mojotone 18watt tremolo 2019

[jlee69]

Hi all,
I am trying to troubleshoot a new 18watt build and wondering if anyone has had a successful build of this https://www.mojotone.com/kits/BritishAm ... fier-Kit_2

I have the 2019 schematic and it squeals like crazy with all tubes in and a speaker attached, no inputs and volume at zero. I started with just the rectifier and everything seemed correct with b+ and pin3 of rectifier. I suspect the input jack wiring, especially since it is different from anything i've seen on this site. Any ideas? Here are some pics... Thanks in advance for any pointers.

[geoff 1965]

hello,
pins 2,6,8 & 9 of an ez81 are internal connections so why is the mojotone layout showing the 5 volt heater supply connected to pin 6?

[JMPGuitars]

At first glance, there's at least 100 things I hate about their layout, though the jacks are not really one of them assuming the connection are made on the correct sides.

The first thing that stands out from your build is that the heater wires are too loose, and should be wound tighter all the way to the sockets.

Heater wiring example:


Going beyond that, you need to verify the layout and schematic with a highlighter as you go through your build. Start with the input jacks, make sure everything is connected where it should be, and the values for each component are correct. Once verified, highlight, and move on to the next component or connection.

Thanks,
Josh

[colossal]

I agree that lead dress, or, the manner in which the wires are laid down is important to avoid unwanted field interaction and oscillation. For an otherwise good build however that squeals right out of the box, try reversing the high voltage leads on your output transformer going to each power tube. You get a 50/50 shot at being in phase the first time. I do also agree at going through the whole amp and carefully verifying assumptions and voltages, not potshotting around hoping that the last change you make is the right one, is the right way to troubleshoot. My gut says your OT leads might be reversed though.

[jlee69]

@geoff1965 - idk why pin 6 but i looked around and for ez81 pin 6 is NC which i assume translates into terminated, but maybe better to terminate it on the lug strip. Is there another tube that could be substituted which uses pin6?

@jmpguitars - thanks for the honest critique, i can see how sloppy my work is compared to a good example. i will go back and redo the heater wires. i would be interested to know what else you think is wrong with the mojo layout. I wondered about the input jacks because mostly i see the resistor across the tip, not between shaft and tip.

@colossal - you mean i should swap blue and brown wires between the power tubes to change polarity?

Thanks alot for the pointers, very appreciated. I combed through this build many times and followed the highlighter rule I read here, then I started questioning whether the layout was correct. I'd like to get to the point where i can not be a slave to the schematic.

[geoff 1965]

the yellow/white 5 volt wire is to give you the option of using a rectifier like an GZ34 and if you look at the schematic in your link it's not connected,i would remove it from pin 6 and wire it to an isolated lug or seal the end with heatshrink.

18 Watt PT.png

[jlee69]

hi Geoff,
The mojo schematic i linked shows the yellow/white connected to pin 6. Is it possible that it was done to accommodate either ez81 and gz34? I agree the safe thing is to take it off, unless it is not a problem and allows use of either rectifier.

[geoff 1965]

double check! it's not connected on the schematic but the layout shows it connected to pin 6?
the GZ34 is an octal tube so won't fit the EZ81 socket.

[jlee69]

that's what i meant . layout != schematic. I understand now, it needs to be removed for my configuration of ez81. thanks.

on the OT polarity issue - is that always trial and error as to what the tubes want to see?

[JMPGuitars]

@jmpguitars - thanks for the honest critique, i can see how sloppy my work is compared to a good example. i will go back and redo the heater wires. i would be interested to know what else you think is wrong with the mojo layout. I wondered about the input jacks because mostly i see the resistor across the tip, not between shaft and tip.

@colossal - you mean i should swap blue and brown wires between the power tubes to change polarity?

The mojo layout appears to me that they designed it specifically to be different than the standard layouts, with no actual benefits from the design changes. Instead they have unnecessarily long leads to compensate for those changes. It doesn't make sense.

If you look at most of the layouts and schematics here, you can see that they're designed in visible sections, where each section has a purpose. Each section lives generally near where it will be connected, and can be directly adjusted without confusion from the layout sloppiness.

The mojotone layout looks like the streets of Boston compared to the streets of New York. One's a clean grid, one's a plate of spaghetti. In case I'm not being clear enough, mojo's looks like the pasta.

Then they have a bad ground scheme, and ground loops on the backs of the pots. AAAAAND then they drew a long lead for the IEC ground, which is supposed to be as close to the IEC jack as possible. The terrible drawing also implies that they're using the PT mounting bolts for ground lugs, which is also a bad idea. IEC ground is supposed to be close the IEC jack and independent of anything else. Same with any other grounds; the lugs should be mounted to the chassis and serve no other purpose besides ground. Transformers vibrate, and nuts can get loose. Even if the nut is just slightly loose, it can cause ground issues or faults.

Don't get me wrong, I like some of the stuff Mojo sells, and they have great cabinets. But their layouts/schematics aren't good, and if you search this site for the amount of tech support we do for them for free, you'll see for yourself.

BTW- I recommend two sticky posts to learn more:
viewtopic.php?f=4&t=25396 <- soldering technique
viewtopic.php?f=4&t=25372 <- ground schemes

Thanks,
Josh

[jlee69]

JMPGuitars - that was a seriously comprehensive answer, not that I expected any different. But just thank you for the time you took to write it. I can see by your examples that you have a first-principles ethic and i agree with everything. I am learning as I go.

One of the many things i had trouble with this amp were finding reference points between the mojo and other layouts. It was difficult to see what parts of the board did what. One of the reasons they gave for the different layout was resistor lead lengths from their vendor were too short to span the turrets.

I will first swap ot leads to hopefully get rid of the squeal, then address the grounding scheme (assume back of pots is bad because if screws loosen then ground to chassis is lost). Power section grounding should be ok, its separate from pre except i should drill a hole rather than have it on the PT.
IEC ground can be moved closer and shortened. And I will redo the heater wires. Thanks again, your comments are very much appreciated!

[jlee69]

oh, and the pot scheme has ground loops too.

[JMPGuitars]

oh, and the pot scheme has ground loops too.

Correct, that's why the bus wire connecting the pots should be removed. Review the ground scheme post and see what works best for you using those guidelines.

[colossal]

that's what i meant . layout != schematic. I understand now, it needs to be removed for my configuration of ez81. thanks.

on the OT polarity issue - is that always trial and error as to what the tubes want to see?

You want to use the Yellow (6.3VAC) and White (0VAC) for your EZ81 rectifier. The Yellow/White is 5VAC and for other rectifiers, including the GZ34. Based on your photo, you are underpowering your EZ81.

Also, the yellow wire (negative feedback) running from the speaker jack back to your Presence pot....you might need to relocate that. That is a sensitive lead and you don't want it routed right next to all the AC (transformer and rectifier).

A couple of those capacitors (specifically that 47n) have taken a nick from the soldering iron. It happens. But you might need to see if they are ok or if they were damaged and replace if necessary.

And, yeah, Mojo should stay out of the amp business. Just stick to selling parts

[JMPGuitars]

Side note: negative feedback loops are not usually needed in these 18watt amps. If you decide to remove the NFB loop, you'd also be removing any issues, and tone change/loss that the NFB causes. The presence of the NFB loop does require your OT connections to the power tubes be correct, but without it, it doesn't matter so much. If you remove the negative feedback loop, you would connect the .01 cap on V2 the NFB was connected on to ground. You can compare to our schematics in the download section.

[geoff 1965]

Josh & Colossal,
send me some of what you two are smoking! i can't see an NFB loop or presence pot on this amp, do you mean the yellow wire to the footswitch?

[jlee69]

Hi Colossal,
The yellow wire in the layout going back to the trem speed pot is from the footswitch, not speaker jack. Are you sure this is NFB?

If i just remove and cap the yellow/white, am i still underpowering? i think that pin4 is getting 6.3V from the yellow attached to it.

I did swap the ot leads and the squeal stopped, but now just alot of noise but i am getting closer with your help.

[jlee69]

any idea why on the input jacks the resistors are going across tip instead of between tip and sleeve? should i just wire those like the schematic in the download section?

[JMPGuitars]

any idea why on the input jacks the resistors are going across tip instead of between tip and sleeve? should i just wire those like the schematic in the download section?

It's an ugly drawing, that's all. Since there's a wire connecting all three of those lugs, it doesn't matter where it goes. You can do it the prettier way we have it, as long as those lugs are all connected correctly.

Bad heater wiring can be a big contribution to noise,as can bad ground schemes. I would start there by cleaning the heater wiring up, and then go through the ground scheme corrections/updates.

[jlee69]

I re-did the ground scheme last night, rewired V6, removed the 5V from PT to rectifier and wrapped it. I plugged my guitar into channel 1 and got clean sound, no noise. Then i unplugged my guitar from the input and the amp started squealing. I swapped back the OT leads and still squeal so i think this is either grounding or the input jack contact isn't making contact when i pull out. I have to do more to isolate the issue.

It was late last night so i couldn't do too much risking waking up the family. I'll try to troubleshoot more tonight. But I think progress has been made .