Mojo input jack wiring for 5E3 kit?

[Ben]

Hey all,

I recently finished a mojo 5e3 kit and have discovered that I did not wire the input jacks correctly. As a result i get some very strong self-oscillating feedback and after some research and some experimentation I am pretty sure the input jack wiring is the cause. This weekend with the help of a frined I am going to open up the amp, drain the caps and redo the input jack wiring but i am still having difficulty understanding the layout diagram, and specifically the connection indicated on the diagram to the #2 jacks from the #1 jacks.

Is this diagram telling me to connect the tip lugs from jacks one to the "switch" lugs on jacks 2? There is no ground connection shown for the #2 jacks is there?

If anyone has any good photos from a mojo kit of the input jack wiring, man that would be great! Ive treid to attach the layout diagram for reference.

Thanks for the help!
-Ben

[loudthud]

Look at post #14 in this thread: http://music-electronics-forum.com/t12667/

We don't know for sure what kind of jacks were supplied with your kit.

Look in this thread: http://music-electronics-forum.com/t13689/

There is a better layout diagram in post #10. I think SwitchCraft jacks were used to make that layout. The terminals are marked G (for ground) T (for tip) and S (for switch). I think Weber uses jacks that have terminals in a different order.

[Ben]

Thanks Loudthud, I really appreciate your help. thats good stuff

Switchcraft "shorting" jacks are what the parts list says I have. They have three lugs and I can see one goes to tip, one to ground and one to "switch" whatever that is.

I did look at that diagram you guided me to before when i was trying to figure out how to wire the jacks. Its actually part of where my confusion comes from because that diagram shows a connection from switch on jack 1 to ground on jack 2. It appears this connection only serves to ground jack 2 and jack 2 is already grounded by touching chassis. Maybe that assumption is wrong tho and that connectio has a greater purpose?

Heres a photo from HOffmans site on the fender hi/lo jack wiring and he doesnt ground jack 2. That further added to my confusion.




sorry, to be so dense. I guess my question now is , is the connection from switch on jack 1 to ground on jack 2 really needed when you are using chassis as ground and dont have insulated input jacks?

[mbratch]

Just in case you weren't aware, the Hoffman wiring assumes the two 68k resistors are connected directly to the jacks. On the vintage layout, as well as the Mojo, these resistors are on the eyelet board.

In the Mojo diagram, and in the Hoffman wiring, the switch and ground on jack 1 are wired together and to one end of the 1M resistor. The other end of the resistor connects to the tip of jack 1, which is also connected to the switch of jack 2. The ground of jack 2 is not connected.

[Ben]

Cool, thats what I thought. So I am only missing the connection from switch to tip on jack #1's in how I have it currently miswired. I guess i didnt understand the symbol depicted on the layout for jacks #1, nor how the jacks themselves worked...embarassingly enough I still dont. I'll learn.


When its not going into self oscillation feedback mode...the basic tone of the amp is there and is totally awesome. Hopefully this is the fix. I shall report back. Thanks for the help again!

[Ben]

5E3 , will you marry me?

Drained the caps and feel comfy doing that now.
Made the connections at the inpuit jacks from switch to ground, then it occured to me the speaker jack needed the same connection.
Fired it back up and got a weird scratchy hissing when playing. Opened her back up , drained the caps again, removed two dog hairs, reseated the preamp tubes and now she works ..and how. gorgeous sounding and will no doubt only get better with a little speaker break in.

Now I am thinking about some NOS tubes. The scratchy hiss seemed preamp tube related maybe? Im told the sovtek 5y3 puts out too much voltage so that will go for sure. I dont wanna go to far down nos tube street. a fellows wallet could take a beating down that street.

Hey Thanks for all the great help!

What should i build next?

[mbratch]

The scratchy hiss seemed preamp tube related maybe?

I thought you proved it was dog hair related? It could have been just a bad tube/socket connection, so maybe the reseating fixed it.

As far as tubes... I find the 5E3 sounds nice with new or NOS tubes. However, you are correct: the new Sovtek 5Y3's generate noticeably higher voltages. You can try it, but measure your plate voltage and bias current. Depending upon your PT, you might be able to use them straight up. In my particular amp, the PT is already a little high, so I made some changes to accommodate it. You should consider a good NOS 5Y3. Also, the only new 12AY7 I've seen is the Electro Harmonix brand, and I find that they can be noisy (I get noticeable hum). So NOS 12AY7 should be on the list. It's good to have at least one spare of everything anyway.

You might also, for fun, experiment with other combinations. The stock configuration is 12AY7 + 12AX7. I've tried 12AX7 + 12AX7 (more gain), 12AT7 + 12AT7 (similar to original configuration in sound, maybe a smidge cleaner?), 12AT7 + 12AX7 (in between the 12AY7 @V1 and 12AX7 @V1 sound), and 12AU7 + 12AX7 (lower gain than original).

Next amp kit? For me it will be something in the Vox/Matchless family. I'm currently thinking Spitfire...

[Ben]

wow, i didnt know you could substititue preamp tubes like that. cool.

This is probably a subject for a different thread but I am woefully ignorant of the Vox sound. I understand the basic fender sound, the sound of a marshall (yes, i know theres variants but you know what i mean eh?)...but Ive had ZERO hands on time with any Vox. The closest Ive gotten is the line6 sim of one. Bring on the adjectives..."chimey"? would that describe an ac30? the sim was one of my favorite sims, it had a gritty edge to the gain that i liked, wasnt chimey....

what guitarists or bands present the typical vox sound?

probably too broad a questiion, like if I asked who used marshalls....

I have a bunch of loud and clean tube amps in the 100 watt range, so whats missing for me I think is a really small high gain recording amp, and a high gain low wattage head or combo. For the later, maybe a marshall 18 watter? and for the former I have no clue whats out there?

Cheers!