50 watt Plexi build troubleshooting

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Kegger87
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50 watt Plexi build troubleshooting

Post by Kegger87 »

Hello all,

I have completed wiring up a Modulus 50 watt plexi and have the amp brought up to voltage and biased. Moving on to testing output, I'm not getting a signal from the input. The amp will pass noise from a slightly scratchy presence knob, and did make an audio pop switching off the standby. There is no hum or unusual noises. This is my second amp I've built, but my first Marshall circuit (first was a successful Fender 5e3 project). The fuses are good and all tubes light up. I'm still fairly new to troubleshooting circuits but I do have a good oscilloscope and proper test equipment. I am aware of the dangers and m extremely safety conscious with the Hv in tube amplifiers so I'd rather sort this out with a little guidance from the experts here. Any help is appreciated!

Thank you,
Craig
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mac dillard
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Re: 50 watt Plexi build troubleshooting

Post by mac dillard »

go here: https://www.diystompboxes.com/pedals/debug.html
build the audio probe. you can connect it to a cd player and use it as a signal injector. start injecting the signal at the input and work your way back up the circuit until you have output. that is where your problem is. It works.
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Daviedawg
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Re: 50 watt Plexi build troubleshooting

Post by Daviedawg »

Good advice from Mac. But first make sure you have wired the input sockets correctly.

Dd
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mac dillard
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Re: 50 watt Plexi build troubleshooting

Post by mac dillard »

most of the time it is a miss wired input socket.
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Bieworm
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Re: 50 watt Plexi build troubleshooting

Post by Bieworm »

It is normal for the presence pot to be a little scratchy. There is always some DC on the pot due to its position in the circuit. I wouldn’t worry about this when it’s only noisy while turning the knob.
50 watt or higher amps often pop when switching the standby switch. There are often large filter caps connected to it that contain a lot of juice… not to worry about either. Big Marshall amps have lived for many decades doing this exact thing
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mac dillard
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Re: 50 watt Plexi build troubleshooting

Post by mac dillard »

a good fix is to just not have the standby switch. Also see recommendations of a resistor and cap across the switch.
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Bieworm
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Re: 50 watt Plexi build troubleshooting

Post by Bieworm »

mac dillard wrote:
Sun 07/30/23 9:11 am
a good fix is to just not have the standby switch. Also see recommendations of a resistor and cap across the switch.
Or simply just don’t use it.
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Kegger87
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Re: 50 watt Plexi build troubleshooting

Post by Kegger87 »

I mentioned the scratchiness and pop just to say that I was getting noise from the speaker, they aren't actual concerns I'm looking to fix. I'm guessing the problem is going to be with the input jack wiring. I scoured the internet and can't find a clear photo or diagram how the 4 cliff jacks should be wired. Does anyone have this?
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mac dillard
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Re: 50 watt Plexi build troubleshooting

Post by mac dillard »

give this a try and see if it helps:https://robrobinette.com/How_Fender_Inp ... s_Work.htm

It also has the Marshall style jacks.
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Re: 50 watt Plexi build troubleshooting

Post by Bieworm »

Here’s a link where they show cliff jacks

https://www.tube-town.net/cms/userfiles ... -v2021.pdf
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Unit_1
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Re: 50 watt Plexi build troubleshooting

Post by Unit_1 »

Kegger87 wrote:
Sun 07/30/23 12:00 am
Hello all,

I have completed wiring up a Modulus 50 watt plexi and have the amp brought up to voltage and biased. Moving on to testing output, I'm not getting a signal from the input. The amp will pass noise from a slightly scratchy presence knob, and did make an audio pop switching off the standby. There is no hum or unusual noises. This is my second amp I've built, but my first Marshall circuit (first was a successful Fender 5e3 project). The fuses are good and all tubes light up. I'm still fairly new to troubleshooting circuits but I do have a good oscilloscope and proper test equipment. I am aware of the dangers and m extremely safety conscious with the Hv in tube amplifiers so I'd rather sort this out with a little guidance from the experts here. Any help is appreciated!

Thank you,
Craig
The standard JTM50 standby has a defect that was corrected in later models (not sure if yours does or not), what is called a hot standby because it shuts down the bias. It was also without the scrubber caps which help to suppress the pop.

It's easy to jumper to correct. Also the presence is allowing DC to get in, but that is also easy to fix.

See the layout below. You'll be redirected to a image site, and then click on that for a higher res picture. This is a layout I made for an amp I had planned earlier this year but I got covid, again, instead. Feeling better....

(edited: actually there's a 'download original image' tab at the top to get the highest res image)

Image
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Kegger87
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Re: 50 watt Plexi build troubleshooting

Post by Kegger87 »

Just an update for everyone, I pulled a total newbie mistake and didn't add the pin 4 to 5 heater jumper on the 12ax7 sockets. After putting on the jumpers I had audio but in oscillation. Reversing the output transformer leads to the power tube sockets fixed that and now the amp is up and running! I still need to work on tidying up the wiring but the amp is quiet and seems to work and sound great at 70% bias. Now that it has a few hours on it I'm going to swap out the JJ's for a pair of vintage Mullards, improve some grounds while I tidy up wiring, and prepare the garage for cabinet construction. The plan is to build the head box and a matching 4x12 loaded with UK greenbacks, and cover in white with gold piping. I've already acquired all of the parts and material, I just need to figure out the best way to use my hand router for the box joints. I'm having a great time learning and building amplifiers, and this half stack will definitely not be my last! Thanks everyone for your help!
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