Mojo British 800 troubleshooting help please

[Atomicholiday]

Hey all! So I just finished this amp yesterday and started testing this morning. This is my first amp build and my skills have a LONG way to go, but it's been a lot of fun so far. Admittedly, I don't really know what I'm doing yet, but I've got the desire and ability to learn, so here goes.

Right off the bat I've got problems while testing. I'm sure it's something obvious, but I'm just not seeing it. I plugged the amp in and checked voltage at the switch and I'm good (120V ac). But when I turn on the power switch nothing happens. No pilot light, no nothing. Checked the fuses first and they're both good. I wired it like the mojotone diagram (unless I missed something). I've checked & rechecked all the wiring and I can't find the issue.

So what's my next step?
Also, while we're at it, any constructive criticism or comments are more than welcome. Thanks in advance for you help!


Mojo wiring diagram
http://www.mojotone.com/Amp%20Kit%20Sch ... art=778096

[danman]

Welcome to the forum! From your description it sounds as if you have wired the power switch wrong. Power at the switch but no power to the PT would definitely be a wiring mistake. It's hard for me to make out the wiring because of the non traditional colors. I checked out the Mojo layout and that thing looks very hard to follow also. Here is a much better layout done by one of our members here...https://drive.google.com/drive/folders/ ... llPYTc5Wms You will notice that it uses single pole switches which are much easier to wire correctly because there are only two solder connections. You will most likely need to remove all of the wires from the power switch and then use your meter to figure out what pole needs to be connected to what. Another problem with all of these layouts is that they follow the old way of placing the fuse in the white (neutral) wire instead of the hot wire. The safer way is to fuse the hot side for the most protection.

Your PT should have two wires for the primary 120vac. The white neutral from your power cord (this is always connected to the wider polarized plug at the end of the cord) should go straight through and connect to one of the PT's primary wires. Since you are using a double pole switch, you will be soldering this wire to one of your switches lugs. The black hot wire from your cord should connect to the fuse holder's center terminal first. Then the black wire will run from the fuse holder's side terminal straight to the switch where it will meet with the other wire from the PT primary. At the switch you should have both wires from your power cord (after they pass through the fuse first)...one wire solders to the top left lug and the other wire solders to the top right lug. Your two PT wires will go to the bottom set of lugs...one on the left and one on the right. This should allow you to power up your amp properly. If the power switch is working in reverse then you simply need to flip the wires.

If you are still having voltage issues, you should be able to follow the voltage through the circuit with your meter. Check all points along the way with your meter set to AC. The high voltage leaving the PT will also be AC until it reaches your rectifier diodes where it will then be converted to DC. The 6.3v filament voltage remains AC throughout the circuit.

[Atomicholiday]

Hey Danman, thank for the reply! I did get it up and running finally. Actually got sound out of it too! YES!

I've got a couple of questions though. A couple things I'm not sure about.

1)With it powered up and running smoothly, the power switch doesn't light up. Not sure what's up there.

2)The presence control is very scratchy sounding and works backwarks? The lower you turn it the higher the presence gets. Also, it doesn't really change the sound much.

3)(most importantly)I'm not sure if the "Mains Select" switch is wired correctly according to the mojo diagram. How do these switches work internally? Should the "110V" marking on the switch line up with the pole? I'm thinking the 120V and 240V wires may be switched.

4)Oh yeah, one more thing. When biasing this thing, what should I expect to see for plate voltage at pin 3 of V4 and V5?

I understand that these are probably very basic rookie questions, but I gotta learn somehow, so thanks again for your help! I'm getting really close to completion now.

[Atomicholiday]

Ok, so now I'm a bit scared here. This was posted on another site (see quotes below).

So are these guys right? Is the Mojotone diagram wrong? Most importantly, is the amp gonna be safe?

"After looking at This, I typed in the circuit diagram from the MOJO tone website.

Looking at the power input alone MY exclamation is "Fu%$ing H&ll" The Fuse is on the Neutral and Double pole Isolator is going to save Your Heirs $$ in Cremation THE GROUND MUST ALWAYS BE CONNECTED TO THE CHASSIS/ GUITAR LEAD/ MICROPHONE / RECORDING STUDIO CARPET."

and this:

Seconded!

In the circuit diagram, the mains fuse is shown connected to the neutral lead. IT MUST BE IN THE LIVE lead.

Also, the HT fuse is shown in the ground circuit! This MUST be connected in the HT circuit instead!

The reason for both these is the same - If there is a short (e.g. through you!), and one of these fuses blows, then you may still be connected to a live terminal, even though power cannot flow in the mains circuit.

see [link]

So you must always have a rock solid connection from anything you can touch through to the earth connection at the mains plug. This is called the protective ground.

see [link] section 15.1

And this:

As pointed out the HT fuse in the neutral line I had another look at the circuit diagram and noticed just how dangerous this is.

Basically what WILL happen if the amp developed a fault between live and earth through a resistor for instance, the current would blow the fuse but the chassis and Guitar lead would now be at mains potential. You wold now touch something else that's earthed, microphone stand say and now You the amp maker get to meet Your maker.

The circuit diagram doesn't show the transformer lead colors so I will have to go by the diagram You posted.

Connect the blue wire from the transformer primary directly to the Neutral pin of the mains socket (G). The HT fuse I would ignore and just connect RED/Yellow wire to chassis. This advice is taken from the Valve Wizard web page and what He says goes. He recommends a fuse in each leg of the the RED wires between transformer and standby switch.

Lastly. I didn't see any wiring fault on the Earth wire, I was pointing out how vitally important it is to make absolutely sure the chassis is connected to the Mains Earth.

[danman]

The older amps were wired with the fuse in the neutral line. Many amps were done this way for years and most of the layouts today still follow this bad practice. The black should be fused for the most protection. The white should connect directly with one of the PT's primary wires, either at the double pole switch or some folks solder the wires directly together. It works fine either way as long as the black is being switched. The MOST important thing is to run a short green ground wire from the earth lug on your power receptacle in the amp, directly to a solder lug connected to the chassis with bolt, nut and lockwasher. This connection is extremely important and must be secure.

Your power switch I am not sure about as I have never used that type before. A quick email to Mojo should get you an answer. The presence circuit will be scratchy when turning the pot because there is a small bit of DC voltage on the pot. If the pot is working in reverse, you may have the wires reversed so check it against the layout I posted.

If you google "marshall mains selector switch wiring" you can find some diagrams that should show you what lugs are what. With out the switch in front of me it's hard for me to answer. You can also use your meter's "beep" function to determine which pole are being activated when rotating the switch. If you have the wiring reversed your voltage will be extremely different. Using the 240v in the US would mean your B+ voltage would be way low. You can also check here for some very detailed instructions on wiring...http://valvestorm.com/sites/default/fil ... CTIONS.pdf These are for a standard 50watt circuit but much of the info will be the same.

The expected plate voltage will vary. B+ will be higher with a cold bias (lower idle current) and lower with a hotter bias. The PT that Mojo supplied with the kit will also affect voltage. There are several PT's available for these circuits and the B+ can range from 380vdc up to 450vdc depending on which PT was used. Mojo should be able to tell you.

[Atomicholiday]

So I think I screwed this thing up royally.

The amp checked out good for all the various voltages, so it was time to adjust bias. Plugged in my bias probes/multi-meters, tubes, and speaker and fired it up. No problems so far and the amp was really quiet.
I realized I didn't have the meters set to the correct setting and without even thinking about, reached over and switched them over from AC to DC/ma. WITHOUT TURNING THE AMP OFF FIRST! Complete rookie mistake and I absolutely should have known better.
As soon as I switched the first multi-meter to DCma, the amp made this extremely loud hum, so I switched it off immediately. And now every time I turn on the stand-by switch I get that same insanely loud hum.
This is with the volume and gain set at 0.
So today I set up the multi-meters and tried it again just long enough to get a reading. I'm getting around 80ma (should be more like 43). Visually everything looks OK, and all the solder joints check out fine.
Any idea what I blew?

[FourT6and2]

So I think I screwed this thing up royally.

The amp checked out good for all the various voltages, so it was time to adjust bias. Plugged in my bias probes/multi-meters, tubes, and speaker and fired it up. No problems so far and the amp was really quiet.
I realized I didn't have the meters set to the correct setting and without even thinking about, reached over and switched them over from AC to DC/ma. WITHOUT TURNING THE AMP OFF FIRST! Complete rookie mistake and I absolutely should have known better.
As soon as I switched the first multi-meter to DCma, the amp made this extremely loud hum, so I switched it off immediately. And now every time I turn on the stand-by switch I get that same insanely loud hum.
This is with the volume and gain set at 0.
So today I set up the multi-meters and tried it again just long enough to get a reading. I'm getting around 80ma (should be more like 43). Visually everything looks OK, and all the solder joints check out fine.
Any idea what I blew?

What do you mean, multi-meters (plural)? You say your switched the "first" meter... and you say "them." How ands why are you using two meters simultaneously? And how exactly are you checking the bias? Through 1ohm resistors on the power tube sockets? If so, switching the meter through to different settings shouldn't do anything dramatic.

[Atomicholiday]

I'm using two bias probes I got from Mojotone. See here:

http://www.mojotone.com/manuals/Hoffman ... =overwrite

I have one multi-meter on each probe so that I can read both tubes at the same time. The way the probe works is pin 8 from the tube base is sent out to a Multimeter set to dc milliamps and sent back to pin 8 of
the tube socket. So I'm reading the current flowing through pin 8 by putting the meter in line with it.

Is this wrong? I followed their instructions to the letter.
Seriously, I'm trying to learn as much as I can here. I know I probably got in over my head, but I've made it this far and I want to see it thru to the end. I need to know what I did wrong and what I screwed up, so I can fix it, learn from it, and not make the same mistake twice. I just need a little help to get there.

[Atomicholiday]

Just a quick update. I got the amp up and running and it sounds awesome! Been rocking it all morning.
Danman, thanks for your input.

[danman]

Great news!! Sorry I didn't see your last post the other day but it looks like you got it figured out.