Mojotone British 800: First Ever Amp Build

[GAStan]

Let me try again...I think I misunderstood.

You won't read continuity of a resistor over a certain resistance value. The fact that you get a resistance value shows the resistor and connections are good.

The continuity check on the meter is useful for verifying wires and low resistance connections. I actually seldom use this setting, prefer to see an actual ohms reading even when checking simple continuity.

 

[ThreeChordWonder]

The capacitor will throw off the ohms reading, the resistor needs to be isolated to accurately check it.

Likewise the cap.

 

[Dr Nasty]

Let me try again...I think I misunderstood.

You won't read continuity of a resistor over a certain resistance value. The fact that you get a resistance value shows the resistor and connections are good.

The continuity check on the meter is useful for verifying wires and low resistance connections. I actually seldom use this setting, prefer to see an actual ohms reading even when checking simple continuity.

That all makes sense to me!

Amp seems like it's coming together pretty well so far. Hoping to have every connection that is not directly connected to the turret board done by the end of tonight.

Realized today that I accidentally sautered pin 8 straight to ground like in the wiring diagram, where I meant to add the 1w 1 ohm 1% resistor instead for biasing.

Also have read about using shielded cable from the preamp pot over to pin 7 of the first 12ax7 to reduce noise. Had already sautered in a jumper cable. Anyone have any experience with this one? Does it really reduce noise that much? Worth making the change before I move further along?

 

[Dr Nasty]

Ok everyone, I'm thinking of replacing the wire I soldered in from the preamp pot over to on 7 of the 12ax7 with a shielded cable as I've been reading that it can considerably reduce noise.

My question is does diameter/capacitance of the shielded cable matter for this application?

I have some high quality best-tronics patch cable that I use for my pedalboard cables.

24 awg
20 pf/ft capacity rating

Would that be ok to use for this application?

 

[BlueX]

Ok everyone, I'm thinking of replacing the wire I soldered in from the preamp pot over to on 7 of the 12ax7 with a shielded cable as I've been reading that it can considerably reduce noise.

My question is does diameter/capacitance of the shielded cable matter for this application?

I have some high quality best-tronics patch cable that I use for my pedalboard cables.

24 awg
20 pf/ft capacity rating

Would that be ok to use for this application?

You could complete the amp with the wire that's already there, and replace with shielded later once it's working. Could be interesting experience to hear any difference first hand.

 

[Dr Nasty]

You could complete the amp with the wire that's already there, and replace with shielded later once it's working. Could be interesting experience to hear any difference first hand.

This is a great idea actually.

I've got pretty decent recording set up. And I have a pretty extensive background in science so all the YouTube comparisons of this kind of stuff always piss me off with how many variables they fail to account for. I've been thinking about starting a channel. Might just start making short little clips of comparisons of that sort of stuff.

Could compare the amount of hum vs the supposed tone suck of a shielded cable.

 

[BlueX]

This is a great idea actually.

I've got pretty decent recording set up. And I have a pretty extensive background in science so all the YouTube comparisons of this kind of stuff always piss me off with how many variables they fail to account for. I've been thinking about starting a channel. Might just start making short little clips of comparisons of that sort of stuff.

Could compare the amount of hum vs the supposed tone suck of a shielded cable.

I'm interested to hear such a comparison, if you record and share!

Maybe you already sorted this out (and it's been mentioned earlier), but here are a couple of things I think are important before you start up.

- Light bulb limiter (LBL), your best friend when starting up a new built, repaired, or modified tube amp.
- "Cap drainer", a resistor to decharge capacitors in a controlled way.
- Shrouded probes and leads, rated for 1000V (noted you already got a Fluke).

Some picture of my set up. Using a sturdy, spring-loaded hook probe (black) that I attach to chassis and leave there. With stackable plugs I connect both DMM and cap drainer here. That way I only need to operate the red probe for DMM or cap drainer (one at the time) with one hand.

 

[Dr Nasty]

I'm interested to hear such a comparison, if you record and share!

Maybe you already sorted this out (and it's been mentioned earlier), but here are a couple of things I think are important before you start up.

- Light bulb limiter (LBL), your best friend when starting up a new built, repaired, or modified tube amp.
- "Cap drainer", a resistor to decharge capacitors in a controlled way.
- Shrouded probes and leads, rated for 1000V (noted you already got a Fluke).

Some picture of my set up. Using a sturdy, spring-loaded hook probe (black) that I attach to chassis and leave there. With stackable plugs I connect both DMM and cap drainer here. That way I only need to operate the red probe for DMM or cap drainer (one at the time) with one hand.

View attachment 157330View attachment 157331

Looks awesome. I really need to get me one of those amp cradles to save myself the frustration. Maybe as a gift to myself if this build is successful.

So I'm about ready to start wiring up the standby switch right now.
Just want to make sure I've got this right.

On the right hand side if I connect that yellow wire that's running to the bias feed resistor to the same lug on the standby switch I should be avoiding those potential issues correct?

Plant to use scotch tape to label the ends before I twist them up so I make sure I don't connect the wrong end

 

[Dr Nasty]

Another quick question.... I just realized I wired that cap/resistor combo to the wrong lug. I just snipped it, sautered a piece of spare 16g wire I had laying around, shrink wrapped that and then will sauter that to the correct lead.

I feel stupid even asking because I can imagine why that would be an issue but just wanted to double check there was any reason other than it being ugly as hell that I shouldn't have done that lol

 

[BlueX]

Looks awesome. I really need to get me one of those amp cradles to save myself the frustration. Maybe as a gift to myself if this build is successful.

So I'm about ready to start wiring up the standby switch right now.
Just want to make sure I've got this right.

On the right hand side if I connect that yellow wire that's running to the bias feed resistor to the same lug on the standby switch I should be avoiding those potential issues correct?

Plant to use scotch tape to label the ends before I twist them up so I make sure I don't connect the wrong end

This does not look correct (if you follow the Mojotone British 800 WD). Both yellow wires should be wired according to the wiring diagram, from the standby switch and to the 1N5408 diodes.

You should take another wire (white if you got) and solder together with one of the red wires on the standby switch. Make this (white) wire 2-3" longer than the yellow ones. You can also twist the (white) wire together with the yellow, since they all run to the same area of the board.

Should look something like this:

 

[BlueX]

Another quick question.... I just realized I wired that cap/resistor combo to the wrong lug. I just snipped it, sautered a piece of spare 16g wire I had laying around, shrink wrapped that and then will sauter that to the correct lead.

I feel stupid even asking because I can imagine why that would be an issue but just wanted to double check there was any reason other than it being ugly as hell that I shouldn't have done that lol

Other than looks I don't think this is an issue, provided solder joints are good. No real voltage in this area, so little risk for damages.

I prefer to orient the jacks so that all lugs are visible (on the side facing the closest end of chassis), see pictures (wiring for a 4-hole Marshall though).

What's the bare wire end sticking out between the jacks? Should be avoided.

 

[BlueX]

Another quick question.... I just realized I wired that cap/resistor combo to the wrong lug. I just snipped it, sautered a piece of spare 16g wire I had laying around, shrink wrapped that and then will sauter that to the correct lead.

I feel stupid even asking because I can imagine why that would be an issue but just wanted to double check there was any reason other than it being ugly as hell that I shouldn't have done that lol

Here's how the original Marshall from '79 looks like

 

[Dr Nasty]

This does not look correct (if you follow the Mojotone British 800 WD). Both yellow wires should be wired according to the wiring diagram, from the standby switch and to the 1N5408 diodes.

You should take another wire (white if you got) and solder together with one of the red wires on the standby switch. Make this (white) wire 2-3" longer than the yellow ones. You can also twist the (white) wire together with the yellow, since they all run to the same area of the board.

Should look something like this:
View attachment 157346View attachment 157347

Ahhh that makes sense. I am almost positive I understand what you're saying.

So that bias feed resister needs to be on a separate lug from the two 1n5408 diodes which need to be tied into the wires. It's starting to make a little more sense to me now. I have not learned about diode rectification yet but I'm assuming that's what those two diodes are for?

So I could just tie the end of the bias resistor to the 1n4007 diode, have them start at that middle lug and tie the white wire into that one I assume. Will take a picture in a bit to confirm.

The extra wire hanging off is just the leg of a resistor I haven't soldered yet because it's going to have other connections tied in that I haven't made yet.

I thought about doing them horizontal so they'd be easier to get to but it just seemed like it would be easy for the lugs to come into contact with eachother if the nut holding the jacks in loosened up.

 

[BlueX]

So that bias feed resister needs to be on a separate lug from the two 1n5408 diodes which need to be tied into the wires.

Exactly! You need to move the 160k resistor from where it's placed in the wiring diagram.

Put the 160k resistor and 1N4007 diode together as in the picture below (silver band on diode facing the resistor). Twist and solder the leads, if you got shrink sleeve you can put that over this joint.

Then you put this assembly where the diode is shown in the wiring diagram.

The (white) wire from standby switch connects to 160k resistor.

 

[Dr Nasty]

This does not look correct (if you follow the Mojotone British 800 WD). Both yellow wires should be wired according to the wiring diagram, from the standby switch and to the 1N5408 diodes.

You should take another wire (white if you got) and solder together with one of the red wires on the standby switch. Make this (white) wire 2-3" longer than the yellow ones. You can also twist the (white) wire together with the yellow, since they all run to the same area of the board.

Should look something like this:
View attachment 157346View attachment 157347

So obviously I will wire it up much prettier than this, but that's the basic idea right?

 

[Dr Nasty]

Exactly! You need to move the 160k resistor from where it's placed in the wiring diagram.

Put the 160k resistor and 1N4007 diode together as in the picture below (silver band on diode facing the resistor). Twist and solder the leads, if you got shrink sleeve you can put that over this joint.

Then you put this assembly where the diode is shown in the wiring diagram.

The (white) wire from standby switch connects to 160k resistor.

View attachment 157401

Literally just posted pretty much exactly this right before I read this response! Thank you so much for all your help!

 

[BlueX]

So obviously I will wire it up much prettier than this, but that's the basic idea right?

That looks correct.

There will be +440 V after the 1N5408 diodes and -40 V after the 1N4007. That's almost 500 V difference. You need to make sure the leads are well separated and cannot touch each other.

Edit: Also, good idea to put the bias caps in place before soldering the parts so you know everything will fit.

 

[Dr Nasty]

Actually finished up the build today.

Kept saying "I'll just finish up the turret board... may as well wire up the pre amp section.... may as well finish up the power amp..." and so on and so on until everything was finished and I realized I hadn't eaten or drank water all day.

Couple of snafus that I fixed pretty quick. Not all of the wiring colors match up with the diagram because I ran out of some. And on one of them I just wired it to the wrong turret and was too lazy to switch it so I just swapped the colors and made sure to label which lugs they were supposed to go to.

Have triple checked everything. Tomorrow I'll go to the hardware store and get everything to build a light bulb limiter so I can test her out

 

[Dr Nasty]

Built my lightbulb limiter today. Decided to add a switch to it so I could cut power quick if needed.

I'm 99% sure I wired it up right but included pictures just in case.

Plugs Hot wire to switch, neutral wire to neutral of the outlet, and ground wire to ground of the switch .

Light bulb socket hot wire attached to the opposite side of the switch so it feeds 120v to the lightbulb when closed. Neutral wire of bulb sock to hot side of the outlet so it should just feed 120v straight through the bulb.


If any of that doesn't sound right please let me know

 

[Chris-in-LA]

As long as it’s wired up like Uncle Doug says, you’re good to go. You don’t need the switch though. You just use the power switch on the amp. Turn the amp off immediately if you have a problem.