New Build: Mojo British 100

[LedZepp007]

Hi all,

(I will preface this by saying that if Metro still sold kits that I would have purchased one. I have nothing but the highest respect for this site and for George).

I am embarking on my first real project: a Mojotone British 100 kit that I am building into a Mojo Bluesbreaker cab. I am completed some pedals in the past and have hacked around in some amps (and generally done a terrible job, I might add). However, I have read quite a bit and worked hard to improve my soldering technique. I felt like I was ready to try a kit without instructions.

I felt like, in order to really create a keeper, I would need to document the build where those who are in the know could critique. Would this be okay to do here?

So, I have received the kit. Looks good, parts are well organized. Board looks good. The layout provided is minimal. I already have some questions about grounding points and which bolts to use where. It looks like Mojo expects 6-32 bolts to hold the massive power and output transformers. I'll probably go to Home Depot and get some 8-32 bolts instead and use nylon lock washers.

In terms of grounding, I was thinking of the following:

1) Using a ground buss on the back of the pots with a grounding point near the inputs
2) Having a power amp grounding point on a lug on one of the power transformer bolts
3) Drilling a small hole near the IEC inlet for a dedicated grounding point.
4) Grounding the output jack to a lug on one of the bolts securing the closest filter cap can

Suggestions?

I had also strongly considered using the Metro build guide, but my sense is that Mojo's layout and George's layout are a bit different in terms of grounding, etc.

Irritatingly, the choke I sourced (3H) doesn't quite fit in the pre drilled chassis holes, so I'll have to enlarge those. Hopefully it is the right choke.

Finally, I am planning on utilizing a LarMar style PPIMV on this amp. I already sourced the parts, but I may have a problem with where to locate it. Since this will be a combo, I would strongly prefer the dual pot to be located on the front of the amp. I considered using the Normal/Low input jack hole, but it seems like that might lead to noise. Opinions?

Oh my goodness, this post just keeps going on. Thanks for your patience. Last thing:

I'm not sure whether I want to build the lead circuit or the bass circuit. Every bass circuit I've ever heard sounds fantastic. However some lead circuits can sound even better. Unfortunately, some of the lead circuits are very ice picky (especially with certain guitars). I have the parts to build either circuit. Anybody have any opinions?

Thank you so much!

[danman]

Welcome to the forum! If you do a search here for "Larry Grounding" you will find a grounding scheme that works very well. It's a little different that the standard Marshall scheme but many have used it with great results. As for which circuit to build, I started with the lead version on my first amp. After a few months I switched over to the bass circuit but now I have settled on a mix of both.

[LedZepp007]

Thanks Dan! Did a search for Larry's grounding scheme. Looks like a good plan. I'll see how I can implement it. I had to enlarge the chassis holes for the choke to make it fit tonight. Then started to mount some other things.

Also, I now have a cabinet and it is sweet. Picture is (hopefully) attached. I know how some feel about attaching logos to the chassis, but it looks totally naked without some kind of logo.

Dan, how did you find a balance between the Bass and Lead circuits?

Thanks!

[Carbia]

Super Lead preamp with Bass power amp works well.

[danman]

I ended up going back to the split cathode arrangement found in the lead circuit. For the coupling caps, I used the bass values (.1uf) for the PI and all the others received .022uf. I am also using a .68uf on the presence pot as I prefer that over the standard .1uf value and like the bass circuit, I do not have a bright cap on the channel 1 volume pot. There isn't much difference between the circuits really and you can find many places to experiment. Experimenting with different NFB resistor values and OT taps will also yield some different tones.

[LedZepp007]

Sweet! Thanks all. I will document my progress.

[LedZepp007]

Okay. Horrible newb question: if my solder joints are appearing overheated (brown spots from burnt flux) I probably need to turn my iron down and totally redo those joints?

[danman]

Not necessarily. Most of the solder that I have used in my builds will leave a brown, crusty residue around the edges of the solder joint. It is just the flux from the soldering wire. I usually clean mine up later by scraping it off with a small screwdriver or using denatured alcohol on the joint immediately after soldering. I believe that Kester also makes a brand of solder called "no-clean" that will leave considerably less residue. If you can, post a few close up pics of a few soldered turrets and we may be able to tell more.

To avoid overheating components be sure to melt a fair amount of solder onto your tip before touching it to the item to be soldered. This extra solder on the tip will allow the joint to heat much quicker and help you to avoid overheating. I usually run my station right around 650 and it seems to do a good job.

[LedZepp007]

Okay. I finally got around to doing some building after finally finishing my licensure.

It ended up being a bit of a bastard child of Mojo and Metro stuff.

My initial voltages are off (however). Here's what I've measured so far that might be relevant:

V1 P1:150.5 VDC
V2 P1: 166.6 VDC
V3 P1: 375.4 VDC (way too high, no?)
HT: 499.9 VDC
Bias: -48.9 VDC
Before 82K resistor: 379.8 VDC
After 82K resistor: 376.3 (this should drop more, correct?)

I'm testing without the EL34s, but with the 12A7s. Heater voltage looks good.

Any ideas? Help would be greatly appreciated. Wall AC is 120VAC almost right on.

THNAKS!

[LedZepp007]

I will also mention that all of the voltages are high for that valve.

[danman]

Those voltages will drop down quite a bit after you get the power tubes installed and properly biased.

[LedZepp007]

I effed up the V3 cathode. Voltages look good now. Biased at 34ma.

It works! Mainly.

I also installed a PPIMV of sorts. Rather than the Lar-Mar style championed here, I chose to use Ken Fischer's "Type 3" MV. It's a 1MA pot that mixes each side of the phase inverter together (I think). Anyway, it works, but I get what sounds like 60-cycle hum when I start to increase the MV. Ideas?

[danman]

Did you twist your leads or use shielded cable? Can you post some pics of the MV and wiring to board and sockets?

[LedZepp007]

Did you twist your leads or use shielded cable? Can you post some pics of the MV and wiring to board and sockets?

It may be a bit before I can post pics of the wiring. I did use shielded cable. Attached is a diagram of the MV.

If I had to guess, it's lead dress. Things got messy in there. Second guess would be that I shouldn't have grounded the shield to the back of the Presence pot.

Thanks!

[LedZepp007]

Okay. So last night I sat down rigged ups Lar-Mar PPIMV and installed it. Still have the same 60-cycle hum that is dependent on the MV. No crackling or popping, just hum after about "2" on the dial that gradually gets louder as the master volume increases. Any ideas?

Thanks!