Panel Thickness

[TriodeLuvr]

I've made a lot of progress on my Marshall JMP 2104-ish build over the last few days. It's a complete scratch build though, not a kit, so I expect a few hiccups before it's done. One thing I'm concerned about at the moment is the overall panel thickness. The chassis is 1/16" aluminum, and the front and rear panels are 1/8". This means the panel components have to pass through a combined thickness of approximately 0.190". This seems OK for the Cliff jacks, fuse holders and most other parts, but the Alpha pots look like a problem.

If I install the pots with no lock washer and no trim washer, the mounting nut threads on just barely flush with the front of the bushing. I can counterbore the front of the panels around the shafts so more threads are showing, and that would give the extra space needed for a lock washer on the back side. However, there's nothing I can do about the length of the control shafts. I'm thinking maybe they don't protrude far enough for the knob setscrews to get a good bite. In fact, the setscrews might be located too far out (when the knobs are positioned correctly) to even touch the shafts.

I won't have the exact knobs for another two weeks or so. Meanwhile, can someone tell me approximately what total thickness is typical in these amps for the panels plus chassis?


Jack

Here's a pic. The distance from the panel to the end of the shaft is exactly 0.4".

[TriodeLuvr]

Well, after looking at datasheets and all the dimensions for a couple hours, I don't think this will work the way I was hoping. The knobs I wanted to use are clones of Davies 2300 chicken heads, so I used their numbers. The bottom line is that if I add a thin lock washer behind the pots, the center of the setscrew will only be 0.020" back from the edge of the chamfer on the Alpha potentiometer shaft. That's way too close. I'll make this work, but I might have to forego the lock washers and maybe search for a different knob. Too bad, I really like the chicken heads.

[crgfrench]

Skip the lock washers and apply a bit of Loctite 242 or Permatex blue thread locking compound

[TriodeLuvr]

Skip the lock washers and apply a bit of Loctite 242 or Permatex blue thread locking compound

I normally use a small internal tooth lock washer on the back side, between the pot and the panel. That's the one I have to omit so the shaft will protrude as far as possible from the front. Nothing I can do about that, there's no way to recess the pot into the backside without the body hitting the panel. Maybe I'm being OCD about the back side washer, but that's just me.

I'm counter boring the face of the panels now. That will recess the nuts and actually provide enough clearance for a lock washer on the front if I want. Or, I might use Loctite blue like you say. I keep that around the shop just for things like this.

Jack

[Henrik76]

Did you drill the chassis for the locking tabs on the pots, or did you snip them off? If you did drill for them, you don't really need any sort of washer on the back. A very thin plain one at the front makes nut-tightening smoother, but it's not really needed there either.
FWIW, I always drill for the tabs if I have a separate faceplate. It's by far the best way to keep the pots locked in position.

Just my 2p.

Good luck with what looks like a nice build! I only wish I could have faceplates like that made, without having to sell the car or part with a kidney.

Henrik

[TriodeLuvr]

I drill for locking tabs in hi-fi gear, but that's usually a single thick panel where I can cut the hole from behind and not go all the way through. It's not absolutely impossible to do that here on the inner panel (chassis), but it's problematic for several reasons. To be honest, I'm also not sure how much the tabs really help. I've seen a lot of equipment over the years where the pots or switches have loosened up, and the tabs only served to keep the control from spinning all the way around. That's a nice feature, but it would be better if the control didn't loosen in the first place. This is the reason I'm fond of installing a lock washer on the backside, and many manufacturers of commercial/industrial equipment do the same (sometimes with the panel also drilled for the locking tab). The only alternative in this case, being as I can't afford to lose the space that a lock washer would take up behind the pots, is to use a thin lock washer on the front and additionally use Loctite blue on the threads. I think those two mechanisms will work together to prevent the nut from turning around on the threads of the pot, and simultaneously from turning on the panel. It's not a direct anti-rotation device like a lock washer on the back, but it should be effective.

I have a serious interest in vacuum tube hi-fi, so I set up a shop at home years ago that lets me construct one-off gear myself. Most of my panels look like this - clear anodized aluminum with engraved lettering. The only services I still need occasionally from outside sources are powder coat (for transformer covers and such) and bare chassis. Most of my hi-fi chassis are made from 10" X 2" extrusions with 1/2" thick front panels, so this amplifier is a major departure from the way I'm used to working. It's a fun project!

Jack