New Build and 102 thoughts:

[Raoul Duke]

So the 220k resistor and 500pf cap would feed one leg of the trimmer and the 1.2m would feed the other - then the wiper would feed the grid?

I’m still on the fence about it anyway. I guess I’m hedging there too. Hoping the trimmer will give me the OD sound I’m after rather than trusting the design of the circuit to deliver the 2nd gen OD (which is what I’m going for).

Realistically, I probably wouldn’t need it given all the data indicating that the layout delivers the 2nd gen “howl” as promised.

I am sold on the PI trimmer. Plus I’ll get to learn something new when it comes time to test and balance it - given there’s no “power amp in”.

[martin manning]

So the 220k resistor and 500pf cap would feed one leg of the trimmer and the 1.2m would feed the other - then the wiper would feed the grid?

No. Here's a schematic for #040, which has the funky OD entrance with different values. The trimmer (as a variable resistor) would replace the 22k (33k in the layout you're looking at), making an adjustable divider with the 470k (1.2 Meg to you). https://ampgarage.com/forum/viewtopic.p ... 834#p57834

I am sold on the PI trimmer. Plus I’ll get to learn something new when it comes time to test and balance it - given there’s no “power amp in”.

It's really no different. You still inject the sine wave at the PI input, or at the top of the Level pot if you prefer.

[Raoul Duke]

Martin,
I stared at the schematic awhile, then looked at your new board drawings - then it made sense. Some days I can follow schematics a little, other days - not so much. Still working toward my 10,000 hours there, lol. Those new drawings are great! That document of yours is a heck of a manual; a virtual “cookbook” of Dumble circuits.

Trying to iron out the push/pull switch on the bass pot. I’d like “pushed” to be the stock 2nd gen wiring and “pulled” to pull the .02 cap out of the tone stack. From what I think I understand - here’s my idea on the connections based on the A, B, C down the left side; D, E, F down the right side.

- connections from J/R switch stay the same, 10k stays connected to lug 1
- .0022 stays connected as per drawing
- .02 cap is connected one leg to terminal C, one to ground (pot case)
- lug 2 (sweeper) is connected to lugs B to E
- lugs A and D are connected and jumpered to ground

How’d I do?

[martin manning]

Trying to iron out the push/pull switch on the bass pot. I’d like “pushed” to be the stock 2nd gen wiring and “pulled” to pull the .02 cap out of the tone stack. From what I think I understand - here’s my idea on the connections based on the A, B, C down the left side; D, E, F down the right side.

- connections from J/R switch stay the same, 10k stays connected to lug 1
- .0022 stays connected as per drawing
- .02 cap is connected one leg to terminal C, one to ground (pot case)
- lug 2 (sweeper) is connected to lugs B to E
- lugs A and D are connected and jumpered to ground

How’d I do?

If I understand correctly, you want to use a pot with a push-pull switch for the bass control, and use the pull function to disconnect the 20n that is connected from the CCW lug to the wiper. Here is one way too do that, using a snip from a data sheet. I'm assuming the switch in the diagram is shown in the "pushed" position. May be useful, maybe not. I see it as a mid/treble boost.

[Raoul Duke]

Thanks Martin!

I can actually follow that mini-schematic. I must be picking it up - however slowly…

[rootz]

It's quite simple Raoul, the 20n cap is still on the bass pot, but you wire it via the switch on the pot instead of directly between the lugs.

I will scribble down some schematics for the OD entrance part. Also a simple mod, that is a crossover between a 2nd/3rd gen entrance and the 220k --> 100k trimmer seen in later amps.

[rootz]

These are some of the variations of entrances I've seen and two that could be done in your amp. Those two do what Martin said if I understand him correctly. The first of those options is the easiest one as it requires the least alteration of what already is there. It also can be adjusted to be totally stock if you set the trimmer to 33k (or was it 22k in your case?). Just replace the 33k resistor in the entrance circuit with a trimmer. Wire the wiper to the junction of the 33k and 220k/500p combo and one of the other two legs goes to the junction of the 33k/1.2meg. Of course the 33k won't be there anymore after the mod. I'd be happy to draw a layout if you want.

[Raoul Duke]

Hey, that’s very enlightening rootz! Thanks for doing that! It really identifies the subtle differences in a visual way for novices like myself. I’ll definitely study these and figure out the best one for this project. Very much appreciated!

Martin, here’s my interpretation of the bass pot schematic above. Did I read and apply it correctly?

IMG_0352.jpeg

Thanks again guys!

[martin manning]

Yes, that looks correct. You can use your DMM to check that the 20n is disconnected from the pot lugs when you pull the shaft out.

[rootz]

Hey, that’s very enlightening rootz! Thanks for doing that! It really identifies the subtle differences in a visual way for novices like myself. I’ll definitely study these and figure out the best one for this project. Very much appreciated!

Martin, here’s my interpretation of the bass pot schematic above. Did I read and apply it correctly?

IMG_0352.jpeg

Thanks again guys!

This looks neat! Definitely a mod I'm going to remember.

[Raoul Duke]

Thanks again guys!

Martin - it should’ve occurred to me to test it with the DMM; which I did upon your suggestion - and it works as intended. Thanks again for that schem!

The bass pot over hangs one corner of the PS board, but I can get a screw driver in behind (at a slight angle) with just enough purchase to snug the board screw down. At first glance I thought I’d have to skip the P/P pot; but it seems like it’ll all work.

The things you figure out as you go…

My tinkering and planning has evolved into “doing” - so I guess I’ll start another build thread soon. Can’t decide which I enjoy more: playing my 102 or starting a new project. Guess I could do both…

[rootz]

Can’t decide which I enjoy more: playing my 102 or starting a new project. Guess I could do both…

You're in the right place if this is what is on your mind haha. Would love to know how your mod works out.