6V6 Plexi with Crossover Distortion [Problem Solved]

[dbharris]

Josh, thanks for pointing that out. I did catch a few errors between layout and schematic, but I did not even think to double check the star grounds. I see the schematic is correct and the layout is wrong. I also looked at your post on grounding. I will rework that this weekend when I'm back inside the amp.

Geoff, that's a great idea. I do have a heavy duty alligator clip lead that I soldered a 220K in the middle of and I use that to discharge the caps before working on the amp or if it is staying outside of the head cab.

[geoff 1965]

Which output transformer and 6V6’s are you using Dan?

[dbharris]

They are Heyboer, but Trinity's take on a clone of the Radio Spares. They are 290-0-290. I am running JJ 6V6S output tubes.

I have another question on grounding and the VRM. I have attached the VRM instructions for clarity. But essentially you move the 1M pull down resistors to the V1 socket and add a cap between the grid stoppers and grid to block DC passing back to the input jack. The instructions say to ground the 1M resistors on the middle tab of the terminal strips that connects to the chassis.

Wouldn't it be better to run a wire back to the preamp star ground? If so, does it matter if I run 2 wires back or tie the ground side of the 1Ms together and then run 1 wire?

[dbharris]

Quick update...I fixed the grounding scheme, it made no difference to background noise.

I finished the wiring for the VRM. I had only briefly tested it before. It definitely works but it noisy and creates some strange noises/unusable settings. So, I've ordered an 8uf and 16uf cap to add a filtering node at the VRM ahead of the B+ filter. I also ordered some marshall style cliff jacks for the speaker jacks.

IMAG1969.jpg

I'm hoping that settles everything out and I'll be able to box the amp up for a few weeks to play it before deciding on whether to add GNFB.

The power at my house is a little wonky too. Still all of the original wiring (house built in the late 1940s) and there is no dedicated earth connection here. Just hot and neutral. Lately it has been intermittent a couple times a day too.

-Dan

[JMPGuitars]

Quick update...I fixed the grounding scheme, it made no difference to background noise.

I finished the wiring for the VRM. I had only briefly tested it before. It definitely works but it noisy and creates some strange noises/unusable settings. So, I've ordered an 8uf and 16uf cap to add a filtering node at the VRM ahead of the B+ filter. I also ordered some marshall style cliff jacks for the speaker jacks.

IMAG1969.jpg

I'm hoping that settles everything out and I'll be able to box the amp up for a few weeks to play it before deciding on whether to add GNFB.

The power at my house is a little wonky too. Still all of the original wiring (house built in the late 1940s) and there is no dedicated earth connection here. Just hot and neutral. Lately it has been intermittent a couple times a day too.

-Dan

Do you have any grounded outlets at all? Your amp should be grounded.

If the electricity has issues, I recommend getting a filtered power strip and/or using a filtered inlet. I use both. I use a Furman SSB6. There's lots of other options too.

I assume correcting the outputs should help. If you still have issues, post a demo video so we can hear the noise.

[dbharris]

Unfortunately, none of the outlets in the house are grounded. They are all 3 prong, but the earth is not connected.

The VRM noises were nasty. The background hum is not a big deal for me, once you start playing it isn't noticeable. If I am touching the bridge or strings of the guitar it gets much quieter too. That happens with all of my amps.

Hoping to be changing houses in the next 1-2 years anyways.

[geoff 1965]

that's an amendment i made to the schematic i worked from,there was no filtering pre VVR so 1st half of my dual can filters the VVR & preamp.
reading through the notes in the VRM link you posted it says on the schematic "B+ in from the standby switch or after the 1st power supply filter cap if noisy" but on the layout it says after "additional" filter cap.
same principal here as zener's working more efficiently with filtered B+.

[crgfrench]

Unfortunately, none of the outlets in the house are grounded. They are all 3 prong, but the earth is not connected.

This is a real safety issue. I would spend whatever to have a qualified electrician come out, pound a copper stake into the ground outside the house and connect your grounds. You can't play guitar from the grave.

[dbharris]

I totally understand the safety concern and I will call around for a few estimates.

[dbharris]

Alright, I forgot to take pics before loading the chassis back in the head cab. But 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 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.

Thanks again to everyone that helped me out!

-Dan

[dbharris]

Somebody on the Trinity forum asked for a clip. I still feel like I have some tweaking to do for this amp. Going to add GNFB in January and then go from there.

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

[crgfrench]

That amp sounds pretty sweet to me, nice job!

[dbharris]

Thanks! I'm still getting used to the Marshall thing coming from fender style amps. It's a lot of fun getting to know this amp.

[ViperDoc]

More of a question than a recommendation: is it not advisable to separate the preamp and power amp grounds? They appear to be all soldered together.

[crgfrench]

More of a question than a recommendation: is it not advisable to separate the preamp and power amp grounds? They appear to be all soldered together.

In the image download/file.php?id=12858&mode=view they look separate. There is a physical overlap of the sections but they don't seem connected. Power ground appears to go to the left star and pre goes way over to the right.

[dbharris]

More of a question than a recommendation: is it not advisable to separate the preamp and power amp grounds? They appear to be all soldered together.

In the image download/file.php?id=12858&mode=view they look separate. There is a physical overlap of the sections but they don't seem connected. Power ground appears to go to the left star and pre goes way over to the right.

Yes, originally going by the Trinity layout they were all mixed together. I did separate them out with jumpers (green wires along the ground bus) where appropriate.

However, I did not perceive any difference in sound, hum, or noise floor following that change.

[dbharris]

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

[ViperDoc]

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

Congrats on a successful build. Any sound bytes?

And this may be rhetorical, but how are your tube sockets being secured to the chassis? I only see locknuts on the tubes that have accessory solder tabs.

[dbharris]

Hey ViperDoc,

I haven't recorded any clips yet after these mods. I need to get Reaper set up on my new laptop and then I'll do some proper recording and post here. Might be a few weeks though.

This chassis from Trinity has threaded holes for the bottom mount tube sockets. I applied some thread lock to the screw head side as well. If they ever get loose I'll add some keps nuts too.

-Dan

[TriodeLuvr]

Is this the typical way turret boards are wired in DIY guitar amps? The component leads are soldered into the turrets from the top, rather than being wrapped around? I've made small turret boards for other projects, but not long ones like these. Even then, the turrets I used weren't hollow, so wrapping the leads was the only choice.

Jack