6V6 Plexi with Crossover Distortion [Problem Solved]

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6V6 Plexi with Crossover Distortion [Problem Solved]

Post by dbharris »

Hello All!

My name is Dan and I'm a new member. I'm sorry to say my first post is to ask for your help. I have build threads at The Amp Garage and Trinity, but was not getting much in the way of suggestions. Someone at TAG suggested I post here. I'll try to keep this brief enough to not be a novel, but give you enough detail to be helpful. This is my first amp build from scratch, the Trinity Custom Plexi. I have modified an amp and built lots of pedals before. I can read a schematic and layout, but don't necessarily understand the "why" of how it all works.

Schematic
18WattCustomPlexiSchematic4.pdf
Layout
18WattCustomPlexiLayout3.pdf
I built the attached amp and while I can get some righteous crunch, the overall sound is brittle, really sharp on the high end. When either channel is dimed the amp will runoff into uncontrollable feedback, with both channels jumped it happens much sooner. There is also a nasty different type of distortion that sounds most present on bass notes, but sort of rides on everything if that makes sense.

Please let me know if I should change my lead dress or start changing the circuit to get her in line. I have chopsticked around to no results. I swapped OT leads and it was virtually impossible to perceive a difference. I put them back. I've swapped tubes around and tried different tubes.

Gut Shots
IMAG1932.jpg
Here are my most recent voltages:
V1
1 - 170.6
3 - 1.478
6 - 129.1
8 - 0.799

V2
1 - 143.3
3 - 0.973
6 - 241.3
8 - 144.1

V3
1 - 173.1
2 - 19.17
3 - 32.14
6 - 181.6
7 - 18.94
8 - 32.14

V4
3 - 393-399 (squeals when tube pin touched by DMM probe)
6 - 366
8 - 24.42

V5
3 - 369
6 - 366
8 - 24.42

After much reading, I thought it may be crossover distortion or blocking distortion. I bought a scope b/c it seemed like that was the only way I could keep chasing down the problem. Here are some screenshots from the output jack of my scope, while injecting a 1K sine wave at the input. There is one clean setting and then the setting where the crossover distortion is most prominent. Adding more volume/gain just pushes it into a square wave after that.

Normal Vol 2, Tone 5
NV2T5.jpg
Normal Vol 8, Tone 5
NV8T5.jpg
Plexi Vol 2, Gain 5, Tonestack 5
PV2G5T5.jpg
Plexi Vol 6, Gain 5, Tonestack 5
PV6G5T5.jpg
Channels jumped Vols 2, Gain either 5 or 10 (can't remember), Tonestacks 5
JVs2T5.jpg
Channels jumped Vols 10, Gain either 5 or 10 (can't remember), Tonestacks 5
JVs10T5.jpg
I can control the hump in the trough here with the treble pot.

Happy to post more pics, scope something else out, take voltages, etc. Just let me know what would be helpful.

Thanks in advance!

-Dan
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Last edited by dbharris on Sat 01/23/21 7:56 pm, edited 1 time in total.
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Re: 6V6 Plexi with Crossover Distortion

Post by crgfrench »

I would resolder at least 9 of the turret connections, some of them look dry.
Screen Shot 2020-10-17 at 10.43.56 PM.png
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Re: 6V6 Plexi with Crossover Distortion

Post by crgfrench »

Probably not the source of your problem, but R24 should be 10W and you have a 5W installed.
I personally would replace that with an Ohmite 12W Brown Devil.
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Re: 6V6 Plexi with Crossover Distortion

Post by JMPGuitars »

crgfrench wrote:
Sat 10/17/20 10:11 pm
Probably not the source of your problem, but R24 should be 10W and you have a 5W installed.
I personally would replace that with an Ohmite 12W Brown Devil.
5W is fine there. That's what I use for my cathode resistor. I only use higher wattage on my 36 watt amps.
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Re: 6V6 Plexi with Crossover Distortion

Post by geoff 1965 »

welcome to the forum Dan,
what value is your phase inverter tail resistor R12? the schematic shows 15K but the layout shows 10K.
the higher plate voltage and squeal on one of the power tubes looks suspect,have you swapped the tubes around to see if the higher voltage & squeal follows the tube? also looking at the stable voltage your plate and screen are very close,i would replace the screen resistors R26/27 with 1K 3W to bring the screen voltage down a bit.
also read the notes on the schematic about C13,guys on the forum have started using the higher value 1000/2200uf on the cathodes
good luck
this is my 6V6 project,trinity plexi mk2 preamp”slight mods” and cathode biased power section with vvr so quite close to yours.
3F17277C-506B-4BE5-A450- CEB7685F4DE3.jpeg
A5CCF72A-CC58-48BA-A6EE-DA0416C44740.jpeg
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Re: 6V6 Plexi with Crossover Distortion

Post by crgfrench »

geoff 1965 wrote:
Sun 10/18/20 8:31 am
this is my 6V6 project
Geoff, what PT is that & where sourced? Thanks
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Re: 6V6 Plexi with Crossover Distortion

Post by Bieworm »

Probably not the source of your problem, but R24 should be 10W and you have a 5W installed.
I personally would replace that with an Ohmite 12W Brown Devil.
5W is fine there. That's what I use for my cathode resistor. I only use higher wattage on my 36 watt amps.
I am using 5W too in my imperial build. I need 250R theoretically.. so I put a 100R + 150R in series. I figured it would be easier to tweak when necessary..
The thing that puzzles me is the cathode cap.. the schematic prescribes 22uf 100V. That's real low, don't you think? I left room for a bigger cap anyway, just in case...
20201018_221724.jpg
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Re: 6V6 Plexi with Crossover Distortion

Post by geoff 1965 »

hello Craig,
bought the PT on ebay,here's the item number 164021680137 it's in the UK though so you would have to consider post & import duty!
i bought a seymour duncan strat pickup for £50 but the post was £15 from the US and they charged me £12 import duty "more than 20%!" daylight robbery!
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Re: 6V6 Plexi with Crossover Distortion

Post by dbharris »

Hi everyone, I really can't thank you all enough for the comments and suggestions!

Going to try to address them all here.

I will reflow those joints on the turret board and add more solder. I tried to make sure I made good mechanical connections before soldering and then didn't overheat any component. None of the joints look cold in person and test ok on my meter. But I had never worked with turrets before and did not know when to stop adding solder it seemed like some spots just kept taking solder, I didn't want it leaking out of the bottom side and making connection to the chassis. It was also introducing a lot of heat. My solder station is generally set to 700 degrees F.

On R24, when I sorted through the parts originally, I did order a 10W too per the schematic when I saw it shipped with a 5W. But the 10W was physically longer than the board, so I did not use it (generic wirewound type similar to xicon).

I will order a couple more cathode bypass caps for the power tubes to see if that feels better. As is, even with the issues I have the amp is very tight feeling. With both SS rectifier and GZ34.

Phase inverter tail is 15K per the schematic, I had reported that layout error over on the Trinity forum.

I just swapped the power tubes around. Now I can get a squeal when touching the screens of both tubes. Easier to induce on V4 still and perhaps a bit quieter overall, but audio memory weeks later isn't always the most accurate. I will order some 1K screen resistors to try that out.

I didn't bother with preamp voltages this time. SS rectifier is in the socket.

Mains 120.7
B+ 388

V4
3 - 375 (the voltage still moves around a bit when I have a DMM probe on this pin but now just between 375-378 and slower than before)
4 - 372
8 - 24.66

V5
3 - 374
4 - 371
8 - 24.66

Geoff, that Plexi Mk2 looks awesome! It's nice to have other reference photos.

Somebody on TAG suggested the amp may be biased too hot.

I did just remember I have a broken pair of JJ 6V6s tubes in an Egnater Tweaker 15 that I never use. Not sure the state they are in, but I will try to swap those in tomorrow and see if there is a difference.

Thanks again for taking a look.

-Dan
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Re: 6V6 Plexi with Crossover Distortion

Post by geoff 1965 »

okay sound's like a plan,i've got the same plate voltage as you and mines biased slightly hotter at 260R with no problems.here's a schematic of the phase inverter/power section i used for you to compare,i upped the bias cap to 1000uf/40V and replaced the 470R screens with 1K's.
6V6_Lite IIb_Conger.gif
notice on the trinity 18W plexi and mk2 they removed the 330pf bright cap from the gain pot and used a 100K resistor to ground to attenuate the signal,might be worth noting.
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Re: 6V6 Plexi with Crossover Distortion

Post by dbharris »

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.

Is there much difference between the two methods shown above? 470K with treble peaker cap-470K to ground vs. just the 100K to ground? For some reason, I ordered 390K resistors from Mouser instead. I know that will be the same 50% signal dump, but with a lower impedance I think. 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?

Thanks
-Dan
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Re: 6V6 Plexi with Crossover Distortion

Post by geoff 1965 »

if you've got a spare 500K log pot patch that across the gain wiper to ground then you can adjust the attenuation between 0-500K and find the sweet spot.
the 1K screens are to protect the screens and if you read the notes on the bias it tells you the higher1000/2200uf tightens up the amp.
i'm still experimenting with that myself and have just upped to 330R to cool the tubes a little.you can buy or make yourself a variable resistance box,they are invaluable for testing/tweaking and if you use wirewound pots you can use them for adjusting the B+ and biasing as well as signal attenuating.
006.JPG
Here’s my voltages with the 330R bias Dan,
Pin3 382V. Pin4 376V. Pin8 27.4
Plate current 40ma. 14W dissipation per tube “and staying there!”
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Re: 6V6 Plexi with Crossover Distortion

Post by dbharris »

Hi Geoff, thanks again for your help!

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.
NRubyMod.jpg
PRubyMod.jpg
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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Re: 6V6 Plexi with Crossover Distortion

Post by JMPGuitars »

dbharris wrote:
Sun 11/01/20 11:22 am
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
Re: PR mod - 1W diodes are fine. 5W are used for voltage dropping the B+ line.

Before you make any other mods, you should ditch those output jacks and use isolated Cliff jacks instead. Then run the sleeve (negative terminal) of the jacks on a single wire back to the star ground.

If you're still having issues with crossover distortion, you should check out the snubber circuit I have on my EF86 Xtra docs (#4). For two 6v6 power tubes you would change the 3K to 1.5K, and the .0047uF to .01uF.

Keep in mind that some crossover distortion is desired. If it's too much, then you want to filter it out. But you should listen to your ears before you pay too much attention to what the scope says.

Thanks,
Josh
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Re: 6V6 Plexi with Crossover Distortion

Post by geoff 1965 »

good to see you making progress Dan,i don't have an oscilloscope "sad violin starts playing in background" so i have to rely on my ears & attenuator box and stick to the proven schematics in the downloads.does the VRM adjust the preamp B+ or just the phase/power section? this is a first for me with 6V6's and the VVR,but i'm getting some nice fat tones and the VVR lets you sweep between headroom & early breakup,i love it!
keep posting your results & good luck
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Re: 6V6 Plexi with Crossover Distortion

Post by dbharris »

Josh, thanks for the advice! I only had 1/2 watt zeners in my parts stash (for pedals) and wasn't sure what was needed for this. I stocked up on extras and other values too in case I wanted to use them to drop some voltage too.

Geoff, Trinity recommends scaling the whole amp. So, that was what I did. It looked like with a couple extra 1n4007 diodes you could choose to scale just the power tubes or power and phase inverter. But I was not sure if that meant changing the dropping string for the preamp to keep voltages where they should be. I'm going to play it like this for a while and if I end up not being happy with the tone, I may try the power amp and PI only.

I'd say my speakers are on the way to being broken in, but probably only have about 10 hours on them. Hoping that will help settle things in too as they relax a bit.
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Re: 6V6 Plexi with Crossover Distortion

Post by JMPGuitars »

dbharris wrote:
Sun 11/01/20 2:04 pm
Josh, thanks for the advice! I only had 1/2 watt zeners in my parts stash (for pedals) and wasn't sure what was needed for this. I stocked up on extras and other values too in case I wanted to use them to drop some voltage too.

I'd say my speakers are on the way to being broken in, but probably only have about 10 hours on them. Hoping that will help settle things in too as they relax a bit.
For voltage dropping, look at the link in my signature.

It's interesting the PR mod helped at all, that's usually only for EL84s.

Please take my advice on the output jacks. It can help with both noise issues and safety. And it's absolutely not optional if you add a NFB.

Thanks,
Josh
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Re: 6V6 Plexi with Crossover Distortion

Post by dbharris »

Hi Josh,

I think I have some isolated jacks somewhere, they might be pc mount that I ordered for a pedal, but I can still make that work. And thanks so much for mentioning it is a safety issue with NFB. The ground wire would go back to the star ground for the power amp and PI, right?

On the Ruby mod, I was at the point where I was willing to try anything that was reversible to see if it helped. My problem was not "fizziness" which I understand is why a lot of people add them with EL-84s. I was reading up more on the mod here http://www.paulamps.com/18watterbuzz.ht ... Distortion where it was explaining more or less that under certain operating conditions the coupling caps were getting charged up. I thought/hoped that the same could be true with my 6V6s.

Thanks again for the advice!

-Dan
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Re: 6V6 Plexi with Crossover Distortion

Post by JMPGuitars »

dbharris wrote:
Sun 11/01/20 6:27 pm
Hi Josh,

I think I have some isolated jacks somewhere, they might be pc mount that I ordered for a pedal, but I can still make that work. And thanks so much for mentioning it is a safety issue with NFB. The ground wire would go back to the star ground for the power amp and PI, right?

On the Ruby mod, I was at the point where I was willing to try anything that was reversible to see if it helped. My problem was not "fizziness" which I understand is why a lot of people add them with EL-84s. I was reading up more on the mod here http://www.paulamps.com/18watterbuzz.ht ... Distortion where it was explaining more or less that under certain operating conditions the coupling caps were getting charged up. I thought/hoped that the same could be true with my 6V6s.

Thanks again for the advice!

-Dan
I would order new jacks. You want solid mechanical connections in addition to a good soldering job.

I just took a look at your schematic/layout because of that question. If I ever tie a part of the PI to the power amp star ground, it's only the part where, in your case, the 220K grid reference resistors connect to ground (but not the LTP portion). There's a couple issues with the layout, but the easiest fix would be to snip the ground bus between the 300R resistor and 33uF cap, and connect the other end to the preamp bus wire. Take a look at the ground scheme thread in my signature.

That 33uF filters the preamp, and should not be tied to the power amp ground. That could lead to noise and other issues. If that's not clear enough ask any questions you may have, or take a look at some of my layout designs and you'll see the correct way to implement this.

Thanks,
Josh
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Re: 6V6 Plexi with Crossover Distortion

Post by geoff 1965 »

another area of safety is you don’t have a bleed resistor,if you fit a 220K 2W metal film between the first positive and negative terminals of the can cap when you switch off the voltage in the capacitors will drain to ground.
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