New MC preamp is comming alive

[Stabist]

Hi!

I'm finaly almost finished with the chassis for my MC preamp - the schematic is based on CJ PV 10 MC module - so it's a tube preamp ...

The chassis is divided into two parts - one for PSU and one for circuitry ...

I just have one queston opened yet - where to put the ground conector - as close as possible to RCA's;? as close as possible to power cord;? somewhere else??

I've purchased my first more serious TT combination (Kuzma Stabi/Stogi Reference/benz micro Glider; ofcourse second hand)- so I' don't have proper experiences yet - it has two ground wires - one from the motor and one from MC head - should I connect them to the same GND conector?

The thing is - preamp itself was allready mounted in temporarily chassis - to test it - but the hum was BIG And it dissapeared when I touched the chassis or when I fiddled around the ground connector ..

Oh - btw - should I conect the ground from 220V outlet to GND conector or somewhere else to the chassis?

Thanks in advance for your help!!!

Oh - and I've made some pictures of an almost finished chassis

 

[Stabist]

another view

 

[Stabist]

and another

 

[Stabist]

And the last one ...

 

[EC8010]

You need the incoming mains earth to connect to the chassis as firmly as possible in order to provide a safety earth bond.

For minimum hum, you want the 0V of your pre-amplifier to connect to chassis by as short as possible a path from the star at the grid leak and cathode resistor. Use twisted pair to go to the phono sockets, and ensure that they do not contact the chassis.

(You would have done better to post this question at the "tubes" forum - it was pure chance that I saw it!)

 

[Stabist]

Thanks for advice!

And you're right - this is really more a question for "Tubes" section ...

 

[Stabist]

Finnaly the chassis is finished

FINNALY!!

After more than 5months!!!! the chassis is eloxated ... Well, this was a saga of some kind ...

So - few impressions:

 

[Stabist]

The sides will be from cherry wood ... so it's a little bit similar to PD's GC chassis - altough in this case it wasn't his artwork that inspired me - I got the idea before - but my realisation was so slow ...

 

[Stabist]

I like the back - I think it looks simple but classy ...

 

[SY]

Some very nice metalworking!

 

[Stabist]

Grounding - pleas help

Hi!

This MC preamp is mine first tube-based project - so I wonder - how to ground it properly to the chassis??
I mean - I put one conector at the back plate for arm and TT's ground - I guess I have to connect it to ground from 230V outlet .. But what else should I connect to that connector?? Maybe also the ground from RCA plugs - but via resistor?? Or???

Thanks!

 

[Stabist]

This is how the interior looks (it's still in experimental phase so wire lenghts are not "optimazed" yet) - on the right there will be tube preamp, on the left allready is PSU. In between an Al plate will come (altough I wonder - does Al plate represents any improvement??

 

[fdegrove]

Hi,

I put one conector at the back plate for arm and TT's ground

If you're using a MC cartridge you probably won't have to ground the arm, in fact I don't have to ground the TT either in my system.
Nonetheless you should experiment with whatever gives you no audible hum.

I guess I have to connect it to ground from 230V outlet ..

I never do that for preamps and similar devices; our local mains outlets aren't grounded anyway except for a few in the kitchen area and places where heavy machines are put.
May be different for you though....

Maybe also the ground from RCA plugs - but via resistor?? Or???

Use circuit ground for that...
Try to find a point with lowest noise by using a scope if you can.
I normally don't elevate the ground by means of resistors in a preamp.
If it doesn't have any groundloops there should be no need for it.

Cheers,

 

[Stabist]

Finished

Playing

No bigger hum - except the specific "hisssss" that all tube-based stuff I've owned had ...
I just have to find the reason for this - when somewhere near my room some stuff goes "on" (I suspect the refrigerator)- it can "pop" in speakers??
And also when turning preamp down - it's better to turn the volume down - othervise it pops very loudable ...

 

[Stabist]

Wood sides still need to be finished ...

Otherwise almost everything is finished ...

 

[SY]

Is that copper foil wrapped around the tubes? If so, do you have something inside it to prevent reflection of the plate IR emission back into the tube?

 

[Stabist]

And this is the TT setup I use ...

 

[Stabist]

Is that copper foil wrapped around the tubes? If so, do you have something inside it to prevent reflection of the plate IR emission back into the tube?

Yep it's copper ... What kind of reflection?? The copper shield is used because without they were to sensitive to hum - I just aproached to around 10-15cm from tubes - and the hum became to heavy ... I didn't know there can be some negative effects??

 

[SY]

Yes, there can be. Tube anodes are hot; being encased in a vacuum, the primary way they transfer heat away is by radiation of IR. You've just wrapped them in a mirror, relatively speaking. What you want is something black and heat conductive between the copper and the tube, and if possible, fins on the copper.

 

[Stabist]

Aha - now I see what you mean - well I'll intend to test them without cooper shield - but during "tests" and whithout cartridge load on the input they just begin to hum ...
But if there will be the need to leave the copper shield in place I'll try to fix the problem you mentioned!!! THANKS!

But then again - I must say that they are pretty cool - comparing with my ex-Unison Research SE amp - it had a pair EL34 and ECC82 - OK I understand for power ones they were HOT - but also ECC's were much hotter than this one here ...