PRF members' tech journal

211
Mesa Fathom (M6 Carbine), cont:I worked on the Mesa Fathom a bit more, since I decided to keep it. The master volume pot was way too sensitive. It went from 0 to full blast in about 2 degrees of rotation. I also didn t like the pull mute being on that pot, because it caused the volume to change when you pulled it.I swapped the 100K linear with a 100K log taper and it s much better. Also moved the mute to a separate mini toggle.I did some detective work about the output stage mods that the factory performs on these to keep them from blowing up. They added some protection diodes to the power rails. I found a photo online of somebody's Mesa work-order that had the parts listed, then I found another separate post with photos of the output board showing where those diodes were connected. I went back and added them to mine.

PRF members' tech journal

212
Might as well spam this thing into a third thread...Ampeg SVT 810 Cabinet:See my earlier post here. Looked like this when I got it:I got out the belt sander and stripped it down to the wood as best I could. Also had to drill, glue, and screw the bottom pieces back together. They were loose. Removed the rusted metal ashtray cup things from the top panel and filled the holes with 1.25" wood dowels. They didn't line up wiht the feet on my SVT II head anyway. Filled all of the extra holes, gaps, and the worst gouges with plastic wood, then sanded everything smooth. Got it looking like this before I put a coat of Behr Deckover on it:This is the second time I've used the Deckover, and I don't think I'm going to actually finish the thing with it. It's too grainy, rough, and flat and it shows every little mark when you touch it. I ordered a gallon of Duratex to put the finish coats on it.I tried cleaning the grille with TSP and a magic eraser, and it was the nastiest, brownest water I've ever seen come off something. Unfortunately, much of the silver finish on the threads came off also, so I'm going to have to scrap that grille cloth. I've got an idea to use outdoor shade cloth from the hardware store for the grille. We'll see how that goes.

PRF members' tech journal

213
Ampeg SVT 810 Cabinet (cont.):Duratex was the correct choice. Applied two thick coats over the first coat of Deckover. Just used an old 3/8" nap roller I had instead of the open-cell foam texture roller. I wanted a little less agressive texture. Turned out great, doesn't show every mark like Deckover.Put the speakers back in, including three new Jensen Mods. The partition behind the top two speakers was missing its fiberglass batting material, so I shoved a $2 pillow from Ikea in there. I had to re-solder the input jack wiring as well. Screwed new rubber feet to the bottom. Need to get new bolts for the new wheels and towel bar handle. I've got metal corner protectors for the front that I pulled off my Trace Elliot cab. Grill cloth should be here Monday. I'm going to drag it into the house before then so I can fire it up and see how it sounds.

PRF members' tech journal

217
Finally got a chance to play it at volume. Looks like all of the original speakers need to be reconed. They're all loose and buzz at high output. I can see the dust caps distort. No dragging in the gap when I push on them, but it seems like the surrounds are tired, as there's way more excursion on them than the new ones. I need to see if I can find a package deal on getting them all reconed.

PRF members' tech journal

218
After doing more reading and listening to youtube clips, I don't think there's actually anything wrong with my CTS speakers (though I did find one of the 6 had partially-detached litz wire). I think they're just super inefficient and have limited excursion for this application. There's a reason they originally spec'ed 2 810 cabs for SVT use: to make up for the inefficiencies and spread the power to make sufficient volume.I'm going to go a different route: sell the CTSs and load the cab with 4 Jensen MODs and 4 Eminence B810s. The Jensens sound pretty close to the CTSs already with that woody upper-mid bite, and the B810s should add extra low-frequency content. The CTSs just aren't cut out for the type of volume I'm trying to squeeze out of them, and I'd be out the same amount of money if I reconed them all. Somebody out there will want them in original condition.

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