mojotone 18watt tremolo build problems

[the1hub]

Hello all,

First amp build so I have no doubt I have screwed something up.

mojotone 18 watt tremolo kit. no offical schematic but i have attached one that i have been working with.

the amp is dead silent with the volume turned down on both channels. turning up the normal channel sounds like it should. a little bit of white noise. the normal channel sounds really good. the tone on the normal channel has a good range.

start the problems.

#1 the tremolo channel...

first off with nothing plugged in just turning up the volume adds 120hz hum. I am pretty sure it has to do with a bad ground somewhere in the tremolo channel. with the volume up on the tremolo channel the chassis becomes microphonic. I can hear most mechanical noise coming out of the speaker.

For trouble shooting:
Rewired all the grounds with no effect.
Rewired all the leads from the turret board to the pots and tube sockets.
Verified twice that all the wiring is correct from the wiring diagram mojotone sent with the kit with my multi meter. (no schematic.... )
Tried multiple routes for the lead from the input jack to V3 pin 2
Tried shielded cable for the lead from the input jack to V3 pin 2
Rewired the heater power supply cables with a tighter twist, no effect.

#2 also the tremolo channel

the tremolo worked before all the rewiring... after the rewire it no longer tremolos. It does add some light gain in the low frequency when the intensity knob is turned up.

Made sure all the wiring is going to the right places...no idea where to start with this one since it was working before.

#3 Again the Tremolo channel

sounds thin compared to the normal channel. I suspect this is related to the grounding problem.

verified cap and resistor values.

I have read threads with similar problems and haven't had much luck

any help is appreciated.

Thanks

[Daviedawg]

My immediate reaction hearing microphonic is that there is a bad joint and/or a short somewhere in the trem channel preamp. The fact that the volume affects it may help to identify where it is. They can be hard to find. Tapping to find the worst response may get a result. It is possible that one issue creates all your faults.

Shorts cause by "whiskers" at the valve bases are a bit of a plague. That could be a cause. But most likely is a cold joint which looks ok but is not sound.

As always great care is required due to the voltages.

Dd

[geoff 1965]

hello,
is the negative side of the 16uf 475V electrolytic on the board soldered to the turret?
listen to Dd and be very careful,have you got a current limiter and capacitor dissipater? they are cheap and easy to make,see "uncle doug" on you tube.
good luck,i'm a newbie like you and have just built my first amp and the guys on the forum have been very helpful and know their onion's!
found this layout for the mojotone,looks like he's swapped the 32/32 voltage dropper for a choke but it might be of use to you.

MojoTone_18Watt_Layout.JPG

[the1hub]

Realized I left out the voltage chart.

@geoff. yes that capacitor is soldered to the board. also i have a dim bulb tester, a bleed resister on alligator clips and a Variac so i am covered for most of the safety side of testing.

[JMPGuitars]

My first suggestion is to ditch that schematic you're using and try and compare your build and layout to the schematic in our download section, hopefully it's not too far off. I only glanced at what you posted, but a couple things I saw looked...off. Ian's schematic is based on the original (without improvements), and there's some updated versions in there too.

Second, their ground scheme isn't going to help with noise. The preamp and power amp should have separate ground connections. The preamp ground should be short, and at its own connection to the chassis somewhere near the inputs.

The ground bus you put in - are all the turrets it's near ground connections? If not, it's too close. If they are all ground connections, it's too far.

This layout is different than yours, but it will give you some hints regarding where to use shielded wire, etc.. (it also has improvements made to the tremolo): http://18watt.com/storage/18watt_layout ... ents_b.pdf

V1 voltages look ok, maybe slightly low.

V2 looks a little high. Measure your resistance to ground (connect to ground somewhere off the board like near the PT) on pins 2,3,7,8. My guess is you'll find a bad connection going towards the ground bus wire which is throwing off the voltages.

V3 pin 6 looks high when compared to http://18watt.com/storage/Voltage-Chart.htm - try swapping tubes and testing the voltage again. If it's still high, check/reflow as necessary all the connections coming off V3 starting with Pin 2 and Pin 3 following the path to ground. Test that, and if still an issue, go through the rest of the connections to V3 starting with Pin 6 and verify the R values again while going over the solder connections. Or do the lazy way and reflow everything going to that tube again all at once.

What are your voltages on pin 2 and pin 3 of V4 and V5?

Finally, make sure to try different tubes in your tremolo position. They're very sensitive to what tube is in there.

[the1hub]

....
Second, their ground scheme isn't going to help with noise. The preamp and power amp should have separate ground connections. The preamp ground should be short, and at its own connection to the chassis somewhere near the inputs.

The closest point to mount a lug to ground the inputs to would be under the nut for the OT. Is that a bad idea? Would you recommend drilling a new hole closer the the input channel?

I wondered about this given that eventually everything would be contacting the chassis at some point any ways. Why does it would matter if it was a star ground or not. I read about the distributed star ground scheme from this book (http://www.tactec.biz/Bookhome.html) I talked it over with a friend who is an electrical engineering professor and he agreed with the book that it would be the best way to keep the ground potential the same. Granted my friend has never built a guitar amp but he does have a lot of experience with very large music systems. Its very likely that i misunderstood him

The way it works now is the grounds for the inputs, pots and preamp sections of the turret board are collected at one point and then attached over one cable to the star ground. The grounds for the trem section match the diagram where its brought to the negative terminal of filter cap and then tied from there to the ground. The PT, OT, filter cap, AC main, speaker jack and the ground coming from the power amp section of the turret board all have their own connection to ground. This is very much a learning experience for me so i really would love to know what is wrong with that and why the other way would be better

The ground bus you put in - are all the turrets it's near ground connections? If not, it's too close. If they are all ground connections, it's too far.

I will post a picture to better show it. It is basically 1 inch above the top of the turrets and 1 inch off the back of the tone pots.

What are your voltages on pin 2 and pin 3 of V4 and V5?

It will take some time to get those. I tore the amp down and ordered new tube sockets. I felt the sockets it came with were too flimsy even after only being soldered to one or two times.

Finally, make sure to try different tubes in your tremolo position. They're very sensitive to what tube is in there.

Interesting thing there is the original tube that was in V3 when the tremolo was working was the same one that was in it when it stopped working. I have since labeled the tubes so i can pay more attention to it.

Thanks for the input

[JMPGuitars]

The closest point to mount a lug to ground the inputs to would be under the nut for the OT. Is that a bad idea? Would you recommend drilling a new hole closer the the input channel?

Mounting nuts are for mounting. You're correct, drilling a new hole is the right answer.

I wondered about this given that eventually everything would be contacting the chassis at some point any ways. Why does it would matter if it was a star ground or not. I read about the distributed star ground scheme from this book (http://www.tactec.biz/Bookhome.html) I talked it over with a friend who is an electrical engineering professor and he agreed with the book that it would be the best way to keep the ground potential the same. Granted my friend has never built a guitar amp but he does have a lot of experience with very large music systems. Its very likely that i misunderstood him

The way it works now is the grounds for the inputs, pots and preamp sections of the turret board are collected at one point and then attached over one cable to the star ground. The grounds for the trem section match the diagram where its brought to the negative terminal of filter cap and then tied from there to the ground. The PT, OT, filter cap, AC main, speaker jack and the ground coming from the power amp section of the turret board all have their own connection to ground. This is very much a learning experience for me so i really would love to know what is wrong with that and why the other way would be better

Star grounds are good, but not everything can go on them. If you connect 100% of your grounds there, you actually add to your noise. An easy demonstration of this is to turn up the volume and gain all the way, there's a good chance there will be a high-pitched squeal caused by the star ground.

For CE standards the AC main absolutely needs an independent connection to ground. I usually drill a hole within an inch or two of the AC receptacle.

The PT coil taps, and the power amp grounds (filter caps, cathode bias cap and resistor, etc... - everything on the power amp section of the board) definitely go together, and you can add the cap can there too.

The OT should only be connected to the speaker jack. Whether or not the speaker jack is also connected to ground is optional. It's required in amps with an NFB (negative feedback loop), but since these 18 watters generally don't have NFB, it's optional. This could be an entire other topic of conversation, but for now, go ahead and connect the output jack to ground. Whether or not you go to the star ground is optional (I usually don't). Your star ground seems like a long enough distance to me that I would probably drill a hole just for the output ground much closer to the speaker jacks.

Connect the pots and inputs to the preamp bus, and give them all a single new connection to ground near the input jacks. Just be careful while drilling, you don't want any of the chassis debris remaining in the amp and causing potential issues.

Running the ground scheme like this also helps reduce radio interference and other unwanted noise. But I'm not a real-life electrical engineer, so I can't really get much more in depth on the explanation than that. There are some proper engineers on here that can give you that info if you want.

I will post a picture to better show it. It is basically 1 inch above the top of the turrets and 1 inch off the back of the tone pots.

Ah, ok. That makes a little more sense. Since you only posted one photo, some things looked weird, but now that you say that, that green wire's perspective angle makes more sense going to the bus.

It will take some time to get those. I tore the amp down and ordered new tube sockets. I felt the sockets it came with were too flimsy even after only being soldered to one or two times.

What did you order instead? Personally I like the Belton "micalex" (aka fancy plastic) sockets.

Interesting thing there is the original tube that was in V3 when the tremolo was working was the same one that was in it when it stopped working. I have since labeled the tubes so i can pay more attention to it.

Yeah, I doubt it was the tube in this case, but I figured I should mention it for when you get it working. The tremolo channels can be very finicky with tube choices.

Hopefully things work out when you get it wired up with better sockets. Before you do though, try looking through the schematics in the downloads section and see how things compare.

[the1hub]

update:

Trem is back working. sounds great.
Trem channel still hums but the hum is slightly less.
microphonic problem solved. bad cable.

replaced a couple of tube sockets with belden... so much better to work on. Toss the one that come with the mojo kit, complete garbage.

updated voltage chart. V2 is still hot

[Daviedawg]

Comparing with my own chart in an amp which has been running for over 5 years, your V1 and V2 look right. But V3 voltages don't look like mine. With the trem off I have 189v on pin1, 145v on pin6. V4 and 5 are in the right area.

Dd

[the1hub]

Comparing with my own chart in an amp which has been running for over 5 years, your V1 and V2 look right. But V3 voltages don't look like mine. With the trem off I have 189v on pin1, 145v on pin6. V4 and 5 are in the right area.

Dd

189 vs 190 is hardly a huge difference. Pin 6 is probably a typo thanks for pointing that out. I will re measure it.

Curious about where you grounded your preamp section including your pots. I switched it from a star ground to a buss ground schem and my amp is slightly noisier on the normal channel. The tremolo channel still has a major ground hum. I have traced the schematic and the layout with a highlighter and everything is where it should be. Volume down on the tremolo channel minimal noise. Volume up on the tremolo channel obvious ground hum. It's driving me nuts. I am about to give up on it.

[the1hub]

updated the voltage chart. thanks for catching that DD

[Daviedawg]

It's more than 5 years. But my recollection is that I have the mains input ground by itself. Power supply ground, amp board ground, input ground which has the screen from the shielded cable as well. The two big ones could be called star grounds as they collect several points. The amp board ground includes the pots. The last earth run on the board for the power valve stage goes to the power supply ground.

I have not suffered from hum after initial fault finding. There is some hiss at high vols which could be my carbon comps.

Dd

[the1hub]

Success!

After rereading Aiken's website I had an ah ha moment. Turns out that the layout that mojotone is fundamentally flawed. I should have cought it myself given how much thought I put in to the grounds. The intensity and speed pots were tied to the negative terminal on the can cap and then on to ground. Looking at richies improved tremolo layout had them going to the buss ground. I grounded them similar to richies and surprise no more hum. I knew better but placed to much trust In mojotones layout.

Thanks for the help

[Daviedawg]

Good news. It is satisfying sorting an issue.

Dd

[roadshow]

Same here,

Followed the schematic and layout I got from MT, and had motorboat issues due to layout per my other post.

Changing the tremolo pot setup from MT to Richie's also cleaned up a lot of weird issues with the tremolo. Still not perfect, but much improved.

[Daviedawg]

You will be pleased to have made progress. You can trust the information in the downloads section.

Dd