SluckeyAmps 1960 Vox AC15 Build

[SkipJohnson]

I have been building tube amps over the last year and a half.  All my builds (10 vintage style tube amps including 2 Champs, 4 Princetons, 1 Bassman, 1 Deluxe, 1 Stereo Champ and a Fender Reverb Unit) have been based on Fender designs.  I have decided my next build needed to be something different and I decided on a British design this go.  I came across Sluckey's 1960 Vox AC15 site (sluckeyamps.com) and decided this was the right build for me to take on.  Further research on this build has lead me here.

I have been researching this build for a while and have collected as much documentation as I could find including the documentation from Sluckey's 1960 Vox AC15 site as well as here on the Hoffman Amp site.  I have found some discrepancies between the documentation (slight design differences, Mod's, BOM lists, etc..) so my first goal is to have a complete set of design documentation and vet it here on the forum before building.  I am hoping some of you can help me fill in the gaps of my documentation.

I want to procure/develop the following design documentation:
NOTE:  I would love to get Visio (.vsd) or DIY Layout Editor (.diy) files for these if anyone has them.  I could then maintain my own version as I make slight changes along the way.
   • Schematic - I have two (2) versions that are but these are workable so I am good here.
   • Power Section Chassis Layout - I could not find anything for this.  This design is pretty different from my Fender builds so this is important because I have never included a standby switch or used this type of rectifier in any of my builds.  I created my own version(s) for this using Visio. I have two versions based on some differing documentation I have.  I will share these to get your input.
   • Grounding Scheme Diagram - I have Sluckey's design to use.  I may also use isolated input jacks as they have been very good in reducing noise in my prior builds.  I will look forward to input from you all on this subject. 
   • Circuit Board Layout - I Sluckey's design to use.  I would really like a Visio or DIY file for this.
   • Circuit Board Blueprint - I Sluckey's design to use.  I also purchase the completed board with turrets from the Hoffman Amp site.  I just need to drill the mounting and wire-thru holes.
   • Complete Chassis Layout Diagram - I usually create a compete Chassis Layout that includes everything (power section, grounding, circuit board, controls, etc.)  I would love to get one if anyone has one, or I may create one myself.
   • Chassis Blueprint - I am comfortable doing this on my own as I have done many.
   • BOM (Parts List) - The two BOM lists I have found are slightly different.  I will create my own version of this as I get the documentation put together.

Here is where I am at on this project:
   • Collected as much documentation as I could find.  I could use some help completing this.
   • I have ordered/received all the parts based on the BOM I have.

Look forward to getting your input and help as this build progresses.

[pompeiisneaks]

I'll be willing to bet sluckey will help on all fronts, since he's a  moderator here and the most helpful person I think I've met :)

I've built the same amp, but did the 30 watt version (ac 30/4) and love it, so I also know a bit about the amp. 

Not sure if you've found this link but there's a long forum thread on the build here:

https://el34world.com/Forum/index.php?topic=17184.0

You can order the turret board for it from doug here on the site (see the link in the above forum thread).

I think you'll love it, the tremolo is really cool, and I love the gain of the EF86 on this thing.

Please post build pictures for us, we love em.

~Phil

[sluckey]

Hi Skip. Welcome to the forum. Glad you're interested in the AC-15. I'll be following your progress with interest.

I have found some discrepancies between the documentation (slight design differences, Mod's, BOM lists, etc..) so my first goal is to have a complete set of design documentation and vet it here on the forum before building.  I am hoping some of you can help me fill in the gaps of my documentation.

Yeah, there were several discrepancies. Several people have built this project using my docs and I thought we had ironed it out pretty good, but always some more gremlins hiding. It seems the more you try to produce accurate docs the more hiding nooks you create for the gremlins. So, let's get those cleared up so you can have a successful project. Start listing particular errors you have found and we'll tackle them one by one. I'll make corrections as needed and then I'll make the revised Visio drawings available to you.

Meanwhile I'll give you something to start thinking about. I don't have a chassis layout for this amp. But, the main thing is to use a 20" wide chassis. My board layout is drawn full scale. So are the tubes and control panel. You will need to treat the board, control panel and tube lineup as one unit. IOW, don't deviate from this part of the layout. I spent a lot of time getting the front panel, board, and tube lineup to work together with a neat and quiet layout. My board and tube lineup will not work well with the original AC15 front panel. You would have a rats nest of wires. I can give you more specifics and you will also be able to see all the dimensions and how they are related when you get the actual Visio file. All other components (mainly power supply stuff) were fitted after the board, tubes and front panel holes were drilled. And I included a lot of pics that should give you some good ideas about how to put it all together, including the power supply. Keep in mind, you will have to work from the schematic to wire the power supply.

So, let's get started. There are several others that will likely join in also. One guy has built quite a few of these so he probably has a lot to bring to the table. Just take your time, measure twice, cut once, and ask questions. And by all means, lets fix the discrepancies in the docs.

[Ed_Chambley]

I have now built 7 of these thing.  15 Watters with an EF86 (which now I add switching values for Vox and Matchless).  Doug has the board, but I like making it.  Loads of turrent to stake.


The Chassis I use is 19" x 8" x 2.5.  This build documentation must work as I have never had to troubleshoot a single one.  Fires right up.  I added a Fender Reverb to one.  Waste of time IMO.  Effects loop, sure.  The amp will quickly move into overdrive, but not noticeable unless Reverb is heavy and then it gets real loose.  I tried additional filtration, but it loses the true old Vox feel.


I had an original when younger and it simply overheated and ate tubes.  I run voltages lower than Sluckey and it makes a difference.  Simply a bit lazy and spongy.  I am closer to 300VDC on the plates.  Almost exactly as the Schematic.  The tone is the same with about 350 Plate volts and does have noticeably more headroom, but I have a 18 Watt EL84 that punches a lot, I wanted more sag with this one.


I built a head version and a single 12 cab and used a Celestion Blue.  Sounds perfect, but I no longer use 2 inputs.  One combine both and volumes work to mix the 12AX7 Tremolo with the EF86 preamp.  Go ahead and plan the slowing of the tremolo, the original design doesn't get slow enough for me.  YMMV!


Welcome and have fun.  Great Re-design!

[SkipJohnson]

Progress:

Since my last post I have been busy creating my own set of documentation using Visio (my design tool of choice).  Thanks to Sluckey for his collection of Visio Shapes my design documents look better than ever.
While creating my own documentation is tedious and time consuming, it does allow me to get a much better understanding of the design and also allows me to make any changes that I may incorporate in the build.

Circuit board drilling and jumper templates
The first docs I tackled was the circuit board drilling and jumper templates.  Even though I bought the completed board with turrets from the Hoffman Amp site, I still need to drill the mounting and wire pass-thru holes.  I made a full size photo copy of the Hoffman board and imported it into Visio.  I also imported the image of Sluckey's board template and superimposed the two on top of each other.  I then placed turret, mounting and wire pass-thru hole shapes in the proper places.  You can easily copy/invert the diagram for a the bottom side of the template.   I then made the jumper templates using the templates.

ZZ-AC15 Circuit Board Component Layout
I then inverted/super-imposed the top and bottom jumper templates onto each other for the component layout diagram.  Again, using Sluckey's Visio shapes and original design document, I recreated the complete component layout.

ZZ-AC15 Chassis Layout
I then used the completed component layout diagram as the start of the Chassis Layout.  Using the original design document, I recreated the Chassis Layout.

<See Attachment ZZ-AC15 Chassis Layout>

ZZ-AC15 Ground Scheme
Using the original design document, I recreated the Ground Scheme.

ZZ-AC15 Power Section Chassis Layout
This design has a few features/components that I have not built before.  These include a standby switch and EZ81 Rectifier tube.  So creating a power section chassis layout was important for me to understand how to wire this section correctly.  As there is no prior design, I relied on the existing schematics available.  The original AC15 schematic has no standby switch.  Sluckey's annotated (original) AC15 schematic has a standby switch located after the Choke, and Sluckey's Detailed AC15 schematics has the standby switch located before the Choke.
Here are the two versions:

<See Attachment ZZ-AC15 Power Section Chassis Layout V1>
<See Attachment ZZ-AC15 Power Section Chassis Layout V2>

Questions:
   - Are both designs for the location of the standby switch correct?  Is one better than the other?  Is one preferred over the other?  Look forward to hearing your replies on this.

Next Steps:
My next step is to create a Parts List and inventory what I have on hand.

Happy Building :-)

[sluckey]

You have been very busy! Nice work! Making these drawings is an excellent way to get intimate with this amp.

Sluckey's annotated (original) AC15 schematic has a standby switch located after the Choke...

I never annotated the original schematic. You must be looking at someone else's work?

...and Sluckey's Detailed AC15 schematics has the standby switch located before the Choke.
Here are the two versions:

<See Attachment ZZ-AC15 Power Section Chassis Layout V1>
<See Attachment ZZ-AC15 Power Section Chassis Layout V2>

I've never seen these two drawings. I assume these are your work? I put the standby switch between the rectifier tube and the choke as seen on my main schematic or in the mod pdf. I never put it after the choke, although it should work fine there as well.

I'd like to suggest a simple change to your power section chassis layout... Connect the IEC Line terminal to the end lug of the fuseholder. This is a safety feature.

And just FYI, I used a 120v neon indicator lamp rather than a 6.3V lamp. That was just my preference. I think most guys have used a 6.3 lamp.

I never heard back from you about the discrepancies in the docs. I guess you were able to resolve these?

Here's a tip about the way I work with Visio. May be useful to you. My board layout has multiple layers. Components on one layer, turrets on another layer, jumpers on another layer, wires on another layer, etc. Doing things this way make it very easy to create the drill guide, and jumper guide. For example, to make a drill guide, just turn off the layers you don't want to see on the board layout, Copy the visible stuff, and paste into a new sheet. Wala, you have a drill guide. Works the same for the jumper guide. This way there are no differences between the sheets.

Looking forward to your next steps. Will you drill and stake your own boards or just buy Hoffman's board?

[SkipJohnson]

Thanks for the quick rely.

Quote
Sluckey's annotated (original) AC15 schematic has a standby switch located after the Choke...

I never annotated the original schematic. You must be looking at someone else's work?

See the attachment for the doc I was referring to.

The two power section drawings were mine.  I do them to make sure I get any unusual wiring right during the build.
I have updated them per your comments on switch location and safety wiring.  thx!!!

I never heard back from you about the discrepancies in the docs. I guess you were able to resolve these?

I'll send you a PM with my notes from my review.  I pretty much have resolved all my questions from this though.

Thanks for the tips on layers.  I have never used the layer feature before, bit I can see how it would be helpful.

Looking forward to your next steps. Will you drill and stake your own boards or just buy Hoffman's board?

I did buy the Hoffman board, but I still need to drill the wire pass-thu and mounting holes as they no longer drill these.  They only drill/mount the turrets.  So I needed an updated template as thier board is a little different than your original.

[sluckey]

See the attachment for the doc I was referring to.

I don't recall ever doing that. Doesn't necessarily mean that I didn't. 

I'll send you a PM with my notes from my review.

OK, thanks.

[SkipJohnson]

Output Transformer Wiring:

I noticed that the Output Transformer (Hammond 1650F - see attachment) has more Primary wires than I am used to in my Fender builds.  Typically the OT's I have used have only the red (B+), blue (plate) and brown (plate) wires.  This transformer has an additional two wires (Green/Yellow, Black/Yellow) indicating  these should be connected to the Screen Grid.  Also, the secondary wiring is different than the OT's I have used in the past.  Usually there is a ground, and one wire for 4ohm, 8ohm and 6 ohm.  This OT has a different wiring scheme defined for each output resistance.
Also, the Grounding Scheme diagram (attached) shows OT Secondary Grounds (Black and Black/Yellow) going to the Chassis Ground.

Here is what I know (or think I know) and do not know in terms of wiring:
Primary's:
   Blue - V7 EL84 pin 7
   Blue/Yellow - ?????
   Red - Circuit Board "A" connection
   Brown/Yellow - ?????/Yellow
   Brown - V8 EL84 pin 7
Secondary's (I am wiring for 8 ohm speaker output):
   Yellow - Output Jack Tip
   Green/Yellow - connected to Green
   Black Yellow - connected to Black
   Green - connected to Green
   Black - Output Jack Ground

Can anybody help me get this right?

[sluckey]

The blue/yellow and brown/yellow wires are screen taps and are not used in this amp. Securely tape up the separate ends and tuck out of the way. DON'T CONNECT THEM TOGETHER! These wires have B+ on them.

As for the secondaries… If you only want a single 8Ω speaker jack... Solder the green and green/yellow wires together, tape the ends, and tuck out of the way. Connect the black and black/yellow wires together and solder to the ground lug on the speaker jack. Solder the yellow wire to the tip lug of the speaker jack. That's all if using a switchcraft style jack. If you are using a Cliff style jack that has an isolated sleeve (ground) lug, then connect an additional wire from the jack sleeve lug to your chassis power ground.

Now, if you choose to have a 4Ω jack as well as a 8Ω jack, just study the big pic of the guts on my website. I bet it will make sense.

[SkipJohnson]

Thanks Sluckey, that is the information I needed.  That completes my design documentation.
I have drilled out the wire pass-thru and mounting holes in my Hoffman Turret board and am now working on installing the above and below board jumpers.

Stay Tuned...……….

[SkipJohnson]

I am including the 6-position 2-pole Presence switch in this build (see attached picture below) with the following settings (per Sluckey's mod notes):
1- n/a
2- .0047uf
3- .0022uf
4- .0015uf
5- 500pf
6- 220pf
I understand how the switch works and get wired.  My question is what is the best way to physically mount the components on the switch?  Seems like it could get very crowded.  Does anyone have pictures of this switch wired up?
Your help is appreciated!!!!

[sluckey]

Look here. Blow it up big so you can clearly see all the jumpers on the switch...

     http://sluckeyamps.com/VAC15/Vox_AC15_mods.pdf

Or, you may prefer this style on my Dual British Lite. It's basically the same except one additional cap and they are physically laid out differently (cleaner I think). Here's a pic...

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

And here's the layout. Blow it up big so you can clearly see all the jumpers on the switch...

     http://sluckeyamps.com/dual_lite/dual_lite.pdf



     

[SleepLess]

Excellent project! I've been wanting to build one for a quite some time now. I'll be following your build and am eager to reading your comments on how it plays and feels!

[clayman]

This is my next build. I am going to use a pro jr, for the iron and chassis and cab. No tremelo

[Tbone55]

I'm salivating following this thread. I've been looking for a donor amp. Maybe my Peavey 110 will be the guinea pig. Or maybe I'll make a new cabinet. Decisions, decisions.

 

[pompeiisneaks]

If anyone's interested, here's the build i did of the same amp, but as an AC30/4, they're identical amps except the 30 watter uses 4x el84 output tubes instead of two:

That's the playlist, there are a ton of videos showing the build process over time.

~Phil

[Ed_Chambley]

Just wanted to mention, I used a 12 position switch for brilliance for mine, but removed the cap from the board.  12 different caps are quite a lot, but this allows me to deepen the tone as well as cut bass and adds flexibility since I use a Wet/Dry setup.  The AC-15 is my main signal without any modulation except I do use the Tremolo/Vibrato some on the AC-15.  Closed back or just slightly open back to me sounds better with the Celestion Blue.  Expensive, yes.  You could get a copy like a Blue Dog or a Warehouse Black and Blue, but they are not that close so if a Chiming Vox with a Blue is what you want, spend the money.


I mentioned I don't build them with 2 inputs anymore, just switching for one or both channels.  There is an advantage to having 2 inputs and being able to mix the 12Ax7 and EF86 and mixing them.  A high gain pentode in the preamp sometimes clashes with distortion and even overdrive pedals, So I use the normal input and mix in the EF86.  This is not often, but can be a problem if you like Fuzz.  You seem to have the experience to determine you preferred tone, just sharing some of the things I have learned from building a few.


This one in the photo is the amp I use along with a 2, 10 Princeton Build.  The 10's help a lot with taming modulation effects.  This is the reason I wanted to be able to pass a bit more lows with the AC-15.  Together the clean tone with Tremolo running on both amps is amazing.  Hit the AC-15 with a little boost and you will start making 80's shredder faces.


Also, Everyone seems to reach for the screened EF86, but the NOS TESLA EF86 with the dark and usually crappy looking plates are a tad more raw sounding.  EL84's vary a lot too and new production will hold up easily to voltages, but Pan Getter RCA have less fizz and are affordable.  If you can swing it, plug in a pair of NOS Amperex.  The have a extremely fluid transition into clipping and very early sustain without much breakup and continue this to full outright overdriven tone ALL which can be controlled mainly with pick dynamics.

This is a true vintage sounding Vox.

[SkipJohnson]

Here is my current status on the project:

Design Phase:
Grounding Scheme - 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.

Chassis Blueprint - I decided to go with a 20" x 8" x 3" chassis.  I like the extra room the 8" depth and 3" height give me.  I ordered a custom built chassis and engraved face plates from Synaptic Amp (www.reverb.com/shop/SYNAPTICAMPS).  Steve at  Synaptic Amps is awesome to work with and worked very hard to get my face plate design perfect.

Pre-Assembly Phase:
I always look for the list of things that need to/can be done ahead of the main build.  See attachment for Pre-Assembly tasks and pictures.  I still am waiting for the face plates so I have not drilled the front/back of the chassis yet.

[Tbone55]

Great videos and great build Skip. That should be one helluva an amp when you're done.

Cheers!

[SkipJohnson]

I have been busy since my last post.
After the preassembly work was completed, I of course found a few more minor preassembly tasks I had forgotten along the way.
Complete Chassis Drilling:
I received my face plates which allowed me to drill the rest of the component holes.
Test Assembly:
Next I did a test fit of all the components into the chassis.  This helped me make sure all the components fit properly, make any adjustments required as well as determine the best order in which to do the assembly as I proceeded through the build.
Circuit Board Assembly:
Now it was time to work on the circuit board assembly.  I learned a great deal during this task as this was my first turret board (all my past circuit boards have been using fiberboard and grommets).  I placed/checked/double checked/soldered all the components on the board working from right to left, carefully trimming the excess wire from beneath the board.  Then I worked the interconnection wires from the back of the board (front chassis side) clockwise around the board leaving plenty of extra wire to work with later.
Filament Wiring:
Working from left to right I cut and test placed the filament wiring for each of the tube sockets.  I was careful to pay attention to doing proper phase wiring using green and black wiring to distinguish between the two wires.  I also kept the twisted wire pairs tight against the chassis.
Power Section Wiring:
This was the most challenging work to date as I was determined to make my wire routing look as good as Sluckey's.  I tried to create a plan on paper (in Visio actually) but found it too much of a task so I started wiring the power section component by component and determining the best routing as I went.  When I was done I came back and used zip ties to hold the wire bundles in place.  In the end I was pretty happy with the overall result.

See my attached PDF for a summary and pictures.

Next I am working on installing my grounding system.

[sluckey]

Lot of progress. That really looks sharp!

[SkipJohnson]

It was a busy weekend and I made it to the final stage of the build.

I installed the grounding system.  Both the isolated front controls ground and the power section chassis ground.
Next I prepped the shielded input connection wires and installed them from the input jacks to the isolated input standoffs.  Then I installed the input 33k screen grid resistors from the isolated input standoffs to V2/V2.
Th volume knobs were next to install.
Now I wired the remaining output transformer wires to the output jack and power tubes.
And finally the biggest task of all, installing the circuit board and connecting all those wires.  Working left to right, I simply soldered each wire to the front panel controls and ground connections.  Then I worked left to right soldering each wire to the back panel and tube socket connections.  This went pretty quickly and easily.  I checked my work as I went along regularly.

See my attached PDF for a summary and pictures.

My next step is to recheck all my work against my design documentation (once again) and make any required fixes.
Then I will perform the first power up and testing and debugging.
Stay Tuned…………………….

[pompeiisneaks]

Whoa that looks great!  I'm excited to hear tone samples!

~Phil

[sluckey]

That amp looks familiar. So does that amp cradle! 

Not just done, well done!

[SkipJohnson]

That amp looks familiar. So does that amp cradle! 

Forgot to mention that I had found the amp cradle DIY on your site a while back and built one.  Works great!!!!!  Thanks!!!!

So I completed my review and first round of tests.

The Good News:
   • No smoke, sparks or funny smells
   • Makes really loud good sounding tone
         Really happy with the volume and tone so far!!!!!

The Bad News:
   • There are a few issues I need to resolve

There are really two issues that are most apparent.
   1) Hum Issue: When the amp is powered up and operating with the Tone (Top Cut), Normal and Vibrato Volume controls all dialed all the way down, the amp produces a loud annoying hum. 
      a. When the Tone, Normal or Vibrato Volume controls are dialed up to about 2.5, the hum goes away.
      b. The volume controls both only behave normally from 2.5 up.
   2) The Vibrato Channel has a few issues beyond issue #1.
      a. The overall volume is less than the normal channel
      b. The Vibrator/Tremolo Effects do not function at all.

See my attached PDF for details of the testing procedure.

Both of these issues have me initially stumped as to where to start working to a resolution.  I have not experienced these specific issue on past builds.  Also, this is the first amp I have built with any on-board effects.

Any suggestions would be much appreciated.

[shooter]

The Vibrator/Tremolo Effects do not function at all.

did you have FS plugged in?  (don't ask how I know:)

[sluckey]

I think the hum and strange behavior of the volume pots may be related and possibly caused by a ground issue.

The vibrato channel is lower volume than the normal channel. Just the way it is. The brilliance switch does not affect the VIB channel. The top cut does affect both channels.

The footswitch must provide a ground to ***ENABLE*** the trem/vib oscillator. If you look closely at my big guts pic you will see that I used a Switchcraft 12A switching jack and its wired to ground the tip when nothing is plugged in. Maybe I'll revise my schematic and layout to make that more obvious. The schematic shows this but if you are not familiar with trem circuits it's easy to overlook.

[pdf64]

Just to note that best practice is to connect the CT of the HT winding to the reservoir cap's negative terminal (rather than to a common 0V point shown in the layout), as that will mitigate for ripple current contaminating the 0V circuit.

[sluckey]

The grounding scheme he used works very well.

[SkipJohnson]

The footswitch must provide a ground to ***ENABLE*** the trem/vib oscillator. If you look closely at my big guts pic you will see that I used a Switchcraft 12A switching jack and its wired to ground the tip when nothing is plugged in. Maybe I'll revise my schematic and layout to make that more obvious. The schematic shows this but if you are not familiar with trem circuits it's easy to overlook.

I replaced the footswitch jack with a switching jack and jumped the ground and shunt terminals.  That did the trick.  The effects are now working. 
I do not know exactly what to expect from these effects as I never played through a Vox Acxx before.  But For both Tremolo and Vibrato I have the Speed control effective from 8 o'clock on up and the Depth control seems only effective from 2 o'clock on up.  And at full speed the effect is not very noticeable.  Does this seem right?

I think the hum and strange behavior of the volume pots may be related and possibly caused by a ground issue.

I am starting to work this issue.  I agree that it seems grounding related which is where I will start my investigation.

[sluckey]

In trem mode the effect should simply vary the guitar signal amplitude (volume). In vib mode you should also hear a noticeable pitch (frequency) shift.

You can replace the oscillator cathode R/C with a red or yellow LED for a stronger oscillator signal. The LED should also give a more constant oscillator signal amplitude across the speed range.

[SkipJohnson]

Almost Done……………………………

Note: Other than the issue below, the amp seems to be working wonderfully.  The tone and volume is great, the effects are working fine (although I may tweak them a bit later to get more pronounced depth) and overall I am quite happy with the build.  I have learned a many great new techniques I plan to apply to my future builds (I keep thinking about building another Fender Champ , hot rodded to the extreme, with a turret circuit board, selective tone biasing, neatly bundled PT/OT wires and custom face plates).

Regarding the strange Volume and Tone pot behavior WRT the HUM:

I have been checking that the grounding system is installed properly and is working as expected.

See attached Ground Scheme PDF.

I verified that the power section part of the grounding system is installed correctly and all the ground points have continuity.
I verified that the isolated part of the grounding system is installed correctly and is indeed isolated by disconnecting its one point of connection to the Chassis (Orange wire between the chassis grounds and the isolated grounds on diagram).

Interesting Revelation but not the obvious solution:
I also worked around the inside of the chassis checking for any unintentional grounds (i.e. ground shorts).  I came across two suspicious ground hits.  These were turrets on the board, one connected to the Normal Volume center pot pin (green wire) and the other connected to the Vibrato Volume center pot pin (blue wire). 
After some investigation I discovered that when the volume pot is turned all the way down, the resistance between the left and center pins is almost zero (of course) and this allows the ground to travel back down the green and blue wires to the circuit board connection.  I never realized that a pot behaves this way but I checked it against several other pots in my drawer as well as in one of my Princetons.
When the pot is turned up a bit (like to 2.5) and the resistance goes up a bit (to something like 35k ohms) this behavior stops.  It turns out that this is exactly when the HUM stops. So the pots also exhibit a level of "slop" between zero and 2.5.  The resistance does not steadily rise to 35 from zero, but rather JUMPS from zero to 35.

So in some way I believe that the strange Volume and Tone pot behavior and HUM is related to this pot behavior.  BUT, I have never seen this problem before and have no clue how to explain it.

Any other ideas from you all would be appreciated.

[sluckey]

Are you using audio taper volume pots?

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.

[DummyLoad]

if you haven't sourced the 1650F OT, then consider the 1650FA - it's a replacement for the 1650F.

 the 1650FA has the secondaries internally connected to provide single wire 4-8-16Ω taps and a common.

https://www.arrow.com/en/products/1650fa/hammond-manufacturing?gclid=CjwKCAjwycfkBRAFEiwAnLX5ITTD-LkCIrOIIZt5R9B41-Nipw1j5x611V2km8eLcf0_WEFm41IOlxoCIiUQAvD_BwE

free shipping if you're in the Continental US!


--pete

[SkipJohnson]

if you haven't sourced the 1650F OT, then consider the 1650FA - it's a replacement for the 1650F.

Way beyond sourcing, but thanks!!!!

Are you using audio taper volume pots?

Yes.  500k Audio Tapered.

So I reworked the grounding scheme in the manner you described. 
Power Section Grounding all to one chassis ground at one of the PT mounting bolts.
Control Section Grounding untied from the power section and bolted to a new chassis ground directly under the input jacks.

No change in the Volume/Tone/Hum behavior though 

[sluckey]

The normal and vibrato preamps have absolutely nothing in common. The fact that they both exhibit the same behavior with the volume pots strongly suggest a less than perfect ground to the pots, maybe even only one gremlin affecting both pots or identical twin gremlins affecting each pot. It's time to share some high rez gut pics, especially clearly showing the ground connection on each pot and how the pot ground lugs are connected to a dedicated preamp ground. The pics in your pdf files are nice for casual viewing but they are very lacking in the kind of detail we need to help troubleshoot your problem. The pics will likely be too big to post directly on the forum so upload your hi-rez pics to the net and just post a link to them.

Meanwhile, try this. Divide and conquer. Pull V1 and V4 to totally disable the VIB channel. Just leave them unplugged until we fix the NOR channel hum. We're gonna work with the NOR channel only. With the NOR VOL pots turned to zero, use a gator clip lead to connect the center pot lug to chassis. Does that kill the hum?

[SkipJohnson]

Result:
Pulled V1 and V4.
Turned Nor Vol, Vib Vol, Tone to 0.
Attached gator clip to center lug of Nor Vol to chassis ground.
Hum still present, but slightly lower volume with than without.  But when the Nor Vol, Vib Vol or Tone turned up to 2.5 the amp is totally quiet.

Pictures:
Wide Shot: https://www.flickr.com/photos/23805934@N02/46519428325/in/album-72157626245738182/
Vol Pots: https://www.flickr.com/photos/23805934@N02/46519430345/in/album-72157626245738182/
Control Panel Chassis Ground: https://www.flickr.com/photos/23805934@N02/46519428525/in/album-72157626245738182/
Tone Pot: https://www.flickr.com/photos/23805934@N02/46519429505/in/album-72157626245738182/

[SkipJohnson]

One other interesting note:
If I attach a gator clip to left or center lug of Nor Vol and simply hold the other end of the gator clip, the amp goes quiet.

[sluckey]

It appears that you are using nylon screws to attach those two 5-lug terminal strips to chassis. Why?

I only see one wire connected to your added preamp ground lug. Where is the other end of that black wire connected?

I would replace those nylon screws with steel screws. Then run a short wire from each of those terminal screws to that added ground lug.

[sluckey]

One other interesting note:
If I attach a gator clip to left or center lug of Nor Vol and simply hold the other end of the gator clip, the amp goes quiet.

If you connect the gator clip to chassis and hold the other gator clip, does the amp go quiet? Do you have any fluorescent lights near the amp?

[SkipJohnson]

It appears that you are using nylon screws to attach those two 5-lug terminal strips to chassis. Why?

That was part of my isolated grounding scheme. Nylon bolts isolate the terminal strips from the chassis.  The idea is to control the low level control panel signal grounds and keep them from getting mixed in with the higher power level grounds in the chassis.  That is the theory anyways.  Here is a link to the article that I got the idea for this grounding scheme.  http://www.aikenamps.com/index.php/grounding

I only see one wire connected to your added preamp ground lug. Where is the other end of that black wire connected?

See attached diagram for the grounding scheme.  There are 2 terminal strips (1-5 lug, 1-7 lug) for the control panel grounds, the 7 lug is tied to the 5 lug with a wire, and the 5 lug is grounded to the chassis.

I would replace those nylon screws with steel screws. Then run a short wire from each of those terminal screws to that added ground lug.

I Am going to re-work the entire control panel grounding and take all the grounds to the single chassis grounding point under the input jacks.

If you connect the gator clip to chassis and hold the other gator clip, does the amp go quiet? Do you have any fluorescent lights near the amp?

No, connecting the gator clip to the chassis and holding the other end of the gator clip has no change in the hum.  There is a fluorecent light nearby, but removing the light does not change the hum.

[sluckey]

If I attach a gator clip to left or center lug of Nor Vol and simply hold the other end of the gator clip, the amp goes quiet.

This still suggest the ground connection to the NOR vol is not quite right. Although your isolated terminal strip ground should be OK. Maybe redoing the control panel groung will fix it.

[SkipJohnson]

I reworked the control panel grounding.  I removed the isolated terminal strips and chassis ground.  I replaced them with chassis grounded 3 lug and 5 lug terminal strips.  See attached diagram.

This has not resolved the hum issue. The only thing I have been able to do to make the issue go away is connect a gator clip to the left terminal of the Vib or Nor Volume pot, and set the other end, un-connected, on the work bench.  Then the hum stops.  Not permanent solution.

Any new ideas would be appreciated.

[sluckey]

That grouns system should work fine. Maybe this hum is not caused by poor grounds. The gator clip on the volume pot ground lug is a puzzle. Do you have a metal bottom plate for the chassis? If so bolt it on. If not, rool out a length of aluminum foil on the bench top. Set the amp on the foil, open side in contact with the foil. Hum any better?

[SkipJohnson]

Do you have a metal bottom plate for the chassis? If so bolt it on. If not, rool out a length of aluminum foil on the bench top. Set the amp on the foil, open side in contact with the foil. Hum any better?

No bottom plate.
Rolled out the aluminum foil and set the amp open side on the foil.  No change. 

[Ed_Chambley]

On the normal volume pot that leads to your 6 position switch, try from the layout you posted remove a and b from switch and exclude for testing. IOW eliminate the switch.  I understand the normal channel preamp is supposed to be separate and if the vibrato channel does the same common thinking says one cannot cause the other to hum.


I assure you both of my preamps change tone when switched.  Does your power trans have a CT for heaters or resistors for a ct? Try lifting and using resistors. If already using artificial ct, check the resistor values to insure they are the same.


Try running a seperate earth ground and connect the gator clip your quiet hand to this. Eliminate  you and the  chassis.


Have you tapped on your tubes while the hum is present? Does it  change? Got another set of El84 known to be good?
Just thinking out loud.

[SkipJohnson]

On the normal volume pot that leads to your 6 position switch, try from the layout you posted remove a and b from switch and exclude for testing. IOW eliminate the switch.

I did that along with pulling the Vib channel tubes so I was just dealing with the Normal channel.  No change.

I assure you both of my preamps change tone when switched.  Does your power trans have a CT for heaters or resistors for a ct? Try lifting and using resistors. If already using artificial ct, check the resistor values to insure they are the same.

I have CT for the heaters.  I will try installing an artificial CT and lift the PT CT and see what happens.

Try running a separate earth ground and connect the gator clip your quiet hand to this. Eliminate  you and the  chassis.

Tried this, no change.

Have you tapped on your tubes while the hum is present? Does it  change? Got another set of El84 known to be good?

Tapping does not change anything.  I actually have another 2 sets (That I had replaced in my Mesa Boogie 20/20 stereo amp) of EL84's.  I tried both sets with the same results.

Appreciate the ideas...…………….

[sluckey]

Just how much of a hum is this? Is it loud enough to be heard while you are playing at bedroom levels? Do you have to raise your voice to be heard above the hum? Is it low enough that you have to listen closely to hear it? Is it 60HZ or 120Hz? Or neither?
 

[SkipJohnson]

Just how much of a hum is this?

Enough to be annoying.

Is it loud enough to be heard while you are playing at bedroom levels?

It goes away as soon as i turn the tone, nor vol or vib vol up past 2.5.  This hum is transient and is constant until it goes away when the tone, nor vol or vib vol are turned up past 2.5.

Do you have to raise your voice to be heard above the hum?

No
 

Is it low enough that you have to listen closely to hear it?

No

Is it 60HZ or 120Hz? Or neither?

Not do not know how to tell the difference.

I am close to just moving on and keeping the tone knob at 3 or higher.  In this state the amp works fine and is quiet.  The volume is way above my expectations, the tone is great, the on board effects sound good.  Then maybe one day I will have an epiphany that will help me solve this WEIRD problem.