Single-channel AB763 Build(s)

[joulupukki]

Sounds good. Thank you. I'll give that a whirl next time I turn the soldering iron on. Also, if and when this ever turns out to be a success, I'll make sure to update my DIYLC file(s) and post here just in case I (or anyone else) decides to attempt this again. Seems like it can't be *that* far off, but who knows. There could be something fundamentally flawed about some portion of this that I've been blind to the entire time. Grasping at wires, absolutely.

 

[King Fan]

I'm not gonna bet against your success but I'm also not putting any money on my idea that it might possibly be the trem wiring. I might put $.02 on the B+ wiring, but I'd have to hunt up a couple pennies; and then reverb seems like it's always on the noise suspect list, too. I'm basically saying keep trying -- and keep looking for smarter folks than me to suggest possibilities.

 

[BigDaddy23]

If you’re going re-run your B+, consider adding the main filter capacitors to the star ground scheme available with your bus.

 

[joulupukki]

Took a longer lunch today. This is my latest hack job (at least it feels a bit hacky, especially since any time I have to go back and redo things it’s a little bit tricky not to touch anything else with the soldering iron).

I moved the reverb transformer back to B+2. The wires from the board to the tremolo are now twisted together. I didn’t re-cut these so it could be a bit better, but it is what it is for now. I twisted the other wire going to the treble pot in with the other two wires that were already twisted. And, of course, I flew the B+ wires overhead.

Hum & Buzz report? Well, the buzz is still there, particularly when I turn up the reverb. But, I do wanna say it’s a tiny bit better than before. For sure, the amp feels usable, especially with the MV control. It’s not as hummy/buzz free like this DRRI I have is (which is still one of my goals – so that I can sell off the DRRI and feel good that I have something better).

One report on the MID-RAW control. @King Fan you inspired me on this in one of your reports on your Bassman Micro. And I do have to say it is totally worth it on this amp! That, combined with the MV I can pull tones (overdrivey) that I’d never be able to do on the DRRI without a pedal. So it’s pretty versatile that way! I’ll have to make a new recording later of the new update.

 

[joulupukki]

If you’re going re-run your B+, consider adding the main filter capacitors to the star ground scheme available with your bus.

You mean, if I end up getting rid of the cap can or even now with the cap can, run that ground to the preamp bus?

 

[joulupukki]

You know what? I wonder if these two runners to the grids of the reverb circuit are problematic? I’ve run out of RG174, but I wonder if I were to run those wires as shielded wires if that may improve / eliminate the noise? There’s also that yellow wire coming off the coupling capacitor from V1B that goes to the reverb filter that I wonder about, but I’d do the first two things that are highlighted first.

Edit: Actually, looking at some original DR amps they don’t do this. So I don’t suspect this is the issue anymore.

 

[BigDaddy23]

You mean, if I end up getting rid of the cap can or even now with the cap can, run that ground to the preamp bus?

Yes - Using the existing cap can, run a wire from the ground tab (on the cap can) to the power supply end of your ground bus...near your bias circuit. Make sure that the PT center tap (? orange wire) is also on that filter cap ground tab.

I think individual caps are better suited to the star ground architecture - if you go down that route, you can separate the different sections of the amp along the bus and therefore their return currents as per Blencowe.

 

[joulupukki]

I made the change late this evening so the ground tab of the can cap is now directly tied with a wire over to the preamp bus. I also changed the terminal strip that I used to have as the power amp ground now no longer have anything grounded at that point. Everything in the power amp now grounds to the bus wire. No detriment that I can tell, but no improvement either.

 

[BigDaddy23]

So the flying/twisted B+ wires and tremolo bundle are next?

BTW, this is a great build and troubleshooting thread. I’m sure you’ll get there in the end, mate.

 

[joulupukki]

So the flying/twisted B+ wires and tremolo bundle are next?

BTW, this is a great build and troubleshooting thread. I’m sure you’ll get there in the end, mate.

Thank you for the encouragement. I hope I can get it there.

I’ve already twisted the trem bundle and the B+ wires are now flying high. That did seem to help a tiny bit and I definitely feel like the amp is usable. The hum isn’t too bad, but it’s that static noise that bugs me. White noise at idle is pretty acceptable as well.

Something I wanted to try just to rule it out:

Remove the reverb transformer from the equation. So, I pulled out V2 (the reverb driver tube) and then disconnected the reverb transformer from B+2. No change in buzz.

These are some questions I still wonder about…

1. Grounding. Even though I’ve checked over and over the different ground connections throughout the circuit, the noise sounds to me like a dirty hum. Not a smooth hum, but full of static. …like it’s picking up some interference from lights or something like that. I thought that it could be something under the board making an accidental connection somehow. I do have a few rivnuts under there for the choke & OT mounts – could those somehow have been slightly connected to a turret or two – by means of solder coming out of the bottom of a turret, like a stalactite? I kind of tested this theory yesterday by loosening the board mounts and raising the board just a little bit and used a chopstick wit’s the amp on to move the board around a little bit. On the preamp side as I barely move the board up and down (we’re talking a 1/16” of an inch movement) something would occasionally pop in the signal like a loose connection, but that was intermittent. But maybe that indicates a loose connection somewhere. I wish it was easy to get a peek at the bottom of the board to see what’s going on down there.

2. I did the headphone test for orientation of the reverb and output transformers before finalizing the layout and have those placed where the noise was at a minimum. But, I never (because of space constraints) ever considered twisting the PT 90°. I see in some Marshall 50W amps that the standup PT is oriented 90° different than how I’ve done it. IF this is the issue, it’d be a bit more involved to make that change. To test that theory I would need to disconnect all the wires and splice on extra wires to make the runs long enough. If that truly was the fix, I’d then need to drill new grommet holes and then have to get rid of the cap can and replace it with separate filter caps inside – probably Nichicon radials mounted to terminal strips.

3. Kind of related to #2 is the proximity of the cap can to the PT. Could that actually be an issue? Not sure. If it was the case, switching to Nichicon radials would solve that issue (I think).

4. I wonder if it would be useful to use / borrow an oscilloscope to trace the signal and see where the buzzing begins?

 

[King Fan]

Well, if you can get hold of a scope, I'm hopeful you might get an answer, and I'm positive you'd learn a ton -- this thread is proof you're a quick study.

 

[joulupukki]

Well, if you can get hold of a scope, I'm hopeful you might get an answer, and I'm positive you'd learn a ton -- this thread is proof you're a quick study.

Hehe. Another learning curve would kick in, I'm sure. ...and just another expense. Although, maybe there's a local ham radio operator who's got one that would let me borrow one. I'll reach out there and see.

Do I have this coupling cap coming from V3 oriented correctly? Seems like the outer foil ought to be on the plate side, right? Not saying this would fix THE issue, but maybe improve things?

For that matter, do I have all the cap foils oriented correctly (layout diagram attached).

 

[joulupukki]

Found a good answer on the orientation of the caps (https://www.tdpri.com/threads/princ...utside-foil-orientation.1081328/post-10912550). Looks like really the only one I have out of place is the coupling cap coming off of the B side of the Reverb Recovery tube.

 

[BigDaddy23]

Getting there!

Did you manage to check the preamp capacitors for DC leakage?

 

[joulupukki]

Getting there!

Did you manage to check the preamp capacitors for DC leakage?

Yes. No leakage detected as far as I can tell. I used my DMM measuring for DC voltage between ground and the side of the capacitors that were opposite the plate. All of them measured 0.00 VDC.

 

[joulupukki]

Here's a few quick short sound demos with MV not all the way up.

Left: SM57 on the combo cab with a Creamback Neo 12"
Right: SM58 on my 1x12 ext cab with a Mojotone Greyhound
Guitar: Fender Ultra Tele

 

[joulupukki]

Argh! I just spent a couple hours reviewing some wiring and making sure the wires to the tubes were good and solid. I double-checked the grounding at the reverb RCA jacks. I raised the board up 1/4” with some nylon standoffs. Everything seemed good, but now I’ve got a definite 120Hz hum when I turn the reverb up. I’ve double-checked that it’s not the reverb driver or recovery tubes (by swapping them). I’ve tried a different reverb tank. I’ve made sure I’ve got the tank wires correct because I’m getting reverb when I play guitar through it, but there’s a pretty loud 120Hz hum. Any ideas on what it could be? Maybe I messed something up when I raised the board? Before raising it, it was only off the rivnuts about 1/8” so I figured it’d be better with a tiny bit more clearance. Maybe I soldered something wrong with the reverb jack grounds? Hmm.

 

[joulupukki]

Found the issue. I had to re-do some wires from the can cap to the board and put the wires … in the wrong spot! I am a dummy. Turning on the soldering iron again!

Edit: Whew. I had B+3 and B+4 connections from the cap can going to the B+4 terminal on the board, B+2 going to B+3 board and nothing (other than the choke) was connected to B+2 board. All solved. Glad it didn’t seem to break anything!

 

[King Fan]

Glad you found it. Yeah, I'm no pro, but I understand that 120 strongly suggests a filter cap problem. That's one reason I was curious if your louder hum was 120, and why I’m hoping your plan to go to separate caps might help. Have you drawn the ground scheme for those individual caps? I always hope someone smart like @Lowerleftcoast will check the grouping of the grounds…. ;-)

 

[joulupukki]

Glad you found it. Yeah, I'm no pro, but I understand that 120 strongly suggests a filter cap problem. That's one reason I was curious if your louder hum was 120, and why I’m hoping your plan to go to separate caps might help. Have you drawn the ground scheme for those individual caps? I always hope someone smart like @Lowerleftcoast will check the grouping of the grounds…. ;-)

Yeah, 120Hz is definitely discernible vs 60Hz. I’m glad that was only a temporary issue, but it was interesting how I could turn off reverb and hear 60Hz and then turn it up and it was an octave higher hum. Glad that was only a temporary thing.

It still has the original hum/buzz that I’ve been trying to get rid of this whole time unfortunately. Weird to me that the 6V battery thing had no effect on it at all, otherwise, I would maybe suspect it would be possibly solved with a humdinger pot.

The other thing I’m curious about is whether it’d benefit from having a balance pot for the bias. But, when I was trying to figure out how to do that with the bias trem I couldn’t ever figure that out, so I left it off.

—

So as far as the grounding scheme if I end up putting in the Nichicon radials instead of the cap can, I was thinking that the first three caps would be grounded to the power amp ground and the last one (B+4) would be grounded over where the cathode for the reverb recovery grounds to the buss bar.

Right now, if you remember my recent change, everything is being grounded to the bus bar as there is no power amp ground. Concerning the hum & buzz this seems to maybe have barely reduced it all.

—

The buzz that I hear is similar to the kind of buzz you get when your guitar is exposed to the room and picking up stray power noise … and when you touch the strings of the guitar that goes away.

But the problem is, that the buzz happens even without a guitar cable plugged in to the input at all. There are a couple of caps (like the bright cap and the reverb filter cap) that I can touch with a finger and that type of buzz increases. But I wonder if it’s somehow electrical interference like that? I’ve tried putting a sheet of aluminum foil over the open chassis with it touching to be grounded to chassis and that doesn’t seem to change it.

I’ve been scheming how to find an oscilloscope because I think I could possibly use that to trace components on the board and potentially discover a bad solder joint or a bad ground situation. There’s one here locally in the classifieds but it doesn’t have a signal generator built in so that would potentially end up being an added expense. Anyhow, what a journey. I’m definitely learning a lot of things, especially how helpful and smart you all are!