Diagnose Static/Egg Frying in Trinity 18 Watt Plexi MkII

[perplexd]

Thanks to all for your feedback! I will start by reworking the solder joints and go from there.
@JMPGuitars Thank you for the soldering references. I chose to put the components in the turret holes because that was how my Hiwatts were constructed, however I don't think that I would do it this way again. I quickly realized the challenges of soldering on both the top and outside of the turret without reflowing one or the other.

Jason

[JMPGuitars]

@JMPGuitars Thank you for the soldering references. I chose to put the components in the turret holes because that was how my Hiwatts were constructed, however I don't think that I would do it this way again. I quickly realized the challenges of soldering on both the top and outside of the turret without reflowing one or the other.

Jason

While I obviously prefer the outside of the turret, the tops of the turrets can be used for perfectly fine solder joints. It simply takes practicing the right way, with the right heat, in the right place. The soldering videos give great examples of what solder joints should look like, how to apply heat and solder, etc.

Don't feel bad about the state of your soldering. Everybody starts somewhere, and all of us can improve. I still go back occasionally and rewatch those soldering videos too. There's a plethora of members on here that can show you their old (and scary) soldering, and the world of difference compared to what they produce now.

Thanks,
Josh

[perplexd]

I was able to get into it tonight and retouched some of the joints around V2 and tone stack and voila, it's fixed! I plan to go through and make sure everything is looking good and retest before I button it back up. Thank you!

[perplexd]

OK, another question about the volume control on Channel 2. It is fine up to ~noon with a quiet shhh sound, then just past halfway, at a very specific point, a hum is introduced and then at 3 O'clock the hum drops off. You can also hear scratchiness in the wiper through this section especially when moved quickly. The shhh gets louder from this 3 O'clock position up to max. Ideas?

[JMPGuitars]

Scratchiness on pots often means you have DC voltage leaking to the pots. The sectional behavior could also be related to a parasitic oscillation. Check for DC voltage on your pots, check that the capacitors aren't leaking, and if that all passes, you can try chopsticking to isolate/solve the issue.

[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?

[JMPGuitars]

Do you own an oscilloscope?

[perplexd]

Do you own an oscilloscope?

No, unfortunately not.

[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?

[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

[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?



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



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?

[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?

[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.

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.

[JMPGuitars]

Did you install R34?

[perplexd]

Did you install R34?

I did not.

[JMPGuitars]

Did you install R34?

I did not.

Try it and see if it affects your issue.

[perplexd]

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.

[JMPGuitars]

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.

[perplexd]

I soldered the 220k in now the hum is back. Ugh.

[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?