Tube preamp build questions

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Hi everybody, I´m new to this forum, I have been reading for a couple of days and i can tell that I will learn a lot here, hopefully some day I´ll be able to help someone also. I´ve already built many stompboxes, power supplies, etc and eventually caught the tube virus. Long term goal: to build my first amp (possibly champ). In the meantime I made a tube preamp. It´s the preamp of the Marshall JCM 800 2204. Schem and layout:
[IMG:800:532]http://i68.photobucket.com/albums/i5/Mo ... 800pre.jpg[/img]
[IMG705]http://i68.photobucket.com/albums/i5/Mo ... layout.jpg[/img]

It works all right, I added an attenuator and a lopass filter at the output for better control, an opamp cab sim, an FX loop. It´s all inside an alu box, including the two transformers wich are in a "sub-box" in an attempt to minimize hum pickup.
Now, I have a hum problem, It´s more noticeable in the low input, in the high input it´s masked by the higher gain, but still there. I tried many things, (input and gain pot wires shielded) mainly moving things apart including the transformers, but the noise doesn´t change. It´s 100Hz hum (220V 50 Hz power here), wich leads me to think it´s post rectifier(s) hum, not mains hum (I already installed UF4007´s but no change)
So, right now i´m thinking two things
1- ground loop(s). What do you think about the PCB layout? It connects all grounds (signal, power)in a way that doesn´t inspire confidence in this regard, at least in my limited knowledge... Solution: put the power supply parts (outlined in red) in a different board and connect it pseudo-star-ground style with only one ground wire to the main board. Will that work?
2- snubber caps across each rectifiers diodes. What value? will that help?

Any help will be appreciated. Thanks

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[Tonegeek]

Welcome to the forum! If you could post some internal shots then we could see the layout and maybe be more helpful. Looks like DC heaters which should mean the problem is not there.

Are you feeding a power amp with a shielded cable from the output of the preamp? Even so you could have a ground loop between the devices. Try lifting the ground on the pre or pwr amp and see what happens. Also if you have a scope you can trace through your signal to where the hum originates. If you don't have a scope, see Luthierwnc advice on this thread: https://tubeamparchive.com/viewtopic.php?t=5905

good luck.

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Thanks for the welcome tonegeek!
Here´s an internal shot (sorry, cell phone pic).
[IMG:800:690]http://i68.photobucket.com/albums/i5/Mo ... gen017.jpg[/img]

At left is the controls panel. All that mess of wiring has been replaced with the shortest possible wiring, as i said, inputs and gain controls wires shielded. In that panel, inputs at the top, standby and power switches at the bottom. At inside bottom, the cab sim PCB. You can see the two power trafos in their own "sub box" that also contains the IEC power inlet. At the top right you can see the out, cab sim out, loop send and ret jacks.
Yes, the heaters are DC powered (7812 regulator). You can see the bronze colored heat sink for it in the preamp PCB in the pic.
The hum is the same plugged into the power amp in of my Fender hot rod deluxe and into my mixer through the cab sim. Anyway I´ll try your suggestion about lifting the ground at one end.
Unfortunately I don´t have a scope, so I can´t trace the signal...
I´ll read the suggested thread.

Thanks a lot for your advice!

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Edit: I´ve already tried to move all wires to see if the hum diminishes/gets louder (input, fx loop, controls, power and standby, all power wires to the PCB, all outs, grounds...no change)

[oldhousescott]

I would be tempted to isolate the heater ground/return from audio ground. That is, keep that ground branch entering from the left feeding the heaters only. Take the audio grounds over to that branch originating from the right side.

Also, I don't see any filtering on the output of the regulator, although it may not be necessary in this application.

[jelle]

Ground loop?

[Tonefishin]

Nice

[mdroberts1243]

I would be tempted to isolate the heater ground/return from audio ground. That is, keep that ground branch entering from the left feeding the heaters only. Take the audio grounds over to that branch originating from the right side.

Also, I don't see any filtering on the output of the regulator, although it may not be necessary in this application.

In my tube pedal, similar to this project, I found I had to reference the ground on the heater supply to the audio ground carefully (e.g. one point only, star ground style) to get rid of hum.

I also recall that these 78xx regulators sometimes need a certain amount of capacitance on their output to be stable... so I would consider trying adding capacitance to the regulator output as oldhousescott suggests.

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Thanks very much people for the help, I´m currently rebuilding the power supply part of the scheme in a separate board to try to eliminate ground loops. I´ll incorporate a cap in the 7812 output. I also added snubber caps (.001 Uf) to the rectifier bridges, we´ll see if that helps too. I´ll report success or failure soon.

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[Mostro]

OK people, I fnally got this thing to work with minimal hum. As I thought the PCB layout was creating ground loop(s). what I did was I removed all power supply parts from the PCB and built it in a new board with each ground of the power supply in a separate wire. I also isolated the heathers grounds from the PCB, also in separate wires, so the only grounds remaining in the PCB were audio signal grounds. Once I had all grounds in separate wires, i started experimenting until i arrived at the best connection between grounds (quasi-star) for the minimum hum. In doing this, i discovered that the biggest offender was the heathers ground, I believe because they are the ones with the most current. You wouldn´t think that they would be a problem with hum here because this thing uses 7812-chip regulated DC heathers, but... live and learn. Now the hum is only perceptible at all-controls-on-10 settings, wich I think is quite reasonable. Nice sound also! Here´s a pic of the thing finished:
[IMG540]http://i68.photobucket.com/albums/i5/Mo ... gen067.jpg[/img]
Thanks everyone for your help.
Now i have to get rid of the background noise at high gain! (just kidding)

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