Finally Building JTM45: Stupid Questions As They Arise

[The New Steve H]

My church buddy brought his Mojo JTM45 kit over, and I am already confused. The capacitors don't have any clear indication of which end is positive or negative. We have yellow Mojo Dijons and a black Tube Dr cap.

Help!

[johnnyreece]

Are you talking about the electrolytics or the signal caps? The 'lytics are the only ones that REALLY matter (IMHO).

[NickC]

I take it you mean non-polarized caps, and are trying to determine the outer foil orientation (for lowest noise in circuit)?

If so, this will be of interest:

http://www.youtube.com/watch?v=VP6ZIHXq ... r_embedded

[The New Steve H]

It turned out the big one was bipolar. I had no idea such things existed. The little yellow ones, I have no clue about. Will check your link.

[The New Steve H]

I just watched that Youtube. I had no idea this was a factor. Someone tell me this: which way (+ or -) does the outer foil point? I can hook these to an oscilloscope.

[johnnyreece]

The way I understand it, the outside foil goes to the low impedance side (not as simple as +/- unfortunately). Don't worry, though; if you get it wrong, it's not hazardous to your amp. Electrolytics, on the other hand...

[The New Steve H]

Okay, I can figure that out. New question: is there a nice, kosher way to mount a resistor between two turrets that are too far apart for the leads to reach? The Mojo board has an 820 ohm resistor which is too short. I can rig something up, but I wondered if there is a best way.

[Phil_S]

I am assuming you are just a little short, less than 1"? I save the long clippings from resistors and caps that have stiff leads. If you don't have something like that, use solid 22awg or larger wire. Make two C-shaped hooks. Put the hooks together and clamp with your pliers. Solder. Cover with heat shrink. Now your piece is the right length.

Some kinds of caps don't have an outer foil, so to speak. I wouldn't worry about it. If you have noise, you can deal with it later.

[The New Steve H]

So just rig the resistors any old way. Thanks.

[ToneMerc]

Okay, I can figure that out. New question: is there a nice, kosher way to mount a resistor between two turrets that are too far apart for the leads to reach? The Mojo board has an 820 ohm resistor which is too short. I can rig something up, but I wondered if there is a best way.

Steve, didn't you go down this "short lead" road before?

http://ampgarage.com/forum/viewtopic.ph ... ight=jtm45

TM

[The New Steve H]

Forgot all about it.

We're trying to use the oscilloscope to figure out whether the cap direction matters, but I'm not picking up anything from my fluorescent lights. Does it really matter?

[johnnyreece]

You'll probably never notice in a JTM45. Unless you're Eric Johnson...

The electrolytics are the ones to pay attention to (I know I've said it before, but it bears repeating ).

[Structo]

The trick that Gabby shows in the video works as well or better than the scope method.
And, it doesn't require a scope.

[Reeltarded]

I still need to put one of those on a pedal board.

[The New Steve H]

The benefit of the scope method is that I don't have to build that thing. The down side is that it doesn't work. Maybe I'm doing something wrong. I'll get out the scope manual.

We got the board populated. The sockets are installed. I decided to quit before doing the heater wiring. I really don't want this thing humming all over the place, so I want to review Merlin Blencowe's heater stuff before I dive into it.

This amp only came with two coils of wire: yellow and green. Weird.

It's a hell of a lot faster than building an amp from scratch, but I miss the personal touch and the ability to mod at a second's notice.