Dumble Overdrive Special #124 build video series

[erwin_ve]

Nice to hear it's working as supposed! Gnarly drive is maybe the 15pF on the master, when set low there is a lot more high pass.

[sluckey]

That's a really crude soldering job on both those speaker jacks. Same for that tube socket just below those jacks.

[pompeiisneaks]

Yeah I touched them up, I think my near vision is getting really bad, I didn't see any of that until I had the photo. That or I need better light. Either way I'll clean them up.

So now to the next issue. After playing it, very loudly for a while it suddenly started doing what I would guess is motorboating? I thought that only happened on old amps when the elytics go out. I've got brand new ones, and they're all showing fine. I started probing around with my dmm on voltage, and got the bias solid right up til after the 220k resistors, Once it gets to the PI output caps side of them it's going all over the place. When I probe one side it ends up almost doubling the wub wub sound and if I probe the other it seems to mellow down a bit, but not disappear, what's making the negative bias go all wonky like that?

~Phil

[erwin_ve]

Leaking coupling cap?
Bad solder joint?

[pompeiisneaks]

Is it common for brand new caps to leak? I reflowed the joints at the PI to see if that was it, and it didn't help. I've chopsticked around and found nothing. Hmm, I'll play with it more tonight.

Phil

[pompeiisneaks]

So I've done some more troubleshooting, I'm completely confused.

I removed the extra jack, and the NFB to try and limit out as much as possible, and suddenly I have horribly low output and it's very distorted. When I reconnect the NFB it starts that wubwubwub sound and the sound comes back super loud and clean. Now, to be clear, when I first removed that bit of solder that was sending my signal to ground, it started doing the typical huge screeching of the OT phase being wrong and the NFB being PFB, so I swapped the leads and all was fine, for a short while, then it started doing this noise. Why would removing nfb stop the bias voltage flying all over the map AND make output sound horribly weak and distorted?

I can't for the life of me figure out what's going on.

~Phil

[pompeiisneaks]

additional weird data. I was probing around checking voltages and suddenly got the noise to disappear. When the DMM is in VDC mode, I have the black clipped on a ground, and then touched the probe to the coupling point of the grid on either side of the PI, the coupling capacitor, and the 1M resistor and I get no noise. Additionally I'm getting 40VDC at that point, which to me seems wrong no? This should be post coupling caps? So does that mean my 0.02 uF is leaking some DC or maybe the 0.1 that's on the opposite side? or is 40VDC normal at the grids of the PI?

~Phil

[pompeiisneaks]

I FIGURED IT OUT!!!!

The layout doesn't show this clearly at all, but referring back to the schematic for the same area, I realized I didn't have a ground connected to the 1uF tantalum and 390ohm resistors in the presence circuit! Tell me you'd get that this is grounded in this screenshot:

NoGround.PNG

DOH

~Phil

[norburybrook]

yes Phil it connects to the send return jacks ground.

There should really be a dotted line across the board connecting it to the solid black line.


M

[sonicmojo]

Looks like you were looking at an old layout down in the thread. Check the one on the first post on the 124 files thread and the dotted line is there. Bummer, but I'm glad you figured it out.

[talbany]

Dumble Overdrive special Complete Layout 52010.jpg

I FIGURED IT OUT!!!!

The layout doesn't show this clearly at all, but referring back to the schematic for the same area, I realized I didn't have a ground connected to the 1uF tantalum and 390ohm resistors in the presence circuit! Tell me you'd get that this is grounded in this screenshot:

NoGround.PNG

DOH

~Phil

Phil
Why did you get the one on page 6

If you read through the 124 Layout thread there were several revisions done, due to the fact that this was the 1st ODS ever documented from the real deal and posted here with the help and eyes of those here at TAG. The one on page 1 is the latest revision.
PS you may want to update your video as well!

On the Layout
Make sure you follow the layout scheme and ground that point to the sleeve connection of the output jack or to the ground portion of the circuit where the global feedback from the tip connection of the output jack is connected. This is to ensure it doesn't flow through a part of the chassis that may contain power supply ground currents since 1/2 of the PI is used for GNFB


Long tails love to see ground
Tony

[pompeiisneaks]

Thanks, I honestly don't recall when or how I chose that particular file, sucks to be me lol. I'll look around and see. Thanks talbany, I'll definitely ground it there, to test I just used alligator clip leads to chassis.

Phil

Sent from my phone using Tapatalk

[pompeiisneaks]

I just figured it out, I used the one from the thread that's not stickied, 124 low plate classic, instead of the stickied one. Maybe that one should have some kind of update or note?

Sent from my LG-V521 using Tapatalk

[pompeiisneaks]

talbany, To be clear it's this post and it IS also stickied:

https://tubeamparchive.com/viewtopic.php?f=5&t=12872

it has the 124 but the old layout and I think that's where I got the wrong one

Thanks so much though for all the outstanding docs and files, leave it to me to go to the wrong place

~Phil

[talbany]

talbany, To be clear it's this post and it IS also stickied:

https://tubeamparchive.com/viewtopic.php?f=5&t=12872

it has the 124 but the old layout and I think that's where I got the wrong one

Thanks so much though for all the outstanding docs and files, leave it to me to go to the wrong place

~Phil

Phil
Yes the sticky posts with the ODS layouts in Dumble files are mine. All the layouts on page 1 of each generation have been updated and should be correct. If you go further back in the thread (Where you got yours) you might run into an earlier revision someone may have posted that may not be up to date. I cannot control what others post. I know the site is disorganized and can be confusing but we just deal with it

Anyway sorry! you got an earlier version by mistake and glad you finally figured it out and hope the layout was of some help!

Let me know if you have any more questions

Tony