New Build: Mojo British 100

[neikeel]

Pics please

[LedZepp007]

I grounded my output jack on a lug attached to a power tube bolt (i.e., right by the heater wires). I wonder if that's the problem? Maybe I'll try to move it when I get home.

[LedZepp007]

It's saying the attachment limit has been reached. So the server ain't letting me upload a pic.

[DG45]

Different build but my speaker output jack ground is also grounded to the lug attached to a power tube socket bolt. No problems but every build is different.

You can upload the photos to a photo hosting site (photobucket etc.) and then just provide a link to the photo, off site.

[LedZepp007]

Ok, figured it out. It's ugly in there. But consistent with Mojo's layout. If I didn't get such a steal on the stuff I probably would've gone with a different kit.

http://imgur.com/a/vpvOS

[DG45]

Ok, figured it out. It's ugly in there. But consistent with Mojo's layout. If I didn't get such a steal on the stuff I probably would've gone with a different kit.

http://imgur.com/a/vpvOS

Did you figure out the problem, or just how to post the picture link

I'm not an expert so I don't have a keen eye for spotting issues right off the bat. But, I suspect that the guys who are good at this stuff will point out the wire routing (and the splices?) in addition to any issues that they may see. This site seems like a great resource for information and help but, unfortunately, it's not a very busy forum ... this section anyway.

[danman]

Does the amp work and play properly other than the hum you are hearing? Are you absolutely sure that it is 60hz hum and not 120hz? That will help alot in determining the source. Can you get some better pics of the preamp socket wiring and input wiring? One last thing, have you biased the power tubes and are they fairly close in current draw? Have you swapped in different preamp tubes to eliminate them as a possibility?

[neikeel]

I have had a good look at the pic you posted, it is VERY difficult to trouble shoot a layout like that with the yellow wire everywhere, although having fling wires everywhere you can see their hook up points, it is likely to be very 'noisy' when you get it up and running.

I cannot se a glaring wiring error (or the 470k instead of 470R in PI mistake that gives a quite amp), as a side point I see no grid stoppers on pin 5 of the output valves.

Can you post a voltage chart (print off a blank one, then scan yours in)?

Probably a good idea to clean up a little and re-sweat some of your joints (use a croc. clip between joint and working part of component to the chassis as heat sink, that way the component does not overheat.)

[LedZepp007]

Thanks for the responses. I figured out how to post pictures, not fix the amp.

It's definitely 60-cycle. I have not tried to pull tubes yet, although that's probably next. Although the wiring looks messy, there is some method to the madness.

I do have stopper resistors, they are covered in heat shrink.

The hum really isn't bad until the MV is cranked.

Another weird thing: I am getting a bit of hum from the guitar that subsides when I touch the strings or bridge. The guitar is definitely grounded, so I'm wondering if this is any indication of what's happening in the amp.

[danman]

When you say that the hum is only bad when the MV is cranked, do you mean that the hum occurs with the MV at 10 or when you are turning it down towards 0. With the LarMar ppimv, it is basically out of the circuit at 10 and as you turn it down towards 0 it attenuates the signal reaching the power tubes. This allows you to increase gain in the preamp, using the channel volumes to achieve distortion, while cutting the signal reaching the power tubes. With the LarMar MV at 10, does adjusting the channel volumes increase or decrease the level of hum you hear?

[LedZepp007]

Individual channel volumes have no effect on the hum. Another weird symptom: the presence pot is scratchy.

[danman]

Scratchy presence pot is normal with this circuit because there is a small amount of DC voltage on it. Marshall changed the circuit a few years later but alot of people still prefer the older circuit. You didn't specify if the hum occurs with the MV approaching 10 or when turning down to 0. How about the power tubes...are they fairly close in current draw?

[LedZepp007]

Ok, poked around with chopsticks and found the culprit of the problem: the V2 socket wiring, specifically the heater wiring. I rewired the socket and no more 60-hz hum. There is still a good amount of 12AX7 hiss when cranked, but I supposed I expected that.

[neikeel]

Ok, poked around with chopsticks and found the culprit of the problem: the V2 socket wiring, specifically the heater wiring. I rewired the socket and no more 60-hz hum. There is still a good amount of 12AX7 hiss when cranked, but I supposed I expected that.

Good.
Hiss is a manifestation of gain and some component noise (all MFs might cure it )

[DG45]

Great to read about problems getting solved.

I built a "45" with all metal films (and MO's) and added a PPIMV. No hum, some hiss, as expected when cranked. I later replaced a few of the metal films with CC's ... 6 total, only on the plates of the pre-amp tubes and PI plus feedback resistor. I did not notice an increase in hiss. I definitely noticed a difference in tone. Reports are that it's supposed to be very subtle, but it wasn't that subtle, and I don't consider myself having keen ears. I ended up going back to the metal films. There was more of what I guess I can describe as an edge to the "crunch" with the MF's that wasn't difficult to notice. I can see why players would prefer the CC's but I preferred the metal films. Could be the circuit, or could be all in my head I guess.

At any rate, I'll defer to neikeel on the noise floor with Carbon Comps as you have quite a few in your build. In my limited experience, just using the 6 CC's where I installed them, there wasn't a noticeable difference in noise compared to using all metal films. So, if you do experiment further with the hiss, maybe using CC's only in some spots would make for a good compromise on tone/noise.

Happy to read that you have it sorted out.