(Slow) Build log for Vox JMI-era AC15 OA-031 circuit

[g1]

Trying to keep all circuitry safe from wrong tube types being installed would be a never ending quest. Then you have to consider, what about correct type tubes with the guide pins broken off?
I don't think it's a legitimate concern.

[dchang0]

Well, I figure I shouldn't make it worse than factory-stock if a person inserts the wrong tube type, if you know what I mean.

That is to say, if it is "X amount of bad" if a guitarist inserts the wrong tube (most likely an EL84 or a 12AX7, since it is likely the guitarist will at least know that an AC15 has those tubes), then if I add the diodes to the tube socket pins, does it make it "2 times X amount of bad" if the guitarist screwed up OR does it increase the probability of something going bad by Y amount?

So far, out of the few amps I've been asked to repair, a surprising number (to me at least) have been "broken" by the guitarist either inserting the wrong tube or breaking the guide key/pins off and inserting the tube rotated one or two positions. In one case, it did result in a scary poof as the main fuse would vaporize immediately upon flicking the switch, so some serious damage was probably averted by the fuse doing its job. I replaced the tube with a brand new one, correctly-inserted, and the amp seemed to be undamaged and worked fine after that--lucky, lucky.

I know I can't prevent buzzed guitarists from impatiently and forcefully shoving tubes into sockets in the dark of a club's stage, and I won't try, but I don't want to make things worse with my wiring choices, if that makes sense...

[g1]

My point is you can't design it to be safe from people putting in tubes with broken guide pins incorrectly. And that is probably more likely than incorrect tube type. So I don't think you need to worry about the other scenario either.
But yes, if it's just a matter of not using the unused socket pins as tie-points, then that is easy enough to implement.

[dchang0]

Ah, I see what you were saying. True, true. Thanks!

[dchang0]

A little more progress. Heater wires are in for the steel chassis--I had to add yet another 3-lug terminal strip (not shown). On the plus side, the terminal strip will make installing a virtual center tap for the heater wires ridiculously easy.

The upper leg of the choke has not been soldered to the positive terminal of cap can half C41 yet. I haven't figured out how to get all the wires at that point onto the cap can's terminal.

Output transformer is not soldered to anything yet, either.

Waiting on diodes to arrive to finish the rectifier wiring.

[g1]

I haven't figured out how to get all the wires at that point onto the cap can's terminal.

Maybe one of those 2 into 1 spade lugs would work for this?

[dchang0]

They make such a thing? Awesome!

Unfortunately, I don't know what keywords to use when searching Digikey/Mouser. So far, typing in "spade lug" pulls up single, crimp-type male spade connectors, no splitters or adapters. Any suggestions on keywords?

I suppose in the end, I could solder a 1/2" length of thick bus wire into the slot in the terminal and then solder the other wires to the bus wire.

Alternately, I could cut some of the heat shrink off the leg of the 22.1K resistor and solder the wires to that, but it seems flimsy and easy to break. Would make changing that resistor out later on tough, too.

[g1]

They seem to make up the names as they go . And some are not sure what's male & female.
Here are a few:
Wire Termination | Power Connectors | Lugs | Terminals | Crimp Tools
Female to Double Male Quick Disconnect Terminal Adapter (100 Pack)
Female to Double Male Tab Chair Adapter

[dchang0]

Awesomeness! Thanks very much! The chair adapter will be perfect.

For anyone reading this thread, I took the keywords from the links g1 provided and searched Digikey, who calls the class of similar items "Terminals - Adapters"

http://www.digikey.com/product-searc...apters/1442878

I'll measure the blade widths and pick the right one(s). I'll move on to the aluminum chassis while waiting on these parts to arrive.

EDIT: Blade width on the F&T 16uF+16uF 450V cap can is 0.165in. That's pretty small; I had to cheat and get a terminal adapter meant for 0.187in. quick detach blades:

http://www.digikey.com/scripts/DkSea...48647369071451

I am hoping the curled-in parts grab the F&T's terminal tab, which is split like a forked-tongue. It is possible they will fit into the middle, empty slot and not grab at all. If that occurs, I guess I'll crush the adapter so it grabs the forked-tongue.

[dchang0]

Added B+ center-tap fuse to ground. Ground is the center lug on the same terminal strip used for the heater wires.

No further soldering on the steel chassis until the diodes and terminal adapter arrive.

[dchang0]

Had to drill one more hole in the aluminum chassis that already looks a bit like Swiss cheese. This hole is for the 1in. P-clamp that holds the F&T cap can.

Due to the large size of this cap can and the way the P-clamp sticks out pretty far to one side AND the fact that the power transformer is going to sit flush against the other side of the chassis, there were relatively few places this hole could be drilled. I lucked out and found a spot between the legs of the power transformer's bell cover.

Another way to position it would have been to flip the P-clamp around so that the legs point to the left and drill a hole near the V2 socket. This would have moved the cap can about 8mm to the left and 8mm down with this particular P-clamp. I chose the current location because the hole is in a relatively out-of-the-way spot if it turns out that I have to move the P-clamp somewhere else, AND I was hoping to use it as the safety ground bolt-down to the chassis, but I've decided to use the nut trapped under the fuse block for the safety ground. That screw contacts the power transformer's bell cover very firmly, so it would serve as a better conductor to even the lower, steel chassis. The trapped nut makes it more likely that the safety ground won't come loose, a lucky accident.



NOTE: I got this P-clamp from Home Depot but do not like it. The hole that is punched in it is very, very large, so large that an 8-32 pan head almost goes through it. I would rather have bought an Essentra Components aluminum 1" P-clamp, but Digikey and Mouser do not stock these in single units. Essentra does ship free samples, so that is one way to get this clamp. In the end, it was a good thing that I didn't get the Essentra one because I needed the clamp to bend upwards to clear the nut for the screw securing the PT bell cover's left foot to the aluminum chassis (see it under the P-clamp in the photo). Home Depot's cheap clamp bends easily.

[dchang0]

Upper chassis now has heater wires and pilot lamp wired.

The sockets I bought aren't clocked the same way as the sockets JMI used, so the layout sort of worked against me. Oh well.

[dchang0]

Finished the interconnects on the upper tagstrip board (nearer the controls). Yeesh--so tedious. The little cross-wires between the tags are so numerous. The photos don't look like much was done, but it took a long time. This would be the downside to using tagstrip boards instead of eyelet boards or turret boards. I imagine sluckey's turret board layout is a lot easier to finish.

I will test the board out with a multimeter and match it to the schematic before starting on the lower board.

[dchang0]

Happy New Year, guys!

I finally got around to working on the build again, this time finishing up the steel chassis with the parts I ordered.

First of all, g1's quick disconnect adapter idea was brilliant, but alas, due to the usual size and shape of the F&T capacitor's solder lugs, the smallest adapter I could find still didn't fit, and forcing it to fit would have been too much work. So I went ahead and just soldered all five leads to the single lug. You can see the quick disconnect adapter in the first photo.



Over on the PT side, I added the diodes and finished wiring up the rectifier. I went with a pair of single diodes instead of a pair of double diodes since I didn't want to drill a hole for another 3-lug terminal strip.



The heater wires and the virtual center tap aren't done yet--those will wait till the joining of the two chassis together. For the most part, the steel chassis is complete minus the wires that run to the aluminum chassis.

[dchang0]

All intra-board wiring for the tagstrip boards is done. Just need to be double-checked, and then they can be bolted to the aluminum chassis.