The Canadian Guitar Forum banner

1 - 20 of 76 Posts

·
Registered
Joined
·
1,202 Posts
Discussion Starter #1
First off, let me say that I am in no way whatsoever an amp tech. Electronics do not come to me naturally and most of the time I feel like I am in over my head. That said, I am reasonably intelligent, good with my hands, mechanically inclined, and not easily frightened by a challenge.

I have built myself a 5E3 from parts. I acquired a partial MOJO kit some years ago in a private deal and had to buy the rest of the missing components individually over the years as well as a few extra tools to complete the build.

I completed the build earlier this week and have been enjoying my new toy up until I tried placing a jumper between the bright and normal jacks. I had been using only the normal jack. It was when I did this and was experimenting with the individual volume knobs that I started noticing something was amiss. The first thing I noticed is that there was no sound when plugged into the bright jacks. The MOJO drawing I downloaded and used to build the amp showed 3 out of the 4 input jacks using the chassis as ground and I wondered about that while building it. I went ahead and wired up a ground to the other 3 jacks and then there was sound from the bright jacks but only at half volume compared to the normal jacks. The second thing I noticed is that the bright volume control does very little when plugged into the normal jack, almost nothing when plugged into the bright jack and shuts off all volume if dialled past 11 on either jack. Using a jumper between normal and bright does not seem to have the same affect as it normally should. In fact, it almost seems as though it's not even there.

This is the drawing I used as my reference to build the amp.
I've been around forums for a long time so I know people will want more information.
I will try to provide whatever I can.
I appreciate any help I can get.

 

·
Registered
Joined
·
1,202 Posts
Discussion Starter #2
I did not follow the MOJO drawing to the letter. There were two deviations.

One, I wanted a mute/standby switch and saw no need for a ground lift. I facilitated that by eliminating the cap, connecting the red/yellow from the PT to one side of the ground switch and the other side to ground. The third contact at the other end of the switch is unused.

Two, I did not get the MOJO 768 OT shown on the drawing and instead used a Mercury Magnetics FTDO-59 so the wire colors are not exactly as shown.
 

·
Registered
Joined
·
1,202 Posts
Discussion Starter #4
This was taken before I added a ground.

 

·
Registered
Joined
·
1,202 Posts
Discussion Starter #5
After.

 

·
Registered
Joined
·
1,202 Posts
Discussion Starter #7
That jack works.
It's the two on the left that are the bright jacks.
 

·
Premium Member
Joined
·
5,153 Posts
I'm no amp tech either, but I find the way Mojo drew the grounding on the schematic you posted a bit unusual. Not just the input jacks - all the way down the line. I'll be waiting to hear from one of the techs on this. Maybe there is some learning here for me. (and for us all) :)

Wondering why they separated the pre-amp grounds from the power amp grounds. What the benefit is and should I be doing it.
 

·
Registered
Joined
·
1,202 Posts
Discussion Starter #9
Excellent question. I was having exactly that discussion with a guy about grounding about a week ago. He was telling me some amp experts believe the preamp section should be grounded in a location separate from the power section. Everything I have been taught tells me that is a recipe for ground looping and that various components all need to be grounded to one location.
 
  • Like
Reactions: Lincoln

·
Premium Member
Joined
·
5,153 Posts
Excellent question. I was having exactly that discussion with a guy about grounding about a week ago. He was telling me some amp experts believe the preamp section should be grounded in a location separate from the power section. Everything I have been taught tells me that is a recipe for ground looping and that various components all need to be grounded to one location.
That's what I've been "taught" as well. In that diagram, mojo has even gone so far as to ground the filter cap for the 12AX7's power supply with the pre-amp stuff and separate from the rest of the filter caps. Interesting.
 

·
Registered
Joined
·
1,202 Posts
Discussion Starter #11
Well, I didn't do that.
I have one area with two strips connected to each other as a central ground location just to the right of the PT.
 

·
Registered
Joined
·
4,830 Posts
Grounding can almost be an art as much as a science. The theory and practice do not always gel. Classic Fender wiring that is often hum free is in some ways technically 'wrong'.
So don't get hung up on it unless you have noise/hum issues. Different (or 'non-traditional') grounding techniques will not make things work/not work.

Your jacks all look correct as far as the wiring goes.
Does everything work correctly without the jumper cable? Lower jacks give different loudness than upper jacks? Bright side is bright, normal jacks normal?

If all that is correct, can you run through this page and say where there are differences? He goes through what happens with the jumper installed: Input Jacks
 

·
Registered
Joined
·
1,202 Posts
Discussion Starter #13
On the normal jacks one is hotter than the other.
I do no perceive the bright jacks to be brighter but that might be because the output is so much lower.
It is significantly lower. Much more than just being noticable. It is enough that those jacks are basically unusable.
 

·
Registered
Joined
·
1,202 Posts
Discussion Starter #15
Check the wiring to pin 7 of the first tube. The bright and normal go to differnt halves of the first tube. Maybe that first stage isn't working properly.
I have no way of knowing what you mean by "the first tube".
If you mean the 12AY7 socket, the bright is connected to pin 7, the normal is connected to pin 2.
Except for the two variations I mentioned, both of which should have nothing to do with this issue, it is wired up as shown on the diagram in my original post.
 

·
Registered
Joined
·
1,202 Posts
Discussion Starter #16
Is it possible the 12AY7 is faulty?
Is there any way that a faulty preamp tube could explain the volume knob issues I explained in my OP?

I do not have an alternate tube to try.
 

·
Registered
Joined
·
1,202 Posts
Discussion Starter #17
 

·
Registered
Joined
·
1,202 Posts
Discussion Starter #19
I tried a 12AX7 known to be good. (a J/J EEC83)
I like the gain better this way.
It seemed too clean for me before.
It was not at all why I wanted a 5E3 in the first place.
Are there any issues with running it this way all the time?

I think I may have discovered one of my problems.
The one associated with the jacks.
After replacing the preamp tube and powering it back up I thought I had made it worse because only one of the normal jacks was working properly and the other one was now like the brights jacks at about 1/3 volume.
Simply by way of accident, while I was pulling the cord out and trying the different jacks, I noticed that for a split second it went back up to full volume.
I decided to try wiggling the cord while plugged into the jack and there was a huge difference.
If I push on the jack in such a way that the tip moves further away from the shorting "thingie" it comes in loud and clear.

Should I attempt to slightly bend the shorting part away from the tip contact on all the jacks and see if that cures it?

I have no way of knowing if it will affect my volume knob issue though.
 
1 - 20 of 76 Posts
Top