then run a wire to the power amp grounds, like you have.

And the green/yellow PT ground wire that goes to the AC power cord wire/ 3rd safety ground wire, is that the heater CT?

If so that should go to the power grounds.

The safety ground chassis connection should always be alone, nothing else connected with it.

I can change this. It is added to my list.

[Willabe]

Did you use Cliff jacks for your speaker jacks? 

[SkipJohnson]

Did you use Cliff jacks for your speaker jacks?

No. Only for the input jacks.

[Willabe]

What is the ground for in the red circle? 

Exactly how many ground connections to the chassis do you have, 2, 3, 4, 5?

[Willabe]

Can we see some pictures of the whole amp, inside? And I'd like to see where the speaker jacks are and whats next to them.

And some pics of the back side of the chassis, showing the PT/OT?