18 watter 19 years later

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higbean
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18 watter 19 years later

Post by higbean »

Hey fellas. I built an 18 watter from (almost) the ground up in 2004. Chassis, turret board and cab. I liked it but the cab I built was too heavy and the Blue/Heritage combo too efficient. I decided to rewire it in a Mojotone chassis and put it in a 1x12. Put it aside for about a year and got off my duff last week to finish it up.

Upon firing it up I have a few issues I was hoping you guys could put some eyes on. Go easy on me as I took about 19 years off guitar to raise a family and I'm just getting back to gigging a bit. :D

So, the trem channel is working fine and sounds really great. The normal channel is what is giving me fits.

With a guitar plugged in with volume on 0 it sounds like the volume is on 10. The volume pot seems to do nothing. It also makes a steady thump in time with the trem settings that I can dial out with the speed and intensity knobs.

I've chopsticked this thing half to death. LOL

I took some readings and used JMPGuitars 18watt Modern Classic test sheet. Everything looks in line except V2 shows 228vdc on pin 1 and 227vdc on pin 6 and V3 was reading 121vdc on pin 1 and 195 on 6.

I used the Mojotone Layout/schematic

https://www.mojotone.com/core/media/med ... 4839db204f

https://www.mojotone.com/Amp_Kits/Briti ... 8W_SCH.pdf

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Bieworm
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Re: 18 watter 19 years later

Post by Bieworm »

Watch out using the voltages test sheet of the modern classic. On your layout V2 is the PI , but on the modern classic V3 is the PI. You have to fill in your V2 values at V3 on the sheet and vice versa for V3 on your layout

About the volume problem on the normal channel.. check the ground connection on the volume pot. I think I don't see your ground connection on the middle lug of the tone pot. That layout stinks. The grounding scheme is not our standard! We don't solder to the back of the pots, but if we do, always turn the pot all the way open to avoid burning the carbon strip in a spot where it's very annoying.
I advise you to rewire the normal channel like the modern classic. It’s different and you’ll probably like it better. But more importantly, the docs from JMPguitars are correct and have proven to work flawlessly

About the tremolo thumping.. it often is a lead dress issue and/or tube specific problem. You will notice the thumping might decrease by moving some dedicated wires and trying several different tubes. It could also be a crappy solder joint too. Roll up your sleeves and chopstick it beyond death :D
classic normal channel pot grounding.png
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"THIS should be played at high volume..preferably in a residential area"

higbean
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Re: 18 watter 19 years later

Post by higbean »

Oh man! I think you nailed it! That pot is 100% not grounded.

I'll look at wiring it like the modern classic for sure. Cusrious what it changes tone wise?

Thanks again!
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Bieworm
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Re: 18 watter 19 years later

Post by Bieworm »

higbean wrote:
Wed 09/13/23 7:42 am
Oh man! I think you nailed it! That pot is 100% not grounded.

I'll look at wiring it like the modern classic for sure. Cusrious what it changes tone wise?

Thanks again!
It is sonically the same tone control as the tremolo channel. Caps instead of the tone sucking resistor. Maybe brighter? I don’t remember actually
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"THIS should be played at high volume..preferably in a residential area"

higbean
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Re: 18 watter 19 years later

Post by higbean »

Ok fellas, it's been a long week of messing with this thing. LOL

I've got the volume pot on channel 1 working and most of the noise is gone, but the trem is still thumping a bit. If I put the Tone knob on the Trem channel on 10 it stops.

Any thoughts?
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higbean
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Re: 18 watter 19 years later

Post by higbean »

Nevermind guys. I got her dialed in. The ground from the footswitch was the culprit.

Moved to a better spot.

I'd like to just disable the trem. I thought grounding out the two wires would do it, but the amp sounds like a Tsunami warning siren when you do that.

Any tips?
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JMPGuitars
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Re: 18 watter 19 years later

Post by JMPGuitars »

Take a look at my layout for the Trem TMB: files/JMPGuitars_18_Watt_Tremolo_TMB_Re ... Layout.pdf

You can see there how to properly wire the jack, and have a switch to enable/disable the tremolo.

Thanks,
Josh
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