mojotone 18watt tremolo 2019

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geoff 1965
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Re: mojotone 18watt tremolo 2019

Post by 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?
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Re: mojotone 18watt tremolo 2019

Post by 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.
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Re: mojotone 18watt tremolo 2019

Post by 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?
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Re: mojotone 18watt tremolo 2019

Post by JMPGuitars »

jlee69 wrote:
Mon 10/14/19 5:20 pm
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.
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Re: mojotone 18watt tremolo 2019

Post by 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 :).
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Re: mojotone 18watt tremolo 2019

Post by colossal »

jlee69 wrote:
Mon 10/14/19 5:14 pm
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.
Ah, you are probably right about that being the trem speed. My mistake. I didn't look too closely and thought it might be a TMB layout or something.

On the rectifier tap, no, you just want to use the tap that is suited for your rectifier. In this case use the 6.3VAC and 0VAC taps. Then heatshrink the end of the 5VAC tap and zip tie it back or terminate it somewhere safe.

That's good you got the OT leads oriented correctly. As Josh said, now it's time to clean up your ground scheme. Carefully check each turret for a good soldered connection while you are hard at it.
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Re: mojotone 18watt tremolo 2019

Post by geoff 1965 »

have a close look at the wiring to the V1 socket,is there a whisker of wire coming from pin2? also check the jumpers for the plate & cathode are'nt touching.
that was a good pointer from colossal with the OT leads,i don't have tremelo but that is one to remember!
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Re: mojotone 18watt tremolo 2019

Post by jlee69 »

just a quick update, things improved a little more last night. I found the squeal happens when i turn the tone pot on normal channel all the way down. I'll post a pic tonight, but i think there is some loop created when the tone is all the way off. If i open the tone knob a bit the squeal goes away, amp sounds great. Does the layout look right? (btw i have a bus bar now so the ground is no longer on the back of the pots)
pots.PNG
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Re: mojotone 18watt tremolo 2019

Post by jlee69 »

I'm going to try this:
Graydon wrote:
Mon 10/19/15 12:02 pm
From my FAQ, you may also want to check this issue.

Q: The normal channel on my amp squeals when the tone control is on zero. What is wrong?

A: This is a well known problem with the grounding issue in the 18watt. Oddly enough, if you use the technically inferior grounding scheme of the original 18 watter, you will not hear this squeal. If you use a floating ground buss which is connected to the chassis at one point (a star ground) you will likely have this issue. The fix is to take the wire that goes from your tone pot to your ground buss and disconnect it from the ground buss. Then attach it to the chassis somewhere near the tone pot, giving that pot its own separate ground. This should stop the squeal.
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Re: mojotone 18watt tremolo 2019

Post by JMPGuitars »

jlee69 wrote:
Thu 10/17/19 10:17 am
I'm going to try this:
Graydon wrote:
Mon 10/19/15 12:02 pm
From my FAQ, you may also want to check this issue.

Q: The normal channel on my amp squeals when the tone control is on zero. What is wrong?

A: This is a well known problem with the grounding issue in the 18watt. Oddly enough, if you use the technically inferior grounding scheme of the original 18 watter, you will not hear this squeal. If you use a floating ground buss which is connected to the chassis at one point (a star ground) you will likely have this issue. The fix is to take the wire that goes from your tone pot to your ground buss and disconnect it from the ground buss. Then attach it to the chassis somewhere near the tone pot, giving that pot its own separate ground. This should stop the squeal.
That issue is covered in the modern ground scheme thread (though it isn't mentioned): viewtopic.php?f=4&t=25372

The preamp and power amp ground busses should be separate. The power amp buss wire connects to star ground. The preamp buss connects to ground near the input jack, along with the input jack. The way I do it is to connect the buss to the input jack ground lug, and then ground the input jack as close to the jack as possible (it's in my linked layout in that thread).

Post some photos of your updated build.

Thanks,
Josh
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Re: mojotone 18watt tremolo 2019

Post by jlee69 »

moving the tone pot ground from the bus bar to farther away got rid of squeal when tone all the way off. the amp sounds awesome, quiet. Thanks to you guys for steering me in the right direction.

I noticed that the speed on trem doesnt vary the speed very much, ill look into that next. i just ordered some new wire and ground lugs to tidy everything up in the right place and redo the heater wires.

best regards!
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Re: mojotone 18watt tremolo 2019

Post by JMPGuitars »

jlee69 wrote:
Thu 10/17/19 5:46 pm
i just ordered some new wire and ground lugs to tidy everything up in the right place and redo the heater wires.
Do you know the power drill trick for heater wires? Clamp one end of the wires to your bench, and place the other end of the wires in the drill and tighten the chuck. Run the drill and it gives really nice twists. Don't over do it though.
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Re: mojotone 18watt tremolo 2019

Post by jlee69 »

i'm aware of that trick, you wouldn't know it looking at my build though :). i did that with one long piece for heaters and then cut them but they unraveled as i attached clumsily to the pins.

i'm also trimming the bus bar back towards the preamp because it doesnt need to span over close to the power side when there are no grounds attached.

Is the trem speed control supposed to be very limited change in speed? the difference between min and max speed is very small. maybe different cap values to change min/max speed?
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Re: mojotone 18watt tremolo 2019

Post by JMPGuitars »

jlee69 wrote:
Fri 10/18/19 7:49 am
i'm aware of that trick, you wouldn't know it looking at my build though :). i did that with one long piece for heaters and then cut them but they unraveled as i attached clumsily to the pins.

i'm also trimming the bus bar back towards the preamp because it doesnt need to span over close to the power side when there are no grounds attached.

Is the trem speed control supposed to be very limited change in speed? the difference between min and max speed is very small. maybe different cap values to change min/max speed?
After you solder one side of the heaters, re-twist anything that become untwisted in the process, as tight as you can, and as close as you can up to the solder tabs.

Good, reducing the unused length means it will be less of antenna.

I'm not a tremolo guy, so I can't help much there. But I will say that 1. tremolo performance is heavily influenced by the tube being used; and 2. if the performance isn't what it should be, highlighter test that section to verify, and also compare it to the improved layout in our downloads section.

Thanks,
Josh
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