5F2-A Princeton in a 5E3 cab

Hi folks!! Long-time lurker, first-time poster, and first-time amp builder here!

I've been researching amp building for a while (probably a couple years on and off), and I've been gathering parts over the last few months (a surprisingly time-consuming but fun aspect of the amp building process!). Most of the parts I'm planning to use are from the Mojotone 5F2-A kit, but I've "upgraded" a few things here and there (more of which later in the thread...).

From reading various threads here on TDPRI, I've learned that one of the best places to start an amp build is by drilling a few additional holes in the chassis. I know I definitely need to drill one hole for the OT (thanks to the legendary build thread by @theprofessor and many knowledgeable contributors!). Also, the chassis does not contain any board mounting holes, so I need to figure out where they should be located and drill those too.

So my first batch of (probably many!) questions revolves around ground connection locations. I would be really grateful for responses on any of the following items:

1. I'm planning to use the power supply, split grounding bus, and heater wiring approach suggested by @robrob on his superb site. I'll need a chassis hole for a dedicated safety ground. Is the location of the safety ground super critical, or can it be located anywhere near where the AC cord enters the chassis? I understand the ground wire should be cut longer than the hot and neutral wires – would there be any concern with looping the AC ground wire and placing the connection on the left side wall of the chassis, near the PT (see 2nd photo below)?
2. I plan to mount a euro-style terminal block to connect the AC neutral wire and PT primary wire together. I'm assuming this block should be located somewhere near the PT, any specific location?
3. I believe the chassis connection for the preamp ground bus is via the input jacks (and possibly the pots?) and therefore doesn't require any additional chassis drilling, but would really appreciate it if someone could confirm this.
4. I believe the power amp ground chassis connection is typically the upper-right PT mounting bolt. Just wondered if there are any advantages to using a separate chassis hole for the power amp ground connection? And if so, would its location be super critical, or can it be anywhere in the vicinity of the PT?
   
5. I know people have had issues in the past with drilling Mojotone chassis and affecting the finish. Has anyone had issues recently?

For anyone who is not familiar or needs a refresher, here are photos of the Mojotone 5F2-A chassis. I've circled 3 holes in yellow that I don't believe are used by the 5F2-A kit (please correct me if I'm wrong! ...mounting holes for a 5F2 choke perhaps??). So I guess these 3 holes could be candidates for some of the above locations to save drilling time.





Thank you so much for your time and for reading this far! I will keep you posted as the build progresses!

 

Thank you, sir – appreciate it! I'll find another spot for the power amp ground – perhaps one of those holes highlighted in yellow if I have the hardware to make it work. There's plenty of room on the side of the chassis for the safety ground because it's going in a 5E3 cab, glad that will work!

Thanks again!

 

Thank you so much, @Tele Slacker! Great to hear you are building the same kit – I'm sure you are going to be finished waaaayyy before I am!! Do you have a build thread?

If I could ask – for #2 (connecting the neutral and primary wires) – it sounds like you are using a terminal strip with a jumper between the outer lugs? That's a neat idea, hadn't thought of that! Yea, I wish Mojotone had included #10 terminal strips, the ones that come with the kit need to be trimmed to fit around a PT bolt...

And for #4 (the power amp ground), assuming you have filament and/or HT center tap wires from your PT, are you using the other terminal strip attached to a PT bolt for your power amp ground chassis connection?

LOL, as you can tell, I have a tendency to focus on details . I hope to find a balance between this and annoying the neighbors with a cranked 5F2-A

 

Thanks, @jimgchord! Just realized that I can't fit the chassis length-wise in my drill press, so if I do go with the side wall safety ground I'll need to drill that hole by hand . The other holes should work fine in the drill press...we'll see!

 

Gotcha, thanks!

 

Thank you so much, @robrob! I've spent many hours on your website

 

Wow, such craaaazy times we are in right now...hope everyone is doing well and staying safe!

I was hoping to make more progress on this build what with self-quarantine and WFH. However, my working day has turned out to be busier (and longer!) than it was before Covid-19. That said, amp building has turned out to be a very welcome distraction during these rather troubling times...

Anywho, a quick update on the chassis holes – I'm pleased to report that there were zero issues drilling into a recent Mojotone 5F2-A chassis. (I haven't drilled the main control panel though, just the side and bottom). I'm also a huge fan of Tapmatic natural cutting fluid – it kept the drill bits super cool.

Here are a couple of before/after deburring shots of the safety ground hole and the OT mounting hole:



My next set of holes will be the board mounting holes. After much debate, I've decided to ditch the eyelet fiberboard that ships with the Mojotone kit and go with a glass epoxy turret board. Not entirely sure why, I think they look sturdier and more robust?? I swung by McMaster-Carr and picked up the board. Next up is figuring out where to locate the holes without putting them under a component. A mock-up is in order!

 

Howdy, folks!! Hope everyone is doing well and staying safe!

Quick update: since there are no board mounting holes in the Mojotone 5F2-A chassis, and I'm going to be cutting my own epoxy board, I'm trying to figure out where best to locate the board mounting holes. My guess/hope is that it doesn't really matter where these holes are located. But life will be easier if they are not located directly underneath a board component

I traced the eyelet board that ships with the Mojotone kit onto a piece of cardboard, and laid out all of the components I intend to use (see pic). (You may notice that I've substituted a few components in the Mojotone kit...mostly because I've read good things about them here, not because I know any better!).

I essentially added a 1/2" of real estate to each end of where the Mojotone eyelet board ends, just to give the mounting bolts a little extra room. I'm proposing putting a board mounting hole in each corner of the board, marked with a faint pencil X. (I read somewhere that epoxy boards are happier when mounted with 4 bolts rather than 2 to avoid stressing the board). Does anyone see any issues with this approach?

Also, I'd really appreciate it if anyone see any issues with the component layout below – I've tried to make all the cap values visible, and orient the resistor bands consistently.

Thanks in advance!!

 

Thanks for chiming in, @jsnwhite619! Huge fan of your work (amp building, guitar playing, cabinet making)

I still have the Mojotone kit filter caps lying around, I might replace the 10uF after I hear how it sounds. I'm planning to insert the filter cap leads into the top of the turrets to make future replacement/switching easier

 

Holy moly, such an epic solo!! Some great 5F2-A tone right there! Thanks so much for sharing

 

Hey there, hope everyone is doing well! So after a 5-month hiatus, I'm finally getting back to this build...

Since my last post, I've drilled a couple of extra chassis holes, cut/drilled/swaged the board, and started mounting the first few pieces of hardware (pics below). I'm using the Steve Luckey build sequence, so I'm starting with the PT primary circuit.

One question I have is regarding the wiring sequence for the black/hot wire from the AC power cord. I know Rob's diagrams typically show this wire connecting to the fuse assembly first (specifically to the center/tip lug). However, I found an Uncle Doug video where he talks about connecting the hot wire to the AC power switch first: . As he mentions, the advantage of this is a reduced chance of shock while changing the fuse when the amp is still plugged into the wall.

My guess is most folks here use Rob's primary wiring sequence. But does anyone have any thoughts on Uncle Doug's sequence? I like the idea of not getting shocked, but I'm curious whether there are any disadvantages to this? If the amp draws excessive current, would the AC power switch potentially melt into a blobby mess before the fuse blows?