ODS 183 NFB

[ViperDoc]

I was just looking at the ODS 183 documents in the files section. The 4K7 NFB resistor is connected to the 4R tap on the layout, but is connected to the 8R tap on the schematic. Which is "correct"? Thanks.

[jelle]

Depends.

I have that same TF-170 transformer and the 4 and 8 tabs are miswired (colors swapped) by the manufacturer. The other one I have is correct.

[ViperDoc]

So one of them is right, and the other is wrong depending on if the transformer was either correctly or incorrectly wired. Thank you so much for that clarification.

[jelle]

Exactly

Sorry I could not help much

[jelle]

Canonically, it would be the 4 Ohm tab in a 100W Dumble Overdrive Special with the 4.7K/390r global negative feedback.

I have had Dumble Overdrive Special #0183 on my bench twice, ruptured an eardrum while letting it morph into feedback from a bass string, and my clone responds like that on the 4 ohms tab, not on 8. (No eardrums were harmed that time, I learn quickly lol), I did not measure the tabs in that Dumble amp.

FWIW....

[ijedouglas]

I think the confusion comes in from the layout. It shows the green tap connected to the GNFB. If you look closely at gut shots, the green tap is shrink-wrapped off but the GNFB wire(green) still carries on to the switching jack. The TF170 wiring is Green=>16/Yellow=>8/Black=>4. I think the author of the document(or 183 documenter) incorrectly labelled the green as the 4Ohm tap. That leaves the black or yellow for the GNFB. My bet (and also seeing this exact same configuration on another Dumble) is that the GNFB goes to the switching jack with the black 4Ohm tap

[ViperDoc]

Look here: https://ampgarage.com/forum/download/fi ... &mode=view

Then here: https://ampgarage.com/forum/download/fi ... &mode=view

The ODS 183 in this layout uses a 1K7 primary "Plexi"OT. The schematic shows using a TF170 "Plexi" OT. I have no idea what wires would arrive out of the box, but the layout clearly marks the 4R tap as green, taped off, and only connected to the 4K7 NFB resistor. Incidentally, on both the layout and the schematic, the 8R goes to the Speaker OUT, and the 16R goes to the Extension Out! WRONG! Right??? I've never seen that before. Unless we agree to only use one at a time. It just doesn't infer that on the back panel?

"Which one of these goes to what output jack?"

"Here, use this coin. My magic 8-ball is broked."

[ijedouglas]

Look here: https://ampgarage.com/forum/download/fi ... &mode=view

Then here: https://ampgarage.com/forum/download/fi ... &mode=view

The ODS 183 in this layout uses a 1K7 primary "Plexi"OT. The schematic shows using a TF170 "Plexi" OT. I have no idea what wires would arrive out of the box, but the layout clearly marks the 4R tap as green, taped off, and only connected to the 4K7 NFB resistor. Incidentally, on both the layout and the schematic, the 8R goes to the Speaker OUT, and the 16R goes to the Extension Out! WRONG! Right??? I've never seen that before. Unless we agree to only use one at a time. It just doesn't infer that on the back panel?

"Which one of these goes to what output jack?"

"Here, use this coin. My magic 8-ball is broked."

The layout is wrong. Pacific TF170 is the transformer. The TF170 wiring is Green=>16/Yellow=>8/Black=>4 / Brown=>Comm

If you look at the actual gutshot of 183 you will see that the green GNFB (from the board) DOES NOT go to the green tap (16Ohm) from the OT. It goes to a switchcraft 13a which is connected to the 4Ohm and 8Ohm taps(GNFB tied to the 4Ohm connection). Here is a picture of an actual Dumble with the same configuration (different tap colors: white GNFB / Yellow 4Ohm / Green 8Ohm / Orange Comm/ Grey 16Ohm (shrink wrapped) ):

PXL_20230730_143845137.jpg

#0183 picture

Screenshot 2023-10-17 084106.png

HAD instructions

PXL_20230730_144739262.jpg

[ViperDoc]

The layout is wrong. Pacific TF170 is the transformer. The TF170 wiring is Green=>16/Yellow=>8/Black=>4 / Brown=>Comm

If you look at the actual gutshot of 183 you will see that the green GNFB (from the board) DOES NOT go to the green tap (16Ohm) from the OT. It goes to a switchcraft 13a which is connected to the 4Ohm and 8Ohm taps(GNFB tied to the 4Ohm connection). Here is a picture of an actual Dumble with the same configuration (different tap colors: white GNFB / Yellow 4Ohm / Green 8Ohm / Orange Comm/ Grey 16Ohm (shrink wrapped) ):

I appreciate the photos, those are nice to see. Since the Fender Twins ran a 4R output only OT (125A29A), this design makes sense. Looks like HAD used a bit less taming (4K7 vs. 820R).

I still don't get the other OT tap combinations between the jacks. If you parallel the 8R and 16R output impedances, you get an overall 5.33R impedance, right? So running a 4 ohm load into a 5.33 ohm OT secondary could be a lot worse, I guess. I've never seen that before.

[ijedouglas]

I appreciate the photos, those are nice to see. Since the Fender Twins ran a 4R output only OT (125A29A), this design makes sense. Looks like HAD used a bit less taming (4K7 vs. 820R).

I still don't get the other OT tap combinations between the jacks. If you parallel the 8R and 16R output impedances, you get an overall 5.33R impedance, right? So running a 4 ohm load into a 5.33 ohm OT secondary could be a lot worse, I guess. I've never seen that before.

Not sure what you mean by less taming. The GNFB is still 4K7 so no difference.

I'm unsure of the parallel speaker cab extension although I know HAD would run a 4Ohm OT into an 8Ohm cab so I don't think he was concerned with a slight mismatch. Look at some of the 2nd and 3rd gen 50W amps. They just about always ran a 4Ohm baseman OT into an 8Ohm EVM or Altec.

[ViperDoc]

I was referring to the 820R resistor on the Fender Twin Reverb vs the 4K7 on the ODS, which would lead to less NFB in the loop given the same OT.

I forgot about the impedance mismatch. It was obviously intentional, I'm curious to know why he did it that way. He must've thought it sounded better.

[Stephen1966]

The layout is wrong. Pacific TF170 is the transformer. The TF170 wiring is Green=>16/Yellow=>8/Black=>4 / Brown=>Comm

If you look at the actual gutshot of 183 you will see that the green GNFB (from the board) DOES NOT go to the green tap (16Ohm) from the OT. It goes to a switchcraft 13a which is connected to the 4Ohm and 8Ohm taps(GNFB tied to the 4Ohm connection). Here is a picture of an actual Dumble with the same configuration (different tap colors: white GNFB / Yellow 4Ohm / Green 8Ohm / Orange Comm/ Grey 16Ohm (shrink wrapped) ):

I appreciate the photos, those are nice to see. Since the Fender Twins ran a 4R output only OT (125A29A), this design makes sense. Looks like HAD used a bit less taming (4K7 vs. 820R).

I still don't get the other OT tap combinations between the jacks. If you parallel the 8R and 16R output impedances, you get an overall 5.33R impedance, right? So running a 4 ohm load into a 5.33 ohm OT secondary could be a lot worse, I guess. I've never seen that before.

I think Ian is saying the 16 Ohm tap is not connected to the jacks. The EXT 13a is a switching jack. With a disconnected EXT, the 8 ohm tap runs to the speaker SPKR jack. Put a dummy jack into the EXT and the 4 Ohm tap runs to the SPKR jack. Similarly you activate the 4 ohm tap to both jacks when you attach an extension cab. meaning you can run two 8 ohm cabs in parallel. I think some of the Supro amps use this kind of switching as well.

[WhopperPlate]

I forgot about the impedance mismatch. It was obviously intentional, I'm curious to know why he did it that way. He must've thought it sounded better.

Ultimately the same reasons why I guys like EVH ran 8 ohm tap into a 16 ohm cab .

IMG_4370.jpeg