#183 B+....

[mrdarwin]

Hum is 100Hz around.
Voltage on pin 5 on power tube is -39.9V.
When I ground the PI, hum disappears, so the problem is in preamp section.
Searching...

[mrdarwin]

Here you can the differents xformer placements...
Do you think the OT is too near of the preamp board and V1?...

[martin manning]

I don't think the OT location is a problem. 100 Hz means that the source is rectified voltage, either B+ or relay power. Try disconnecting the relay power supply.

At what point did you ground the PI to stop the hum? Is your loop return jack making good contact with the preamp output? You can try putting a patch cord from send to return to check.

[mrdarwin]

Disconnected completely xformer psu for lamp and relay power. No changes.
When I ground the jack loop return, hum disapears, jack loop send, hum disappears. But if I ground input, hum here. Bad grounding somewhere?
Connection is good between send/return.
I changed V1, and get less noise...

[larsvictor]

What happens to the hum if you turn the (preamp) volume down (grounding grid V1b) ?

[martin manning]

What about OD vs. clean? If you have hum in both modes then the issue is in the clean (V1) section. Larsvictor's suggestion will then isolate it to V1a or V1b.

On the bright side, looks like this Marshall PT option is a good one. Voltages are in line, as is heater current capacity, it fits the chassis cut-out, and circuit mods are simple.

[mrdarwin]

What happens to the hum if you turn the (preamp) volume down (grounding grid V1b) ?

Hum disappears....

Hum is present only in OD mode

[larsvictor]

Hmm...No hum with preamp volume down is a hint that the hum enters the signal path before V1b. But then it should be present in both modes. Is it just amplified with overdrive engaged ? Please check it out with OD disabled and full master volume. Can you still hear it ?

Disconnected completely xformer psu for lamp and relay power. No changes.

Hum is present only in OD mode

?...normally with no current through the OD-relay the amp is in clean mode. If there´s hum only in the OD mode you obviously decided to use the Ceriatone design for the OD-relay (in Ceriatone amps the n.c. position is: OD engaged).

Ground lug 3 of the bass pot with a patch cable (bypassing 10k resistor), turn bass pot fully down, r/j to rock, PAB disabled. What happens to the hum with treble fully down ?

[larsvictor]

Ground lug 3 of the bass pot with a patch cable (bypassing 10k resistor), turn bass pot fully down, r/j to rock, PAB disabled. What happens to the hum with treble fully down ?

Sorry, error in reasoning. This will not really isolate the spot. But maybe this: Bypass the 4M7 resistor at the midboost switch with a patch cable to ground. If you keep the switch open (= midboost on) with PAB on and there is no hum the problem is located in the output section of V1a. If there is still hum the issue is behind the midboost switch (hum should dissapear then with midboost off)

[mrdarwin]

finally back...
rectifications:
when preamp volume down: hum still here
when OD disabled and full volume: hum is here, so I can say hum is amplified in OD mode
yes, the relay is Ceriatone mod

[mrdarwin]

Ground lug 3 of the bass pot with a patch cable (bypassing 10k resistor), turn bass pot fully down, r/j to rock, PAB disabled. What happens to the hum with treble fully down ?

Sorry, error in reasoning. This will not really isolate the spot. But maybe this: Bypass the 4M7 resistor at the midboost switch with a patch cable to ground. If you keep the switch open (= midboost on) with PAB on and there is no hum the problem is located in the output section of V1a. If there is still hum the issue is behind the midboost switch (hum should dissapear then with midboost off)

hum is here in zall situations

[mrdarwin]

I've got a Ceriatone Dumble, not a kit but an amp built by Nik.
I can hear the same hum but not as loud as the 183.
I have to try to plug my amp in another place...

[larsvictor]

Well, it seems that the problem is located around V1b.

Since the hum is
- still there with no power in the relay,
- still there with preamp volume down,
- amplified by the 2nd tube and
- dissapearing by grounding the send jack (= master volume down)
there is nothing left but V1b and surrounding components.

[67plexi]

I've only built 15 D style amps but lead dress is important
And a Ceraitone has nothing close to a Dumble amp.
My #183 build used exact parts as the real one no hiss no hum.
Possible cold solder joint or ground loop issue.
You may want to try a on/on/on switch on the rock/jazz switch
that will bypass the tone stack.
Next I ask never use Dumble and Nik in the same sentence please.

[mrdarwin]

Thanks.
I'm gonna look around V1b, and find a way to bypass tone stack.

I've built 8 amps based on Ceriatone layout (with personnal mod, not dumble based amp) with nice sound.
Layouts are clear and precise. When I look on Nik's layout and other layouts, I can't see rezally major differences. I think the most important things about an amp is the quality of components and the time you spent wiring... but I c'm curious about other opinions and arguments.