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Hi Hankules,

I really appreciate your feedback, thank you! I did previously play with the lead dress to see if I could induce any changes and nothing seemed to make a difference. For what it's worth, I tried to mimic the lead dress in this thread https://www.trinityamps.com/phpbb/viewtopic.php?f=12&t=3029 and from a picture of an original JMP 50 in Dav Hunter's amp book.

I received some suggestions on other forums to adjust the circuit to minimize crossover distortion. I've ordered some parts and hope to tackle that this weekend. Once I can get that under control, I am going to keep searching for parasitic oscillation, which I think I may have in the Plexi Channel, and lead dress should be the answer to that.

I am thinking that the amp is biased a bit too hot, at least for the particular set of power tubes I have. By my calculation she is putting out about 28 watts.

I'll keep posting updates for the sake of posterity.

Thanks!

-Dan

I spent the weekend substituting parts, inspecting with the scope, listening to the amp, rinse and repeat. I also touched up the solder joints, but there was no change, so I think they were fine before. I took voltages after every step, no unexpected readings. I tried a different set of power tubes, no change.

I changed the cathode resistor for the power tubes to 330 Ohms. This had very little to no impact on the cross-over distortion. It is biasing to about 95% now. I think I could go back to 300 Ohms and it would be fine.

I added a smallish CDE Mica 1KV cap between the power tube plates. This had a noticeable effect on crossover distortion and taming the brightness somewhat. It did not minimize crossover distortion, but delayed the onset a bit. Still not a huge change. This was more apparent in the normal channel. I started with 220pf, then went up to 330pf. There was not much difference between the two, so I kept the 330pf in. I wonder if trying a smaller cap may be advisable?

Next, I swapped the screen resistors to Vishay metal film 3 watt at 1K. This created about a 5 volt delta between the plates and screens. I could not perceive much difference in sound or feel after this change, but kept it.

Next, I swapped the cathode bypass cap for the power tubes to a Vishay 1000uf 63V cap. The amp already felt quite stiff, but this seemed to tighten the low end response. I also have a 1500uf that I may swap in.

I then proceeded to chopstick the amp again while hooked to a dummy load and 1K sine wav monitoring through the scope. I was not able to perceive any differences in lead dress around the tube sockets, except with getting the grid wires too close to the speaker jacks causing extra hum. When I touched the shielded wire between the plexi gain pot and V2, I could sometimes change the character. I resoldered that joint and it sounds the same as before. I think eventually that connection would've gone bad and it was not a strong enough mechanical connection. I may run a new section of shielded cable here anyways for peace of mind.

In summary, the brightness is more in check but still requires non-typical tonestack settings to dial in a good sound. The plexi channel still runs off into uncontrollable feedback at settings on the Gain and Volume of about 3/4 and up. Crossover distortion is still present. But I wonder if this is perhaps typical of the power amp design anyways?

I'd like to be "done" with the amp next weekend. I plan to install the VRM and I might try dumping some signal after the gain pot. I am a little hesitant to do that b/c the gain control is warmer sounding than the volume control, so I am typically running that up higher. But I would like the controls to be usable over the full range.

I was referred to two methods of signal attenuation? After the wiper of the gain pot insert a 470K with treble peaker cap-470K to ground vs. just a100K to ground? For some reason, I ordered 390K resistors from Mouser instead of 470K. I know that will be the same 50% signal dump. The 100K to ground dumps more signal to ground as the gain pot is run higher, right? So, at full on it would only pass about 10% of signal. Or am I mixed up here?

Open to any other suggestions.

Thanks
-Dan

I implemented the Ruby mod, which nearly removed all of the crossover distortion. Because the highest cathode voltage I had recorded was about 25.5 VDC, I went with a 3 diode array: 20V and 6.5V zener in series (both 5 Watt) and back to back with a 1n4007. That was connected in parallel with each 220K grid leak resistor with the 1n4007 side going to ground. Some pics of both channels on my scope. This was the most prevalent I could make the crossover distortion with a SS rectifier. It is even less apparent with the 5AR4. Sounds much better now.





I then installed the VRM and tested it. All good. But I did not have time to rewire the input jacks and V1 socket. So, I'll be tackling that next weekend. And then tidying things up in the amp with zip ties and thread lock, etc. I plan to play the amp for a week at least after that before doing any other mods.

I am still considering reworking the amp to incorporate GNFB similar to what a real plexi would have. That requires several changes: channel mixing resistors and moving both channels to 1 input on the phase inverter, changing the tail resistor, adding the NFB resistor and feedback tail resistor, and phase inverter plates. Or if I don't do that, I may try to make the gain range more usable on the plexi channel. Maybe I'll be happy with it and leave it as is. Who knows....

I'll try to grab a short audio clip next week.

-Dan

Glad to hear the Ruby mod worked for you. I was planning on doing the same mod to my Plexi, but I also read that removing the EL84 shared cathode bypass cap would remove most of the crossover distortion (which it did, while sacrificing a little gain). I may re-install the cap and go with the Ruby mod instead..

Hi Dave, the Ruby mod was very easy to install and made a huge difference for me. From what I understand reading about these 18 watters, sometimes the cross-over distortion sounds very harsh and sometimes it is present on a scope but not to our ears. Science is weird!

I lost the shoulder washer for the VRM pot and when I emailed Stephen for the part number he offered to send me a replacement. Trinity offers such high quality kits and most importantly customer service!

To provide an update on this...the VRM worked as installed but made lots of weird noises (crackle, buzz, light saber, etc.). I added a 16uf F&T cap at the vrm as the first filter node. Grounded to the power amp star. Cured all the weird noises. I also placed a 2W 220K metal oxide resistor in parallel with that cap to drain power when turning the amp off.

I changed the speaker jacks to Marshall style Cliff isolated jacks. Ran a ground wire back to the power amp star. Maybe lowered the background hum a bit.

Lastly, I added the 2 470K resistors as a voltage divider after the gain pot. No treble peaker cap. Since I tend to run that pot fairly high I am considering adding the same network after the volume pot too.

I forgot to grab pics before putting the chassis back in the head cab. I will do that the next time I crack her open.

I think I will still add global negative feedback and make it switchable. But going to enjoy playing the amp as is for a month or so.

I am liking the sound quite a bit. I am probably attenuating volume something like 8-9 decibels with the vrm and that also warms up the amp. So far have only plugged a guitar straight in, I will connect it to my pedal board this weekend. I run the pedal board with the 4 cable method on my mesa amp, so need to rework a couple things to put it all in front of this one.

Thanks again to everyone that helped me out!

-Dan

To anyone trying to keep track, here are my deviations from schematic/layout:

1. Screen resistors are 1K 3W Vishay metal film
2. Power tube cathode resistor changed to 330 Ohm 5 Watt Yageo wirewound
3. Added 330pf 1KV CDE silver mica cap between power tube plates
4. Power tube cathode bypass cap changed to 1000uf 63v Vishay
5. Ruby Mod, added 3 diode array x2: 20V and 6.5V zener in series (both 5 Watt) and back to back with a 1n4007 connected in parallel with each 220K grid leak resistor
6. VRM installed with F&T 16uf 475V filter cap, 220K 2W metal oxide resistor in parallel with cap, grounded to power ground
7. Replaced speaker jacks with Cliff jacks grounded to power ground
8. Added 2 470K 2W Yageo carbon film resistors as voltage divider after gain pot
9. Moved grounds on the 33uf filter cap and 15K tail resistor for the phase inverter to the preamp ground. Connected jumpers as necessary to make the other power amp grounds.

Dan, how are you liking the sound now that you've figured out some of your issues and had some time with it? Would love to hear some samples of it, if you can get any recorded.

I like the sound quite a bit. IMO you are going to adjust the tonestack to your guitar and room for bass response and brightness. But the sound is kind of sort of the same anywhere on the tonestack. The VRM changes the character of the amp a good bit (to be expected), but really makes it useable in your home. The real trick is to ride your volume knob on your guitar.

I'll try to record a short clip this weekend with strat and les paul for you.

That would be fantastic! I have not been able to find a single clip of the Custom Plexi. Deciding between that and the sIII, so this would help a great deal. Glad to hear you're enjoying it.

Put together a quick clip...please excuse the sloppy playing.

https://youtu.be/h20X0cRYa1w

The Ocean is Neck/Bridge (LP), Middle (Strat)

Sunshine of your Love is Neck (LP) with volume rolled down to 8 for main riffs and and 10 for solo with tone on 0 for solo only, Neck (Strat) and same vol/tone changes on guitar

Pawn Shop is Bridge (LP) Bridge (Strat)

Then I play some open chords and roll the volume back on the guitar to clean up the tone

Last, is playing each channel separately and then jumpered together.

-Dan

Thank you, Dan! Sounds great, honestly. Love the white tolex. Don't neglect that Lone Star Special small world I guess because that is currently my main amp (clean channel used as a pedal platform).

Thanks, I'm glad it was helpful. It really is a fun amp to play. A good bit less forgiving than the LSS with no reverb, active loop, or master volume and a stiffer feel overall.

The LSS really is a fantastic amp. It does that hot rodded fender thing and has every feature you could need especially with a handful of pedals you can achieve nearly any sound.

It really is. The cleans on the LSS can sound really good and chime-y but not too much where drive/distortion pedals sound tinny or fizzy. Perfect pedal platform.

Enjoy your Custom Plexi. I'm still deciding what I want to do, I might just build that and the sIII!

Quick update, I spent some time working on the amp today and I am very satisfied with the results. I know the guts are a bit sloppy since I have chopsticked around so much and done many rounds of mods, but all of the connections are solid and sound is great.

I started out chopsticking. Was able to determine all of the instability in the amp was due to a micorphonic tube in V1. It seemed good back when I was rolling tubes, so I guess it must have been a little microphonic and got worse as I played the amp and it got rattled around. It was a new Mullard long plate. Now I have an EHX 7025 there. All good and not running off into feedback at the drop of a hat.

I alligator clipped in a couple small snubber caps across the plate resistor for the second gain stage. Settled on 470p and soldered it in. This really helped calm the ice pick harmonics of this amp.

Then, I made all of the several changes necessary to add Global Negative Feedback on a switch...PI plates, feedback resistor, changes to long tail pair part of PI, moving where the channels meet the PI and adding mixing resistors. I fired up the amp and it wasn't howling but was definitely out of phase and unstable. Switched the OT leads and all good.

This sounds like a big box Marshall to me! Nice and tight even with lots of gain. Now that I have the fat boost and GNFB switches there is a lot of variation that can be dialed in with those and different amounts of gain. Still cleans up incredibly well with the volume knob on a guitar. I am calling this one complete. If there is anything I might have done different, it would be going with a 220p or 330p snubber since the GNFB kills some high end too. But the difference is not worth opening her back up for that one cap.

-Dan

I listen to Lots of music that should be paid for on ITunes on soundcloud... The site SSM has music for everyone and I am currently working on a youtube channel where I compose soothing piano music for meditation and relaxing. Also, I share some samples to the SSM source

After reading a lot of your advice, guys, I decided to try it also through the Ruby mod. I didn't expect it, it turned out so easy. Very quickly and easily installed, I got to work. Highly recommend. I don't know, maybe this might not work for everyone, but it helped me. I've got rid of a lot of problems with sound and not only. By the way, there is also a lot of interesting and useful information in this regard, which helped me a lot - https://artistpush.me/blogs/news/how-to ... your-music

That sounds like a solid burn-in process! Have you noticed any tonal changes after the 25-hour run? Also, are you planning to test it with different speaker cabs? I’d love to hear more about your findings. By the way, if you’re gathering feedback, maybe a survey gift cardcould be a great way to encourage responses!

Really appreciate you documenting the process so thoroughly — it’s a goldmine for anyone tackling similar Plexi-style builds. Sounds like you've been super methodical, and it's paying off. The Ruby mod implementation is especially interesting; great call using the zener + 1N4007 setup — impressive results on the scope.

It’s also good to hear the brightness and crossover distortion are under better control now. Sometimes dialing those in is a balancing act between power amp design and speaker interaction. Regarding your point about the gain control sounding warmer than the volume , I’ve noticed that in a few builds too, so dumping post-gain with care makes sense. A 390K to ground should still give you a decent attenuation curve, especially if you’re experimenting with how much signal to dump and where it affects feel the most.

Hey Dan,

Wow—this whole build journal has been a fantastic read. Thanks so much for documenting your process in such detail. It's not only insightful, but also a great reference for others tackling similar 18-watt or Plexi-style builds.

The Ruby mod implementation and your observations around crossover distortion were especially interesting. It’s wild how subtle changes—like the zener + 1N4007 array—can drastically clean up the power section. I had also heard mixed results depending on how “harsh” the crossover distortion is to the ear, and your scope screenshots really bring that point home.

Also love your iterative approach—trying different values, listening, testing on the scope, making careful changes like upgrading screen resistors or experimenting with cap values. That kind of methodical, ears-plus-tools strategy is what makes for truly dialed-in amps.

Regarding your brightness issue and tonestack settings, it's funny how many of these circuits seem to come alive only in a narrow range of knob settings. I’ve run into similar situations where the gain control is far more tonally pleasing than the volume, especially when using MV or VRM setups. Adding that 2x470K voltage divider was a clever move—sometimes even small signal dumps post-gain can really help the usable range feel more natural.

One thing that stood out was your attention to grounding strategy—reworking the Cliff jacks, isolating grounds, and moving key components like the PI tail resistor grounding point. Those things are so often overlooked, but clearly made a difference in hum and noise floor. Really shows your attention to detail.

Looking forward to seeing what you decide on regarding GNFB. Making it switchable sounds like a best-of-both-worlds idea, especially if you want to compare feel and dynamics on the fly. Also curious to hear how it behaves once your pedalboard is in the chain.

All in all, awesome work. This is exactly the kind of thoughtful, real-world amp-building content that makes these forums so valuable.

Really appreciate you sharing all the details — super helpful for anyone working on similar builds. The Ruby mod results are impressive, and your filtering fix for the VRM noise was a smart move. Love how methodical you’ve been throughout. Looking forward to hearing how it handles with your pedalboard in front. Great work!