My birthday prezzy from Madame Filtercap this year was a luvly new 10" Weber alnico speaker. Not to put too fine a point on it, it's a 3.2-ohm 20-Watt model 10A125-O. Time to make some long-awaited... er... "improvements" to my silverface Vibro Champ. Let the games begin.
Here's the Vibro Champ with the grille and speaker removed, and the new speaker. On the old speaker you can see the alnico magnet structure. The new one is similar underneath that bell-shaped cover, but larger.
Replacing the VC's 8-inch speaker with a 10-incher is not a new idea by any means. But it is challenging because some of the goodies hanging down from the chassis -- especially the output transformer and the "cap can" -- are taking up space that the speaker basket needs. Some people deal with this by mounting the new speaker off-center to the left. I didn't want to do this, though, because the particleboard grille frame is wider on that side to support the Fender badge, and the triangle of board under the badge will block part of the speaker. Why install a larger speaker and then cover it partway?
The other approach is to move the speaker =forward= in the amp by reverse-mounting it to the baffle. (In other words, mount the speaker so the back of its rim touches the baffle, not the front.) Easier said than done, because now the speaker would stick too far forwards and make it impossible to re-install the grille. The answer is to reverse-mount the speaker on a baffle-within-a-baffle, so that the speaker's front is flush with the front face of the original baffle. That moves the speaker forward about a half-inch overall -- hopefully enough to get the basket away from everything.
The outside dimension of the speaker is just a hair smaller than the height of the original baffle. So I can't just cut a hole. I'll need to remove the center of the baffle and leave two wings to mount the new baffle. I decided to make an oval hole so that I can slide the new speaker/baffle unit towards either side if I have to. I traced the outside of the speaker onto the old baffle.
I left as much as possible of the old baffle to support the new baffle, along with the velcro tabs that hold the grille frame on. I cut the new baffle from plywood, with a hole just slightly larger than the inside dimension of the speaker rim. I also cut three gasket rings: two from very dense mat-board (the stuff you frame pictures with) and one from some thin double-wall cardboard that can "crush" a little bit to conform to the baffle. The gaskets help the speaker seat evenly, and they serve as spacers to get the front edge of the speaker flush with the old baffle face. Here's everything so far. (Two of the rings are glued together already.)
New speaker, reverse-mounted on the new inner baffle. I trimmed the gaskets at the baffle edges so they don't hit the cabinet or the chassis.
I test-fit everything with the chassis in place. The old cap can still contacted the speaker basket unless I slid the new speaker/baffle off-center. Too close for comfort. Time for my evil plan.
As part of the project, I'm replacing the filter caps with a new cap-can anyway. The can sticks down from the chassis like a vacuum tube. Its top fits up against a hole in the chassis, and tabs coming out of the top of the can are bent/soldered to the inside of the chassis to hold it in place. They use some kazillion-watt iron to do this, and it's impossible to melt all that solder again. I ground through the old tabs/solder with a Dremel tool and pulled off the old can.
Now to get myself some breathing space above the speaker, I jacked up the new cap can on some metal stand-offs. I screwed the bottoms of these through little holes I drilled on each side of the big cap-can hole. The new can has insulated "ears" with screw slots, perfect for screwing into the tops of the standoffs. Here's my exclusive, patented hi-rise cap can from inside the chassis....
... and from outside. I'm holding the old can in place in the background for comparison. I gained almost an inch of extra space.
Here's the new can, wired in place. The black ground wire goes from one of the four grounding tabs to one of the huge lumps of solder that was holding the old can in place.
And now everything fits with the speaker centered! There's about a fingertip-worth of clearance between the cap can and the speaker. The transformer has room too, especially because one of the slots in the speaker basket sits right below it.
The 5Y3 rectifier tube on the left has plenty of room over the bell cover. The original RCA 6V6 tube had a decent amount of room. The new JJ 6V6 in the photo is longer. I can just =barely= get it into the socket with the chassis in place. I might eventually slide the inner baffle to the left a half-inch to help things out. All I need to do is unscrew the baffle, slide, and screw it in again.
Front view, sans grille. It looks a little odd because I left so much of the original baffle in place. I might trim the gaskets down a bit if I want to slide things farther. (The Thundering Twin-o-Lux in the background has a baffle-in-a-baffle job too... another story. I often remove the grille when recording. Yay 70's Velcro.)
It sounds great so far. The JJ 6V6 absolutely kills the old RCA tube for lows and volume. I re-installed one of the old RCA 12AX7s, and that's way better-balanced than the spiky-sounding Groove Tube I had in the preamp. The GT in the vibrato circuit is just fine. Nice vibrato.
I'll play the amp a bunch to break the speaker in a bit and get used to how it sounds. A bunch of maintenance chores and mods await. Much mirth shall ensue. Tune in later for more top-quality TL/DR right here.
