Diagnose Static/Egg Frying in Trinity 18 Watt Plexi MkII

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Re: Diagnose Static/Egg Frying in Trinity 18 Watt Plexi MkII

Post by perplexd »

https://imgur.com/5bipYrY

In this one, all knobs at noon and I am bringing up the volume knob. You can hear the hum come in as I pass about noon, and then I roll it back and forth before turning it the rest of the way up and then down.

I looked for DC voltage after each of the tone circuit caps and the .002mF cap in front of the gain knob. On the wiper of the volume knob the most I saw was briefly ~20mV as I turned it but it always settled to basically zero. How much DC voltage are we talking about?
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Re: Diagnose Static/Egg Frying in Trinity 18 Watt Plexi MkII

Post by JMPGuitars »

Do you own an oscilloscope?
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Re: Diagnose Static/Egg Frying in Trinity 18 Watt Plexi MkII

Post by perplexd »

JMPGuitars wrote:
Tue 08/02/22 11:54 am
Do you own an oscilloscope?
No, unfortunately not.
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Re: Diagnose Static/Egg Frying in Trinity 18 Watt Plexi MkII

Post by perplexd »

Coming back to this, I've been reaching out to find someone with a scope with no luck so far. I am wondering if you think based on the last video I posted, that I should just go to an amp tech who can diagnose it?
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Re: Diagnose Static/Egg Frying in Trinity 18 Watt Plexi MkII

Post by JMPGuitars »

That would be the easy way, but what I said previously is still true. You can likely correct it chopsticking it. Place the volume knob into a noisy position, and start moving wires with a wooden chopstick, tapping components, etc. You may find some clues.

The other option is you can also buy an oscilloscope. There are some great scopes from Siglent in the $250 to $500 range that would work great for this kind of stuff. Buy it from somewhere with a good return policy, and you can decide to keep it or not later. ;) This is a good article on using a scope for audio testing if you go that route: http://www.r-type.org/articles/art-125.htm

Thanks,
Josh
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Re: Diagnose Static/Egg Frying in Trinity 18 Watt Plexi MkII

Post by perplexd »

So. I decided that it would be good to have an amp tech figure this and go through the amp to make sure everything was good, but I picked it up today and basically he couldn't figure it out. He was convinced it was related to a grounding issue and tried moving a few things around with little improvement. Now it is a little messy, so I will be cleaning it up. With that, I have a few questions. I read the grounding sticky and it seems that the layout follows that pretty close except for some of the power section is on the preamp buss. Should I modify this as shown in the redlines below?

Image

Also, should I reverse the input grounds so that Channel 1 goes to the buss and Channel 2 to the chassis?

Image

Finally, I hadn't done this since I cleared up the solder joint issue, but I just removed V1 and V2 and the hum behaves exactly the same way. I think this means that the issue lies somewhere from the tone stack to the phase inverter?
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Re: Diagnose Static/Egg Frying in Trinity 18 Watt Plexi MkII

Post by perplexd »

Hello again, I am back into this build as another issue has come up on Channel 2. The overall output level dropped and it has very little bass response, almost all mids.

I started walking through Channel 2 and noticed that I did not wire the input jacks exactly as shown. In the image I have crossed out the ground connection and added redlines where I ran buss wire through all 4 ground lugs. I cannot see how this would change the function, as I think it only adds a redundant connection to the switched ground lug, but I wanted to double check?

Image
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Re: Diagnose Static/Egg Frying in Trinity 18 Watt Plexi MkII

Post by perplexd »

Update: I have been very carefully through the amp, checking solder joints, cleaning up things etc. and it is mostly working very well now except for this lingering issue of a hum on the volume control of v2. I have quoted the post below with a video of the symptom. Again it does this with noting plugged in and even when no v1 or v2 are installed.
perplexd wrote:
Mon 08/01/22 6:57 pm
https://imgur.com/5bipYrY

In this one, all knobs at noon and I am bringing up the volume knob. You can hear the hum come in as I pass about noon, and then I roll it back and forth before turning it the rest of the way up and then down.

I looked for DC voltage after each of the tone circuit caps and the .002mF cap in front of the gain knob. On the wiper of the volume knob the most I saw was briefly ~20mV as I turned it but it always settled to basically zero. How much DC voltage are we talking about?
I have noticed that the v3 tube may be part of the equation. I had a Sovtek 12ax7LPS in there and when I swapped it with other short plate tubes the effect was quieter and in one case it shifted the start point from 50% up to about 60% of rotation.

Chopsticking, I have found that touching/tapping the coupling capacitors going into the phase inverter from each channel is audible through the speaker. Don't know if this is typical? I also noticed that when taking the voltage measurement going into v3, the hum stops and I get radio in the speaker for whatever thats worth.

Thanks for all of your help.
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Re: Diagnose Static/Egg Frying in Trinity 18 Watt Plexi MkII

Post by JMPGuitars »

Did you install R34?
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Re: Diagnose Static/Egg Frying in Trinity 18 Watt Plexi MkII

Post by perplexd »

JMPGuitars wrote:
Thu 02/16/23 5:18 pm
Did you install R34?
I did not.
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Re: Diagnose Static/Egg Frying in Trinity 18 Watt Plexi MkII

Post by JMPGuitars »

perplexd wrote:
Thu 02/16/23 5:26 pm
JMPGuitars wrote:
Thu 02/16/23 5:18 pm
Did you install R34?
I did not.
Try it and see if it affects your issue.
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Re: Diagnose Static/Egg Frying in Trinity 18 Watt Plexi MkII

Post by perplexd »

JMPGuitars wrote:
Thu 02/16/23 10:49 pm
perplexd wrote:
Thu 02/16/23 5:26 pm
JMPGuitars wrote:
Thu 02/16/23 5:18 pm
Did you install R34?
I did not.
Try it and see if it affects your issue.
OK, I put in R34 and the issue seems to be gone! Smooth all the way up the pot travel. Even with the long plate phase inverter tube.
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Re: Diagnose Static/Egg Frying in Trinity 18 Watt Plexi MkII

Post by JMPGuitars »

perplexd wrote:
Fri 02/17/23 11:29 am
OK, I put in R34 and the issue seems to be gone! Smooth all the way up the pot travel. Even with the long plate phase inverter tube.
Nice! I figured that would do the trick.
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Re: Diagnose Static/Egg Frying in Trinity 18 Watt Plexi MkII

Post by perplexd »

I soldered the 220k in now the hum is back. Ugh.
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Re: Diagnose Static/Egg Frying in Trinity 18 Watt Plexi MkII

Post by JMPGuitars »

I just listened to your clip. It sounds like you might have a parasite trying mess with you. With R34 in place, do you still get radio signal testing voltages?
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Re: Diagnose Static/Egg Frying in Trinity 18 Watt Plexi MkII

Post by perplexd »

I am not getting any radio signal tonight. I get about 53 volts on each coupling cap going into the phase inverter rather than the 52 and 62. I found that the voltage at the bias resistor drops about 4-5 volts as I swing the channel 2 volume control, it does not drop at all when I swing the v1 volume control
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Re: Diagnose Static/Egg Frying in Trinity 18 Watt Plexi MkII

Post by JMPGuitars »

perplexd wrote:
Fri 02/17/23 9:15 pm
I am not getting any radio signal tonight. I get about 53 volts on each coupling cap going into the phase inverter rather than the 52 and 62. I found that the voltage at the bias resistor drops about 4-5 volts as I swing the channel 2 volume control, it does not drop at all when I swing the v1 volume control
Ok, so it sounds like R34 at least helped a little.

My suggestion is to get the volume knob in a position that the issue is obvious. Then you can try chopsticking some wires and see if that has an affect on it. For example, see if moving that grid wire into different positions with the chopstick has any affect on the hum noise.
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Re: Diagnose Static/Egg Frying in Trinity 18 Watt Plexi MkII

Post by perplexd »

Okay I think we are on to something. I found that the yellow wire connected to Pin 2 of V3 affects the hum and was able to get it to go away by pulling it out away form the other wires. Checked the voltage on the bias resistor while while turning volume pot and it doesn't drop anymore. It finally sounds better when volume is at 6 or above, like more headroom. So maybe either a solder issue on the tube socket or lead dress?

BUT now I hear a high frequency ticking when the guitar is plugged in either channel. It stops when the guitar volume is all the way down or unplugged.
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Re: Diagnose Static/Egg Frying in Trinity 18 Watt Plexi MkII

Post by Daviedawg »

It could be guitar finding RF pickup from lights, a computer or related gear or even a watch. Move your guitar away from anything and switch off lights to see what happens.

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Re: Diagnose Static/Egg Frying in Trinity 18 Watt Plexi MkII

Post by Bieworm »

I was taking a new build nearly apart once… turned out to be the 20 foot Planet Waves cord I was using
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