Dumble "hum along"

[didit]

Hello -

Late last year I bought an attempt at a #102 from another member here. Fired it up when time allowed over xmas break. Immediately obvious it wasn't going to give immediate gratification. Set aside while finishing other bench space occupying projects. Found and fixed cold solder and a few other hot messes. Down to hopefully the final tweak to working as advertised, which is persistent 120Hz hum from the PI. Pull from any another socket and the hum disappears. Put a few known good tubes into there, including a 12AT7 from a quiet amp. So now I am looking for best advise. New to debugging any Dumble design. I suspect a poor ground, and if visual trace is correct this PI design finds ground via wire that loops from under, then over and then soldered onto the effects loop jacks -- i.e., the red wire in this close-up in that section of my amp. And yes, original soldering is occasionally crude, but that joint seems solid.
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#102 detail.jpg

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Best .. Ian

[ericlc]

Regarding the PI ground, I believe that is possible if that wire is connected to ground side the cap and resistor of the presence circuit.

[erwin_ve]

Hi Ian,

The fx loop looks odd to me-: is there a 220k/250pF connected to ground?( falls a bit outside the pic).
I suspect this is a #124 style fx loop with wrong wiring.

[didit]

The fx loop looks odd to me-: is there a 220k/250pF connected to ground?( falls a bit outside the pic).
I suspect this is a #124 style fx loop with wrong wiring.

Hello --

Thanks. There is paired cap + resistor in that general spot. This might be the exact clue. Will take a look, compare with schematic and get back to you. The OD entrance was a muddle of none that I could find and I corrected that, so care in matching #102 precisely was evident non-objective.

Best .. Ian

[didit]

As FYI --

Best I can determine the layout used to build out this amp is here --

https://ampgarage.com/forum/viewtopic.p ... 15#p218899

And as noted in the posting has "treble reduction network" on the loop jacks. It's too hot in the workshop today to take a closer look confirming actual build.

Best .. Ian

[stelligan]

As FYI --

Best I can determine the layout used to build out this amp is here --

https://ampgarage.com/forum/viewtopic.p ... 15#p218899

And as noted in the posting has "treble reduction network" on the loop jacks. It's too hot in the workshop today to take a closer look confirming actual build.

Best .. Ian

I have a fine sounding amp from that layout. That network or t-filter at the loop jacks de-nuts the amp for me as it cuts too much treble. It is wired with 2 switch jacks not 1. Has 2 caps and 1 resistor so, this is not as in that layout. Looks like a cold solder joint there as well. Using red wires for ground is unusual/confusing....

Yeah. I would yank that loop wiring and wire as real 102 here: https://ampgarage.com/forum/download/fi ... &mode=view

Happy hunting..

[didit]

Hello -

Thanks sir. Will rewire/remodel looping jacks per your recommendation. Looking at my photo it appears the builder only partially followed as the there is one plain & one switching jack installed. So far avoided lifting the main preamp board, and hope to continue that. Hence probably keeping that red wire despairing any confusion. We will get there. It's close already.

Best .. Ian

[daniboy79]

When I received my Ceriatone OTS FM50 ME kit, I studied the layout, schematic drawings and some hi-res photos of an example build for a few days before embarking on the build. I decided to copy the lead dress exactly as observed in the photos. I am convinced that this is the quietest amp that I have ever come across.

Maybe you can study some of the layout drawings as posted earlier and follow the ground point suggestions. All the best!

[amplifiednation]

More pics!

Is the red wire ground?

Not much to the loop wiring in these, it either connects or it doesn’t.

Does either of the volume controls make the hum increase/decrease?

[didit]

More pics!

I can do that. Traveling on the job but back home on the weekend. Will update with more photos.

Is the red wire ground?

Yes. Solidly soldered to the ground on the jack. And the meter confirms it is connected correctly on the board. Plan on ensuring the loop jacks are properly fixed for ground on the chassis as measured a few too many milli-Ohms there on the meter.

Not much to the loop wiring in these, it either connects or it doesn’t.

Absolutely, which makes this puzzling.

Does either of the volume controls make the hum increase/decrease?

Just the ‘master’ and it goes up and up. At even 12 o’clock the hum is quite loud.

Thanks Taylor

Best .. Ian

[didit]

Hello -

Got a bit of shop time in recent days. Reseated the "loop" jacks and reflowed solder all through the PI cathode path to ground. And happily that hum is significantly reduced. Running clean, the amp is now reasonably quiet. Next step probably a full rework precision power supply, which I'd planned from the outset. Reflowing all solder seems in order. There is significant hum when the overdrive is engaged. Doubtless this was there all along, just masked from hum in PI. Will now trace out where the probable problem spots are for grounding that.

So significant positive progress to report. Any guidance or suggestions always welcome.

Best .. Ian