Monday, January 6, 2014

Harmony B3500 update: Cab and Cord

Finally posting about updating the Harmony B3500 amp: fitting a 15 in. speaker to the cab, and upgrading the AC to a three-prong cord.

Upgrading the AC Power Cord

Most pre-mid 1970's amps don't have earth-grounded chassis. It can be a MAJOR safety problem when playing guitar. Guitars generally tie the strings to the ground wire on the amp, effectively "grounding" the player, too...if the chassis really is at earth ground, that is. Believe me, sometimes it's not (Zzzaaappp).

A correctly wired three-prong cord helps to assure that it really is earth grounded (it's not foolproof--there's no guarantee the wall receptacle has a ground connected).

For my own reasons, I chose to wire the Harmony B3500 "modern style"--rather than run a cord through a stress relief and hardwire, I used an appliance-style power cord receptacle. The type for a removable power cord--most modern amps use these. I had some on hand, I wasn't worried that I'd ruin the "vintage" value of the amp (doesn't really have much), and it's a little more flexible, length-wise. So sue me. The wire nut was already there, I just reused it.

Here's the mod from the backside. Suppose I should rivet this instead...

Also added a fuse holder to the AC input. There is a fuse soldered directly to the circuit board, but this is a better setup.The fuse holder also perfectly filled the hole from the old cord (the previous entry to this blog shows the original AC wiring for comparison, btw).

Fitting a New Speaker to the Cab

Since I had an old Utah 15 in. speaker that was unused, the cab seemed perfect. And it worked great.

I used a sheet of 5/8 ply for the baffle board. Sorry, I lost the pics of the 1x2 cleats I installed. But I do have "before and after." Painted flat black, of course...

I used four T-nuts for the screws to attach the speaker. Always seemed like the best solution here. Used a Sharpie to blacken the T-nuts, which worked so-so.

Here's the finished install (again, sorry, lost a bunch of photos).

Yeah, I'll clean up the grill once the weather is decent again in the spring. But everything works well. It's relatively light and portable for gigging.

The speaker itself...not exactly great for guitar, though it sounds OK for bass. There's a reason it wasn't pressed into use before...

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