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 Post subject: Re: Custom Plexi Build
PostPosted: Wed Oct 21, 2020 12:45 pm 
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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


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 Post subject: Re: Custom Plexi Build
PostPosted: Mon Oct 26, 2020 2:45 pm 
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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


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 Post subject: Re: Custom Plexi Build
PostPosted: Sun Nov 01, 2020 12:26 pm 
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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.

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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


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 Post subject: Re: Custom Plexi Build
PostPosted: Sun Nov 01, 2020 6:17 pm 
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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..


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 Post subject: Re: Custom Plexi Build
PostPosted: Mon Nov 16, 2020 12:08 pm 
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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.


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